Life in the Universe

CSIR GOLDEN JUBILEE SERIES LIFE IN THE UNIVERSE M S CHADHA BAL PHONDKE LIFE IN THE UNIVERSE M.S. CHADHA BAL PHONDKE...

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CSIR GOLDEN JUBILEE SERIES

LIFE IN THE UNIVERSE

M S CHADHA BAL PHONDKE


M.S. CHADHA BAL PHONDKE

National Institute of Science Communication Council of Scientific & Industrial Research Dr. K.S. Krishnan Marg, New Delhi-110 012 India

Life in the Universe M.S. Chadha Bal Phondke © National Institute of Science Communication (erstwhile PID) (CSIR) First Edition: January 1994 Reprinted: October 1995 February 2000 ISBN: 81-7236-084-3

CSIR Golden Jubilee Series Publication No. 11 Series Editor

Dr. Bal Phondke

Volume Editor

Purnima Rupal

Cover Design

Pradip Banerjee

Illustrations

Pradip Banerjee, Neeru Sharma, Sushila Vohra, Malkhan Singh, J.M.L. Luthra and K.K. Bhatnagar

Production

Radhe Sham, Seema, Sukamal Mondal, A.K. Anand, S. Bhusan, Sudhir Chandra Mamgain and G.C. Porel

Designed, Printed and Published by National Institute of Science Communication (CSIR) Dr. K.S. Krishnan Marg, New Delhi 110 012, India

Foreword The Council of Scientific & Industrial Research (CSIR), established in 1942, is committed to the advancement of scientific knowledge, and economic and industrial development of the country. Over the years CSIR has created a base for scientific capability and excellence spanning a wide spectrum of areas enabling it to carry out research and development as well as provide national standards, testing and certification facilities. It has also been training researchers, popularizing science and helping in the inculcation of scientific temper in the country. The CSIR today ia a well knit and action oriented network of 41 laboratories spread throughout the country with activities ranging from molecular biology to mining, medicinal plants to mechanical engineering, mathematical modelling to metrology, chemicals to coal and so on. While discharging its mandate, CSIR has not lost sight of the necessity to remain at the cutting edge of science in order to be in a position to acquire and generate expertise in frontier areas of technology. CSIR's contributions to high-tech and emerging areas of science and technology are recognised among others for precocious flowering of tissue cultured bamboo, DNA finger-printing, development of non-noble metal zeolite catalysts, mining of polymetallic nodules from the Indian Ocean bed, building an all-composite light research aircraft, high temperature superconductivity, to mention only a few. Being acutely aware that the pace of scientific and technological development cannot be maintained without a steady influx of bright young scientists, CSIR has undertaken a vigorous programme of human resource development which includes, inter alia, collaborative efforts with the University Grants Commission aimed at nurturing the budding careers of fresh science and technology graduates. However, all these would not yield the desired results in the absence of an atmosphere appreciative of advances in science

and technology. If the people at large remain in awe of science and consider it as something which is far removed from their realms, scientific culture cannot take root. CSIR has been alive to this problem and has been active in taking science to the people, particularly through the print medium. It has an active programme aimed at popularization of science, its concepts, achievements and utility, by bringing it to the doorsteps of the masses through both print and electronic media. This is expected to serve a dual purpose. First, it would create awareness and interest among the intelligent layman and, secondly, it would help youngsters at the point of choosing an academic career in getting a broad-based knowledge about science in general and its frontier areas in particular. Such familiarity would not only kindle in them a deep and abiding interest in matters scientific but would also be instrumental in helping them to choose the scientific or technological education that is best suited to them according to their own interests and aptitudes. There would be no groping in the dark for them. However, this is one field where enough is never enough. This was the driving consideration when it was decided to start in the 50th anniversary year of CSIR (1992) a series of profusely illustrated and specially written popular monographs on a judicious mix of scientific and technological subjects varying from the outer space to the inner space. Some of the important subjects covered are astronomy, meteorology, oceanography, new materials, immunology and biotechnology. And many more interesting topics are to be covered in the forthcoming titles. It is hoped that this series of monographs would be able to whet the varied appetites of a wide cross-section of the target readership and spur them on to gathering further knowledge on the subjects of their choice and liking. An exciting sojourn through the wonderland of science, we hope, awaits the reader. We can only wish him Bon voyage and say, happy hunting.

Preface In a way, the Earth, enjoys a rather unique position in this vast universe. It is the only planet known so far to have supported life; at least the carbon-based life needing water and oxygen for survival. Since, as Copernicus established a few centuries ago, neither the Sun nor the Earth is unique in any other respect and since there is no dearth of stars with plenetary systems of their own, the question naturally arises as to why the Earth alone should give rise to life? And continue not only to support it but also help it evolve into a multi-splendorous entity? Attempts to find satisfactory and cogent answers to these questions have been made since millenia. Some scientists had, in the process, opined that life was seeded on this planet from somewhere else. But that does not answer the question: How life originated in the first place? It merely shifts the site of origin. Others had invoked the existence of a 'creator' responsible for the genesis. That solution to the vexing problem was shattered by Louis Pasteur, who so very convincingly demonstrated that living organisms do not arise spontaneously; they arise from preexiting organisms. Pasteur, thus, put us back at square one. But steady investigations, painstaking at times, painful another, have started slowly lifting the veil from this eternal mystery. Following that trail of search is as thrilling as the mystery itself.

Acknowledgements Looking at the series of beautiful, low cost and easy to read books being brought out by PID, New Delhi, one of us (M.S.C.) asked the other (B.P.) whether we should not attempt a book on the origin of life jointly. The answer was a spontaneous "Yes" and it was decided that this could be one of the books in the CSIR Golden Jubilee Series. Enthused by the opportunity, we started on this joint effort and it has been an enjoyable experience. This task has been accomplished through a series of meetings whenever an opportunity arose for the two of us to be in the same city at the same time. This book has evolved slowly but steadily from the first draft which was heavy on facts to the present version, which in our opinion is interesting and easy to read. The draft of the book was perused by Dr. V.G. Kulkarni, Director, Homi Bhabha Centre for Science Education and Dr. K.B. Sainis of Molecular Biology and Agriculture Division of BARC and their valuable comments helped us to improve the text. We acknowledge our thanks to them. Ms Purnima Rupal has excelled herself in providing editorial assistance and so has Mr. Pradip Banerjee and his group in doing the art work. Their dedicated and enthusiastic effort is highly appreciated. Mr S.K. Nag has overseen both editorial and production work of the book. He deserves a special mention. Thanks are due to the staff of BARC and PID who offered secretarial and technical assistance in writing this book. M.S. CHADHA BAL PHONDKE

Dedicated to the International

Society for

the Study of Origin of Life (ISSOL)

Contents What is Life?

... 1

The Universe

... 10

Building Blocks

... 26

From Dust Thou Comest

... 37

The Emergence

... 50

Cooking the Soup

... 63

The First Steps

...72

The Family Tree

... 85

We are Not Alone!

... 96

Glossary

... 109

haron KATCHALSKI (1913-1973), the famous Israeli Bio-physicist and the brother of the equally eminent scientist, Ephraim KATCHALSKI (b. 1915), who later became President of Israel, was asked to define Biophysics. The year was 1969 and Katchalski was presiding over the third International Congress of Bio-physics held in Boston, USA. He said /'Biophysics is like my wife. I can recognise and appreciate her but cannot define her".

A

What Katchalski said of Biophysics is equally true of life. It defies definition. What is life is a question man has asked himself for centuries. Very early in his understanding man realised that the world was divided into two categories of entities, the animate and the inanimate, the living and the non-living. Only by observing the two very intensively was man able to draw some inference about the essential differences between the two. These perceived differences, in turn, contributed to the definition of life Consequently, at one time, a living being was thought to be the one which grows. But then it was seen that even crystals grow. If life is in those that move, water

2


The world is divided into the living and the non-living

moves as in rivers and streams. On the other hand, trees dc not move and yet belong to the living world. ARISTOTLE (384-322 B.C.), the legendary Greek philosopher, once remarked that if one started a journey from the simplest of matter, say an atom, towards the most complex organized system like the human species, one cannot identify a single point on the road where the non-living world ends and the living world begins. Despite all the scientific advances made since then, many scientists would tend to agree with Aristotle. Indeed, as Katchalski had observed, it is easy to identify an animal as living and a stone as non-living; but it becomes almost impossible to state in precise terms what is living and what is not.

WHAT IS LIFE?

4


That is why many gave up defining life in a precise manner. Instead, they offered certain analogies to describe their concept of life. The British biochemist Norman PIERI (b.1907) found that a mineral can be identified with certainty as non-living. On the other hand, a tree or an animal can be said to be surely belonging to the living world. However, he saw that such a clear cut distinction cannot be made when it comes to a virus. Wendell STANLEY (b.1904) had, in fact, crystallized the tobacco mosaic virus which brings about disease to the tobacco plant. The crystals could be kept in his laboratory like one keeps a stone or a block of wood. One could call it non-living at that stage. However, if the very crystal was placed on the leaf of a tobacco plant, it could infect the latter thus showing signs of life. Pieri, therefore, found the question of defining life similar to that of defining an acid, which can be said to be different from a base and vice versa. However, there is water that is neither an acid nor a base. So, to overcome these conundrums scientists defined acids and bases in terms of the concentration of hydrogen ions in a liquid. Life, likewise, will have to be defined in terms of either the constituents or characteristic attributes of a living system. Several such representative features of life were suggested by different scientists from time to time. At today's level of understanding, we recognize living things as those which have the capacity to grow, replicate and repair themselves. But there is one more attribute that is quite unique to life forms and which has sustained life on this Earth for the past three and a half billion years. Living systems are capable of generating basic changes in their inherent constitution and inheritable characteristics. These alterations — the mutations — occur at first in a random fashion. Besides, the living system can sustain these mutations and carry them forward from generation to generation. Those mutations that can survive in the existing environment thrive and continue to hold their own. In turn, they give rise to a new organism. It

WHAT IS LIFE?

5

Living things can grow, replicate and repair themselves

is these processes of mutation and natural selection that have resulted in the bewildering variety and complexity of life forms in the world today. This process of finding a cogent answer to the question 'what is life?' has made it clear that there is a continuous link from the simplest non-living system of an atom of hydrogen to the most complex living system of the human species. Erwin SCHRODINGER (1887-1961), one of the founders of modern quantum physics, emphasised this observation in his classic book What is life? Published in 1944, the book provided a basis to look for characteristics of life in its physical and chemical constituents. It thus reiterated the view expressed

6


by Neils BOHR (1885-1962) that the nature of life could be explained on purely physical basis. There was no necessity to invoke a mysterious 'life force' to explain the existence of life. These thoughts have left a deep impression on all subsequent efforts, including those currently underway, aimed at finding how life originated on this Earth. The early ideas about origin of life were steeped in metaphysics and religious thoughts. These even gave rise to the hypothesis that God created the entire spectrum of living organisms. Modern scientific theories, however, now accept unanimously that life forms arose on the Earth as a logical consequence of the physicochemical interactions that have been incessantly going on ever since the universe came into being. This idea of life arising in a way from non life has been in vogue for centuries. Aristotle had propounded such an idea. Even the Rig Veda postulated that life started from the primary elements, while the Atharva Veda postulated the ocean as a cradle of all living things. The dark ages in between, however, appear to have distorted these apt concepts. This, in fact, led to the enunciation of the

WHAT IS LIFE? 17

idea of 'spontaneous generation'. This premise suggests that life forms originate from the inanimate world 'suddenly and spontaneousObservations of ancients about the emergence of worms from mud, maggots from decaying meat and mice from old linen had helped them in such beliefs. During the later part of the 17th century, using the newly discovered

Redi's Experiment

microscope Robert HOOKE (1635-1703) and Anton van LEEUWENHOEK (1632-1723) observed the existence of micro-organisms in decaying vegetables and even stored water. They were unable to explain their origins. People, therefore, took this observation to support their notion that living organisms arise spontaneously out of nowhere. However, Fransesco REDI (1626-1697) showed in 1668, that if the jar of meat was covered with muslin no maggots would emerge as no flies could lay eggs on the decaying meat. Despite this, people continued to believe that living organisms appear spontaneously. It was only in 1860 that Louis PASTEUR (1822-1895), through simple experiments, dealt a death blow to this theory of spontaneous generation. He showed that if two swan-neck flasks were heated, so as to kill

8


Haldane put forward the concept of 'Primordial Soup'

all living organisms that may be present, their contents remained free of living organisms even later. However, if the neck of one of the flasks was broken the contents did not remain sterile in it, while in the other one they continued to be sterile. The foundation of a modern approach to the origin of life was laid by Charles DARWIN (1809-1882). In 1871, while writing to his friend Hooker, he said, "if we could conceive in some warm little pond, with all sorts of ammonia and

WHAT IS LIFE?

9

phosphoric salts, light, heat, electricity, etc. present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed which would not have been the case before the living creatures were formed". For more than 50 years this idea remained dormant. However, in 1924, Aleksandr OPARIN (1894-1980), a Russian biochemist published a small book entitled Proiskhozhdenie Zhizny. In this book, he pointed out that the properties so characteristic of life must have arisen during the process of the evolution of matter. In 1928, John Burdon Sanderson HALDANE (1892-1964), the British biologist who spent later years of his scientific activity in India, wrote a classical paper, The Origin of Life. According to Haldane, when UV light acted upon the Earth's primitive atmosphere a variety of organic substances were made including sugars and protein-like materials. According to him, before the origin of life, these substances must have accumulated until primitive oceans reached the consistency of a hot, dilute soup. He, thus, gave the concept of Primordial Soup. All these theories had one strong common thread, that life was invariably linked to the nature of the universe. The seeds of life were sown at the time the universe came into being. Gradually, as the universe, which itself was quite nebulous to start with, started taking shape events took place which were inevitably to lead to the emergence of life. The origin of life can thus be traced to the origin of the universe itself.

s a child grows and looks at the world around, its innate curiosity is aroused. The fascinatingly diverse surroundings tickle the child's growing imagination. Prompted by its keen powers of observation, a number of questions occur to the fast learning toddler. It tries to find the answers to most of these questions through self-experience. And when this does not resolve the issue the child runs to its parents and other elders. One question which every little one asks, sooner or later, is "Where did I come from?"

A

The Universe

This is a common, everyday version of an eternal question that has confronted man ever since he first appeared in this world. Even as he went about his daily routine of hunting and gathering the direly needed food for himself and his clan, he collected evidence that told him that there are two kinds of entities that inhabit the land known to him. One kind, like him, can grow, move, and needs food and water to maintain itself. It is easily affected by the natural elements The other kind remains rooted to one place and does not grow. Neither does it need any food to continue to exist. The

THE UNIVERSE

11

12


elements too have little, if any, impact on it. He must have realised that there is something different in him, the living, than the other, the non-living. Another realisation that came early to man was that he was but a part of the universe. Lying on his back at the end of a tiring day his eyes would have traversed the limitless sky above containing countless numbers of shining, glowing and apparently steady celestial bodies. Distant and hence mysterious though they looked, he felt that they somehow influenced his well being. He, therefore, endowed them with divine powers. As his understanding grew, he held them responsible for the making of all that he saw around him. Our ancient books of knowledge contain no dearth of references suggesting that everything on this Earth owes its existence to the Panchamahabhutas, Prithvi (Land), Aap (Water),Te; (Fire), Vayu(Air) and Aakash (Space). This enunciation embodies the thought that man, and indeed the living world, is a part of this universe and not different from it. But the universe is so vast that there are some hundred billion (1011) galaxies, each with an average of a hundred billion stars. Thus, in all perhaps, there are as many as 11 11 2" 10 xlO ,that is 10 , or ten billion trillion stars. Our own Sun is one such star with as many as nine planets orbiting around it. Besides the planet Earth, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto constitute our solar system. In addition, there are 53 satellites, thousands of asteroids and billions of comets. 1

The immense spread of the universe can be further appreciated if one starts to measure the intergalactic and other distances. Familiar units of distance, like kilometre, are grossly inadequate to express these stellar distances. Instead, they have to be measured in terms of the speed of light. In one second, a beam of light travels 300,000 kilometres or can go round the Earth seven times. So 300,000 kilometres is equal to one light-second. It takes light eight minutes to traverse the distance between the Sun and the Earth. One can, thus,

THE UNIVERSE

13

Pancliamahabhutas

14


say that the Sun is eight light minutes away from the Earth. The distance which light covers in one year is nearly 10 trillion (1013) kilometres. This is called one light year. In our own galaxy, the distances between our Sun and other discernible objects are of the order of 15,000-100,000 light years. Furthermore, the 100 billion stars of the milky way are not arranged symmetrically; they lie in a flattened disk that extends for 100,000 light years from the central region. To reach out to these distant objects, astronomers made use of telescopes. In 1610, Galileo GALILEI (1564-1642) O u r Milky W a y used the telescope for the first time to make astronomical observations. He discovered that the planet Jupiter was not alone, but had several satellites like our Moon. He also observed that there are hills and valleys on our Moon. Many improvements by other scientists and astronomers followed over the decades. Powerful as they were, even these instruments could not give enough details of the stars, as these were extremely distant and faint. But another discovery came to the scientists' aid. Astronomers made use of the fact that many stars, including our Sun, send out radiowaves. These could

THE UNIVERSE

15

Copernican universe

be picked up and processed to reveal secrets of the transmitting celestial body. So, radio-telescopes came into vogue for the first time about 30 years ago. They could tell how hot a star is or how fast it is moving. These powerful telescopes and radio-telescopes have unraveled the secrets of the outer space during the last few decades. A closer look at the planets in our own solar system has been made possible as well as details about the topography and the nature of atmospheres of several planets have been provided. The mission of man to the Moon and pictures taken by man-made satellites moving around and away from the Earth have brought forth spectacular images of the Earth we inhabit. All these have more than vindicated the truth about the planetary system as enunciated by Nicolaus COPERNICUS (1473-1543) around 1510. Furthermore, several of the old theological beliefs like flatness of the Earth and movement of the Sun around the Earth have been buried for all times to come.

16


Giant metre-wave radiotelescope

Man has also come to realize that the formation and evolution of a planet is intimately related to the evolution of its Sun, a star. This is because the stars themselves are the manufacturing plants for all the chemical elements. Our Earth was formed at the same time as the Sun, the other planets and small bodies of the solar system about 4.5 billion years ago from primitive material in the interstellar space. Naturally, the relative abundances of the chemical elements, probably the chemical compounds into which they were combined, and perhaps the detailed physical properties of the pre-Earth material were established by processes taking place in space before the Earth actually came into being.

THE UNIVERSE

17

T h e Big B a n g

But how was the universe itself born? Scientists now believe that in the beginning there was only a singularity, an entity which had no volume but had a very high density. The result of this extreme, almost infinite compaction was very high temperature and almost unbearable pressure leading to an enormous explosion — the Big Bang. Initially, the explosion caused only radiation of a mammoth amount of energy and spread of gaseous material. The elemental products of the Big Bang, which created an expanding

18


UjLLtyjf.

I 'I

i

I

"/"»

Chandrasekhar and his limit

universe, were hydrogen and helium, but no carbon. The first generation of stars was formed out of these light elements. The life history of a star is extremely fascinating, although scientists are acutely aware that their knowledge of it is rather limited. Whatever little is known, they owe it largely to the India-born Nobel laureate Subrahmanyam CHANDRASEKHAR (1910-1995). There is now a general agreement that some 100 million years after the Big Bang the primordial gas, which was thrown about as a result of the outburst, gave rise

THE UNIVERSE

19

to galaxies. These originated out of the randomly colliding gas molecules. Gravitational attraction could coalesce and keep together at least some of these molecules thrown against each other. Thus a stable cloud of gas, which did not disintegrate further, was formed. The cloud was more or less homogeneous, a smoothly dispersed entity, not unlike fluid in motion. It is common experience that any moving fluid turns turbulent if its velocity crosses a certain limit. Since the gas cloud, that is the prototype of a galaxy, is moving circularly, the turbulence leads to breaking up of some particles in it along the outer edges. These whirling particles, colliding with each other, could coalesce and condense together if the gravitational attraction was strong enough. The process has been likened to the formation of hailstones in the Earth's atmosphere. Such condensates constituted the embryonic protostar. This nucleus could attract matter from its surroundings to grow bigger. But as it did so, the gravitational attraction from its centre led matter on the periphery to collapse inwards, increasing the density at the centre. This kind of a chain reaction increased the rate at which particles moved inwards, generating intense heat. This shrinking of an embryonic star is mind-boggling. For example, our Sun has an estimated diameter of some 1.4 million kilometres. But, astronomers believe that during the Sun's embryonic stages its diameter must have been as large as trillions of kilometres. The rise in the temperature of its core can then be equally dramatic. When the temperature reaches about 10 million degrees, the nuclei of hydrogen gas collide together with such great force that the electrical repulsion which keeps them apart is easily overcome. This is the process of fusion by which hydrogen is converted to the next heavier chemical element, helium.

20


Stellar furnace

When protons and neutrons, the individual nuclear particles, come together to form an atomic nucleus, the combination is more stable. It also contains less mass than the combined mass of the independent particles. The 'missing' mass is converted into energy, either as heat or light and is radiated away. Likewise when hydrogen nuclei are thrown together and get converted into helium nuclei, some mass is converted into radiant energy. A thousand tonnes of hydrogen are converted into only 993 tonnes of helium and seven tonnes of mass is transformed into energy. This is the process by which stars like our Sun generate the energy which they radiate.

THE UNIVERSE

21

This commissioning of the stellar furnaces, which infuse 'life' in a star, also stabilise it. For the inward gravitational pull is now counterbalanced by the outward pressure generated by the fusion reactors at the centre. So, further collapse of the star is thwarted. But the hydrogen fuel that stokes these furnaces is not unlimited. Sooner or later it gets exhausted. When that happens, the furnaces shut down, but only for a while. Because, with their shutting down, the outward pressure which was preventing further shrinkage of the star also ceases to act. This brings about a rise in the central core density with a concomitant increase in its temperature. This goes on until the temperature reaches a level where helium nuclei can combine into the next heavier element, carbon, with the 'missing' mass once again metamorphosing into energy. The stellar furnaces are back in harness, bringing stability to the star. In a time period shorter than it took hydrogen stores to be used up, the helium supply too gets exhausted. The fusion reactors are shut down until the gravitational collapse raises the temperature to a level where carbon nuclei can fuse. And this phenomenon keeps on repeating with the core of the star being successively made up of such heavy elements as silicon, sulphur, argon, calcium and finally iron, the most tightly bound element. The stellar furnaces down their shutters for good once the iron core is formed. With the closing down of the fusion reactors there is no more the outward pressure to resist the gravitational pull. Further collapse, therefore, occurs very rapidly. The tremendous amount of gravitational energy that is generated sends massive shock waves towards the periphery of the star causing it to explode violently — becoming a supernova. The violence of this explosive event can be so great that the luminosity of the star increases thousand or million fold, so much so that a single star may produce, albeit briefly, more light than an entire galaxy. It is a spectacular sight indeed!


22

Supernova1987A

But the supernova also gives rise to a lot of fragments of the exploding star. This graveyard of the dying star becomes the cradle for a new star to evolve. Our Sun is one such second generation star built out of cosmic dust that was left over by stars that had burnt and exploded billions of years ago. Helium, carbon and oxygen created in the first generation of stars returns to a pool of elements in the interstellar medium. They participate in the formation of heavier elements by more complex nuclear reactions that take place in the second arid later generation stars. A full complement of stable isotopes of carbon, nitrogen and oxygen, some of which return to the interstellar medium, are thus generated. Therefore, it is obvious that the elemental and isotopic composition of the interstellar medium is constantly changing. When the solar system was formed from the interstellar

THE UNIVERSE

23

Libby's concept of radiodating very old objects

medium 4.5 billion years ago, our galaxy was already about 12 billion years old and its composition was well evolved. Our Earth is of the same age as the solar system. It is now possible to determine fairly accurately the age of ancient rocks and that of the Earth itself. Willard LIBBY (b.1908), the American Nobel laureate, was the first to think of a novel method of determining the age of very old objects. He observed that carbon exists in nature in three independent isotopic forms 12C, 13C and 14C. Of these C and C are stable isotopes, while 14C is radioactive and hence decays over a time period.

24


When living objects, like plants absorb carbon in the form of CO2 from the air, they show no preference to any of the isotopes. In other words they ingest, say, both 12C and 14C equally. So, the proportion of 12C to l4 C in any living being should be the same as that which exists naturally in the atmosphere. This would be so since the chemical proportion of the two isotopes are identical. The proportion of the two isotopes inside the plant would continue to be the same as in the environment as long as the plant lives. When the plant dies, however, the amount of 12C continues to remain the same but that of 14C starts decreasing due to its continuous decay. It takes 5,568 years for the radioactivity of l4 C to be reduced to half its original value. It is obvious that at the end of that time the ratio of l2C to l4C would also stand reduced to half its value in the living plants. Thus, by measuring the ratio of the two isotopes in an old fossilised object and computing its relationship to the ratio in the environment, the age of the object can be easily determined. This technique of dating an object, based on the measurement of radioactivity of carbon in it, is useful for studying specimens up to about 40,000 years old. The measurements of radioactivity of other elements that decay at a rate far slower than that of carbon can prove useful for much older samples. Several such methods dependent on radioactive decay of uranium, rubidium and plutonium are now available. These permit fairly accurate determination of the age of objects billions of years old. The isotopic composition of lead (Pb) at the time of the formation of solar system can be accurately determined by measurements of Pb in certain meteorites. It is assumed that the source of lead on the-Earth has always been the decay of uranium (U) isotopes " U, * U, and thorium Th. The ratio between the parent uranium and the daughter lead can give an idea of the age of the sample. Based on such measurements, the age of the Earth is found out to be 4.5 billion noo

IOC

J

THE UNIVERSE

25

years. On the other hand, the oldest known rocks are about 3.8 billion years old. In the time between its birth 4.5 billion years ago and that of the oldest rock at 3.8 billion years the Earth became the round sphere that it is. Simultaneously, the rocks of the crust became solid and the ordinary geological processes familiar to us today started playing their role. Yet, what that earliest stable Earth was is a question still beyond our grasp.

t was an early spring day in 1953. Two young men, obviously in high spirits, burst through the doors of Eagle, a typical English pub on the banks of the petite Cam river in Cambridge. In an excited voice, they announced to the whole gathering that they had solved the riddle of life.

Building Blocks

Young brash students playing pranks is not unknown in Cambridge. But this was no empty boast. For the duo was that of Francis CRICK (b.1916) and James WATSON (b.1928). And they were talking about having discovered the three dimensional structure of the biological molecule, deoxyribonucleic acid, DNA. The discovery received due recognition with the award of the Nobel Prize nine years later. Ever since, DNA has come to occupy centre stage in the theater of molecules that constitute life. There are other chemicals, some more important than others, that contribute their might to sustain life. But none can perhaps be considered as crucial as DNA. For it is the repository, in coded form, of all the information that is necessary to build chemicals and make them behave in a well

BUILDING BLOCKS

Francis

Crick

27

James

Watson

28


orchestrated manner, so that a living being functions like a well oiled complicated machinery. What is more, DNA, ensures that the living organism reproduces itself giving birth to progeny which carries the same features and traits that characterise the parents. It is not for nothing that DNA is dubbed the molecule of heredity. DNA best illustrates the ingenuity of nature in designing a structure that is most aptly suited for the function intended to be carried out by a chemical. It is made up of two chains of nucleotides coiled around each other in a double helix, much like mating serpents holding each other together in a twisted fashion. It can alternatively be looked upon as a spiral staircase where the hand rails are made up of a chain of sugar and phosphate molecules and the steps are made up of a pair of nitrogenous bases. The sugar in the two strands of the DNA double helix is a deoxyribose molecule. There are four different bases: Adenine, Thymine, Guanine and Cytosine. Adenine on one strand always links up with Thymine on the opposite strand. Likewise, if the base on one strand is Guanine, the other strand pairing with it is sure to have Cytosine at the corresponding site. There are several advantages of such a structure apart from its aesthetically highly pleasing appearance. The bases are only four in number. But they can be arranged on a strand in a large number of different sequences, thus creating an endless variety of DNA molecules. In turn, this is reflected in the fascinating diversity of life forms that inhabit the Earth. Since the four bases pair up in a unique fashion, the two strands of the helical molecule become complementary to each other so much so that if the sequence of bases on one strand is known that on the other can immediately be deciphered. Indeed, nature employs this very stratagem for replication of this molecule. This ensures that hereditary

BUILDING BLOCKS

29

ABCDEPG HIJKLMN 0PQ.R5TU VWXYZ

CAT DOG BALL POY EGQ WOLF ZEBRA FISH APPLE ROSE

Alphabet of life consists of but four letters

information is passed on from one generation to the next in a highly faithful manner. The structure also provides a mechanism by which mutations or changes in hereditary information can be brought about. Change or even deletion of just a single base on a strand significantly alters the sequence and hence the characteristic feature of the DNA molecule. This can be best explained by an analogical situation in English composition. Consider, for example, the sentence

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THE BAIL FELL OFF THE STUMPS A handful of English language letters are arranged here to make a meaningful sentence. The sequence of the letters is extremely important to convey a message. Suppose now only one of these letters is changed. Instead of the letter T in the sixth place, let there be the letter 'L'. The sentence, will now read as THE BALL FELL OFF THE STUMPS The sequence of letters still makes sense but now carries a totally different meaning. It becomes clear now why DNA constitutes the molecule of heredity in a majority of life forms. But a few simple organisms have opted for the other nucleic acid, the ribonucleic acid, RNA, as repository of hereditary information. There are a few differences between RNA and DNA. First, the sugar moiety is that of ribose in RNA rather than the deoxyribose in DNA. The RNA also carries four nitrogenous bases, three of them identical to those in DNA. The only different one is Uracil which replaces Thymine. Naturally Adenine pairs with Uracil. Even in those organisms where DNA is the hereditary molecule, RNA plays important roles. RNA is most essential particularly for translating the coded information in DNA for building other important molecules needed by the organism for maintaining the crucial life process. The nucleic acids are the ultimate self-replicating molecules. These are stable but mutable structures which provide the genetic heritage in every living species. They form the genes and are concerned, directly or indirectly, with the production of the specific protein molecules. The protein molecules, in turn, are responsible for the cellular structure and some of these are enzymes which are responsible for facile biochemical reactions in living systems. These are highly efficient catalysts which control the chemical activities

BUILDING BLOCKS

31

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H a e m o g l o b i n — a high molecular weight protein

of the cells, including their own synthesis. They are also responsible for the synthesis of other proteins, several key cellular molecules and even nucleic acids. The proteins, thus, are also important molecules. They too are made up of long chains of small constituent units called amino acids. As the name implies the amino acids have both an amino group at one end of the molecule and carboxylic acid group at the other. There are 20 different amino acids. Of these, 15 are neutral, that is their positive and negative charges are balanced. Three are basic (lysine, arginine and histidine) which have an additional basic moiety and the other two are acidic (aspartic acid and glutamic acid) which have an additional acidic group. The amino acids are linked to each other through peptide linkages in a repetitive fashion. The resulting proteins can have very high molecular weights.

BUILDING BLOCKS

Different molecules have different constituent elements

33

34


Besides nucleic acids and proteins, other molecules important to life are carbohydrates and lipids. These have different levels of complexity but are formed from simple building blocks. No matter which of these important molecules one looks at, it is seen that they are all made up of a few simple elements. In nucleic acids, for example, only five elements, namely carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and phosphorous (P) are present. Likewise, proteins contain mostly C, N, H, S and O whereas carbohydrates and lipids are made up of C,H and O. The beauty in the architecture of life forms lies in the fact that a great deal of diversity can result from a few common components. It would be useful to remind oneself of a dictionary of words made up from only a few alphabets. The words put together in different manner have given us numerous languages of immense variety. The similarity ends here because the words in a dictionary do not act as parents giving birth to the progeny of more words. In living systems, however, DNA molecules have the information content which dictates the nature of the progeny. One could also remind oneself that the architectural buildings of enormous variety result from the same bricks and mortar, except that like words buildings too do not reproduce themselves while a living cell does. Another remarkable feature of life on the Earth is the unity in terms of the basic molecules that constitute living beings. In spite of the enormous diversity in the form or figure of living beings the basic molecules are common to all living matter, whether these are microbes or mammals. These molecules together provide the bricks and mortar of life. In turn, these basic building blocks are composed of atoms which are formed during the life activities of a star. The atoms have inherent chemical properties. Their ability to link with those of the same element or with those of the others, is

BUILDING BLOCKS

Amino Acids

Carbohydrates

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Proteins

Nucleic Acids

Life arises from simple elements

guided by their electronic configuration. Carbon, which is tetravalent, can bond to four monovalent hydrogen atoms to give methane. Nitrogen, which is trivalent, can bond with three hydrogen atoms to give ammonia. Likewise, a divalent oxygen joining with two atoms of hydrogen gives water.

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Hydrogen cyanide can be formed from one atom each of carbon, hydrogen and nitrogen. Phosphorous bonded with four oxygen atoms can give a trivalent phosphate ion which when linked with three hydrogen atoms gives phosphoric acid. This is very elementary chemistry, but it is important. It leads one to recognize that the ability to give rise to more complex compounds like purines, pyrimidines, sugars, nucleosides, nucleotides and finally the nucleic acids. Likewise simple molecules can join together to yield complex amino acids which, in turn, link up to build proteins. Under suitable conditions simple non-complex molecules like methane, carbon dioxide, carbon monoxide, ammonia, hydrogen cyanide and water can give rise to a variety of organic compounds. These, in turn, can give rise to even more complex compounds which finally result in intricate systems leading ultimately to life.

unday 28 September 1969, barely two months after man first landed on the Moon. It was an ordinary day in the sleepy small town of Murchison, some 140 kilometres, north of Melbourne in Australia. Spring had just about set in and, with the first blooms of the season trying to catch everyone's attention, the air was invigorating.

S From Dust Thou Comest

Suddenly, at around eleven in the morning, the clear blue sky above was rent with ominous thunder. A moment earlier a bright flash was seen extending right across the sky from horizon to horizon. It was so bright and overpowering that people as far away as Canberra, the Australian capital almost 400 kilometres to the east, could also see it. Even as they stood transfixed, their eyes glued to the fireball in the cloudless sky, it started descending towards the land below. Those nearer Murchison saw the bright sphere glow with yellow colour and moving downwards at a steep angle. Those even closer felt that it was more orange than yellow and tinged with a silver rim. And it was not spherical but had a dull orange conical tail. As it rushed headlong


38

The Murchison meteorite

towards the Earth it left a trail of blue smoke behind. People of Murchison watching this awesome event, their mouths agape and daring not to bat an eyelid, distinctly recalled that hardly a minute or two had passed when this dreadful object suddenly exploded and fragments started raining on the ground like shrapnel from a bomb explosion. What had happened was dramatic enough. If the citizens of Murchison felt that the skies were falling on them, they could not be faulted. Nonetheless, the event was not so unique. For it was a meteorite, a very large one no doubt, but still only a meteorite that had fallen on the Earth. This is not so uncommon. Several small, medium and large meteorites keep bombarding the Earth all the time. Meteors in the language of common man, are the 'shooting stars' of the night sky. However, they are not stars. They are

FROM DUST THOU COMEST

39

A shooting star

40


solid bodies that roam around in our solar system. They may at times but rarely be as large as some planets. Usually however, they vary in size from microscopically small dust particles to a largish satellite of a planet. These meteors orbit around the Sun in irregular fashion. As a result sooner or later they fall within the strong gravitational pull of a planet and hurtle towards it. It is estimated that some 100,000 tonnes of such solid matter from space shower the Earth every year. Most of it, however, burns due to friction once it enters the Earth's atmosphere. It thus gets destroyed. Only the large bodies retain sufficient mass to reach the ground in several fragments called meteorites. The Murchison meteorite was one such large body that fell on the Earth in recent times. To that extent it was commonplace. But its detailed analysis brought out its unique nature. Those who were able to reach the site of its landing within a very short time unambiguously reported noticing a smell like that of pyridine or methanol. Without the presence of organic matter this would not have been possible. In fact, presence of organic molecules was confirmed on detailed analysis of the pieces of this meteorite carried out in many different laboratories. Scientists at the Ames Research Centre of the National Aeronautics and Space Administration (NASA) in USA, under the leadership of Cyril PONNAMPERUMA (1923-1994), found that this meteorite contained hydrocarbons. But more interestingly they found the presence of six amino acids commonly found in proteins familiar to us. Lest some doubting Thomas should suggest that these came to be there because living matter from the Earth had contaminated the fragment when it landed, there were twelve other amino acids that are not seen in natural proteins of earthly origin. Amino acids are so structured that they can occur in two forms, which could be considered mirror images of each other. Amino acids that go on to build naturally occurring


Amino acids occur in two forms — left-handed and right-handed

41

42


Fragment of the Murchison meteorite

proteins on our planet are all left-handed. But the amino acids detected in the Murchison meteorite were of both the right-handed and the left-handed forms in more or less equal proportions. It was but logical to conclude that these complex organic molecules had been formed in the extra terrestrial region. The Murchison meteorite samples were collected within a very short time after it landed on the Earth. Thus, the chance


43

that they could be contaminated by earthly organic matter was minimal. The results of detailed analysis of these samples, using highly sophisticated techniques, confirmed suggestions and theories which studies of other meteorites, particularly those called carbonaceous chondrites, had thrown up. Several such carbonaceous chondrites had reached the Earth earlier. The first of these to be properly recorded and studied was found at Alais in France. This had landed in 1806. Since then a large number of these have been found and analysed in different parts of the world. The more important ones are from Orgueil in France dated 1864, Murray in Kentucky, USA, of 1950 vintage and Allende in Mexico which landed in 1960. Most of these chondrites have been found to be 4.5 to 4.6 billion years old. That is also roughly the age of the Earth. They, thus, provide evidence of what must have transpired at the time the Earth was born. Clear evidence thus can be found that such complex organic molecules which lay the foundation of life do get formed as a result of chemical interactions constantly taking place in the universe. Such a contention is also supported by information provided by other maverick visitors from space, the comets. These are considered to be made up of condensed water vapour and dust that abounds in the vast space. In common parlance, they are, therefore, often labelled as 'dirty snowballs'. They normally come from the outer realms of the solar system which are very very cold. Consequently, most of the material in them exists in the frozen state. Unlike meteorites which reach the Earth and hence can be 'handled', comets have to be observed only from a far off distance. During their eccentric sojourn around the Sun they sometimes come relatively close to the Earth. At that time they can be studied in some detail. Such studies have shown that they contain organic molecules like ammonia, methane

44


and carbon dioxide. Besides, some other molecules like HCN are also present. The Murchison meteorite fragments, no doubt, provided conclusive evidence for the existence of organic compounds in the extraterrestrial reaches. However, suggestions that the vast space between stars, which was earlier thought to be empty, is teeming with organic compounds, have been forthcoming ever since astronomers started looking at the celestial bodies with their powerful radio telescopes. Aided by this useful ally, astronomers could now venture where no one had gone before. They could not only train their telescopes on much more distant regions of the universe but even at that interstellar space which had held little interest earlier since it was thought to be occupied by some inconsequential dust. The radio waves easily penetrated these vast streches that were opaque to visible light. They were astonished by what they found. The space which was thought to be empty or of little interest was teeming with molecular clouds. Each molecule emits a characteristic radio signal. For example, when simple water molecule tumbles in space, the positive and negative charges in its interior rotate rapidly in a specific pattern. This alternating pattern generates radio waves that are specific to the water molecule. Thus, by studying the properties of the radio waves, the molecule responsible for their generation can be identified. These tell-tale signatures of different molecules are first determined in the laboratory. For this purpose microwave radiation of a specific frequency is passed through a cell containing vapour of a molecular species. The molecules then absorb characteristic radiowaves. Just as no two individuals possess identical fingerprints, no two molecules have identical spectra of absorbed waves. These laboratoryderived spectroscopic patterns can then be used to search for the molecules present in the interstellar medium.


45

EES

Arcttirus,

More than 80 types of organic molecules have thus been detected in interstellar space, mostly within giant molecular clouds. Then there are a large number of unidentified spectral lines resulting from radio observations which indicate that the list is incomplete. The compounds already identified are not only simple ones consisting of two or three atoms but also more complex species containing as many as eleven atoms. Amongst the

46


Molecules in interstellar space

diatomic entities are H 2 , CH, OH, CN, CO, and CS, The triatomic compounds and ions include HCN, H2O, H2S, SO2, HCO + and I^H"1". Some of the higher molecular weight compounds include NH 3 , CH4, CH3OH, CH 2 NH 2 , CH3CH2OH and CH3CH2CN. It is quite likely that compounds like glycine, the simplest amino acid may also be discovered one day in the interstellar medium. This is because some of the familiar compounds which have been shown to be precursors of amino acids or nucleic


47

acid bases are present in the interstellar region. These are ammonia, water, formaldehyde (HCHO) and hydrogen cyanide (HCN). Besides, a number of hydrocarbons, both saturated and unsaturated, compounds containing only one carbon atom like methane (CH4) to compounds made up of more than one carbon atoms as also carbon monoxide, cyanogen, formamide, acetonitrile and many others have been characterized. True, the concentration of such compounds is very low. But since interstellar regions stretch over enormous areas, the total amount of these compounds in the interstellar medium is enormous. How did all these complex organic compounds, so different and so abundant, get formed? Could they be there in these extra terrestrial reaches as a result of some life forms? That seems hardly likely because of the extremely inhospitable environs of these interstellar spaces. Not only that there is little atmosphere, the temperatures also are very low. Moreover, these regions are constantly bombarded by cosmic rays and other intense radiations. So, the only plausible way in which they could have been synthesised is through chemical reactions. At the beginning, when the universe itself came into being, there was no chemical element. However, the violent cataclysmic events at the time of the Big Bang initiated the chain of interactions leading to the formation of these elements. The simplest and most abundant of the elements is hydrogen. It was formed in the first millionth oe a second after the birth of the universe. Before the formation of stars there were only these gaseous clouds of hydrogen. The synthesis of other heavier elements, so essential to life, took place in the furnaces raging within the stellar interiors. These were then thrown outside in two different ways. Some stars on aging gently shed part of their material. Thus, late in their life some of the heavier elements are ejected

48


Molecules come together in a meaningful way to form functional structures


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in the intersteller space. Then during the final death throes the star explodes catastrophically to become a supernova. At that time all its contents are thrown aside violently. They then constitute the gas and dust that lies in the space between stars. The bricks thus lie scattered in sufficient abundance all over the universe. But how are they brought together in a meaningful way to form functional structures that ultimately result in the edifice known as life? Scientists now have sufficient evidence to believe that the very inherent properties of these chemical elements and the restless nature of the universe propel these elements towards forming first primitive and then more complex organic molecules. Even at very low temperatures such as less than minus 173 degrees celsius and at very low concentrations in the gas phase, collisions between two species are possible. In relatively dense clouds and higher temperatures, the process would take place with even greater vigour. Further, reactions can be initiated by collisions of ubiquitous, high energy cosmic rays with helium. These collisions can produce positively charged, reactive species which can initiate reactions leading to the formation of interstellar molecules.

rchbishop James USSHER (1581-1656), an Irishman from Armagh, was a theological scholar. After a detailed study of the story of Genesis as given in The Bible he concluded that the world, in all its features including the biosphere, was created in a space of six days beginning at 9 a.m. on October 26, 4004 B.C.

A

The Emergence

Such legends can be found in different mythologies belonging to various civilizations. According to a Hindu belief, life emerged through the navel of Lord Vishnu, as he reposed in the tranquil ocean of milk, the Kshirsagar. A lotus leaf bloomed out of the navel on which the first child made its appearance. Another legend credits Samudramanthan, the churning of the oceans carried out by the Devas (Gods) and Asuras (Demons), with giving birth to living matter. Such beliefs held sway till Louis Pasteur in the nineteenth century convincingly demolished the concept of spontaneous generation. Moreover, a large number of highly convincing evidence had already been accumulated demonstrating that the Earth was 4.5 billion years old,

THE EMERGENCE

51

far far older than the mere 6000 years proposed by Archbishop Ussher. Yet, there still are a number of adherents of the hypothesis that some 'supernatural power' created the world, indeed the universe, in one stroke. Despite strong scientific evidence to the contrary proponents of this theory find it difficult to abandon the concept. The diehards amongst them merely dismiss scientific arguments out of hand without engaging into any rational dialogue. Those not made of such sterner stuff tender a milder variation of the same theme, that of spontaneous generation. According to this theory, life arose out of inanimate objects. To that extent it may appear to conform to the available evidence that suggests that chemicals essential to life can be generated from simple inorganic molecules as a result of

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People believed wierd theories about the origin of life like geese appearing from fruits of a tree

some chemical reactions. But that prima facie similarity is deceptive. For, the scientific database gathered painstakingly from analysis of extraterrestrial matter indicates that the synthesis of complex organic molecules took a long time and did not occur in a single event or a single moment. The essential, and crucial, difference in the theories of spontaneous generation and chemical evolution lies in this important fact. Moreover, as first Francesco Redi and later Louis Pasteur showed, the so called spontaneous generation was actually growth of living forms from pre-existing parents or eggs. As the latter were microscopic, they were not noticed when they made their entry into the inanimate bodies. It thus became increasingly difficult to accept both these theories which traced the origin of life to non-physical proces-

THE EMERGENCE

53

ses. Scientists, therefore, seriously started the search for alternative more credible, explanations of how life emerged on the Earth. Svante ARRHENIUS (1859-1927), the renowned Swedish chemist, put forward the concept of universal life. The idea was first mooted 2500 years ago by the Greek Philosopher ANAXAGORAS (500-428 B.C.). According to this concept, life had always existed somewhere in the universe, in some form or the other. Arrhenius extended these ideas and introduced the concept of 'panspermia'. He came up with the revolutionary concept that life was seeded on the Earth from some other region of the universe, outside our solar system. This would imply that life evolved somewhere else, travelled long distances across somewhat hostile stretches of space and then innoculated the Earth. The concept of universal life has some common features with the steady state theory of the origin of the universe. The latter does not accept that the universe came into being as a result of, and only after, the Big Bang. Hermann BOND! (b.1919), Fred HOYLE (b.1915) and Thomas GOLD (b.1920), who proposed the steady state theory, suggested that there was no specific beginning to the universe. It always existed. Further, as the galaxies moved away from each other in the ever expanding universe, new galaxies were formed in the intervening space. Thus, the universe would look more or less the same at all times. For formation of new galaxies, of course, dust and particles, that is matter, would be needed. Bondi and his colleagues propounded that matter was being continuously created. But most of the available astronomical data have not supported this theory. It met with another major hurdle in 1965. Arno PENZIAS (b.1933) and Robert WILSON (b.1936) were at that time trying to perfect an antenna system that was to be used for satellite communication. When they tested

54


their system they detected a faint but persistent signal. To get rid of it they tried changing the direction of the antenna but failed to eliminate the noise. Initially they thought that the noise originated from a defect in their instrument. So, they made numerous attempts to correct this deficiency, alas

Bondi, Hoyle and Gold propounded the Steady State Theory

THE EMERGENCE

55

without any success. Only then did they realise that the signal was the signature of an all pervading background radiation originally predicted by George GAMOW (1904-1968). Subsequent studies with more sensitive and sophisticated instrumentation demonstrated that the radiation had the

Steady state model states that new matter is continuously created in the intergalactic space


56

mammmkx:

-.

|l

Penzias and Wilson with their horn antenna

a temperature of approximately 3 degrees Kelvin, that is 270 degrees below zero Celsius. This crucial observation removed many doubts about the Big Bang theory of the origin of universe. Moreover, it also disproved the steady state theory since the latter could not explain the existence of this persistent radiation. With this the concept of panspermia, though very intriguing, also lost support . Moreover, whether living organisms can survive the adverse environment of interstellar space is an open question. This was precisely what was asked by many scientists when Fred Hoyle and Chandra WICKRAMASINGHE (b.1939) revived the panspermia hypothesis

THE EMERGENCE

57

in 1970. They were more explicit than Arrhenius in stating that life in the form of microorganisms arrived on the Earth from distant reaches of the universe. They insisted that even today we get such uninvited microbial visitors. Nonetheless, there is very little experimental evidence to support this explanation of the origin of life. However, even if it is accepted, for the sake of argument, that life was seeded on the Earth from somewhere else, it does not tell us how life originated in that part of the universe in the first instance. Thus, the theory of panspermia fails to provide a cogent answer to the basic issue of emergence of life in the universe. Oparin tried to address this question by suggesting: "there is no fundamental difference between a living organism and lifeless matter. The complex combination of manifestations and properties so characteristic of life must have arisen in the process of the evolution of matter." He proposed that life originated under a strongly reducing atmosphere rich in hydrogen (H2), ammonia (NH3), water (H2O) and some hydrocarbons. According to Oparin, more complex molecules were produced from those which rained down into the primordial ocean. These molecules underwent further transformation to give rise to the first living system. In a way, he was extending the argument first put forward more than half-a-century earlier by Thomas HUXLEY (18251895). Speaking about the physical basis of life Huxley had observed that life crucially depended upon certain molecules which were themselves inanimate. Yet, when they came together and organized themselves in a specific manner, they gave rise to the rudiments of a living organism. Oparin's proposal was also consistent with the theory of descent of man and origin of species proposed by Charles Darwin . By the early twentieth century Darwin's theory had come to be widely accepted. That explained how the fascinating diversity of life forms was generated starting from primi-

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Inanimate molecules organize in a specific manner to give rise to the rudiments of a living organism

THE EMERGENCE

59

tive simple organisms. Darwin had also talked about the "warm little pond" which was the cradle of life in the universe. That thick solution, rich with molecules which could then give rise to more complex systems, could get formed only if the cloud of dust and gas containing hydrogen and helium present at the beginning of the universe gradually got transformed by chemical evolution. In support of these ideas, Nobel laureate Harold UREY (1893-1981) had proposed that the first atmosphere of the Earth did not contain oxygen. This was necessary since only under such reducing atmospheric conditions can chemical reactions giving rise to biologically interesting molecules take place. But life, as we see today, is critically dependent on oxygen, so much so that the latter is called Pranavayu. Naturally then, life as we see today, could not have existed at the time the Earth was born. Some chemical changes would have had to take place to make the atmosphere conducive to life. The theory of chemical evolution provides an explanation of how this could have occurred. This concept envisages that the reducing atmosphere at the beginning of the Earth was also accompanied by such sources of high energy like ultraviolet rays or electric discharges or intense heat as is usually experienced in volcanic eruptions. Under these conditions atoms linked up to form simple compounds like methane, ammonia and water. These interacted to build first some water soluble compounds and then molecules of increasing complexity. Some of these were highly evolved organic molecules which were precursors of life. Since these molecules were not being used as there were no life forms to do so, they went on accumulating until their concentrations reached a stage described by Darwin as the "warm little pond" or "primordial soup". The process continued till molecules that can replicate themselves appeared, giving rise to primitive life forms.

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The first living molecules; Harold Urey (inset)

The emergence of oxygen in the atmosphere and its subsequent enrichment is also explained by these hypotheses. In the primitive atmosphere, oxygen was not present in the free form. However, it was present as water vapour and in the form of oxides of carbon, sulphur and the like. It could be freed only by two possible processes. One is that of photolysis of water in which a water molecule comprising two hydrogen atoms and an oxygen atom splits, thereby liberating the oxygen. Such a reaction could have occurred under the influence of the high energy sources. The other process is a more complex one and requires the mediation of living organisms. Some primitive micro-organisms can bring about a chemical reaction between carbon dioxide and water converting them into carbohydrates and liberate oxygen as green plants of today do. Such primitive organisms have the ability to survive in oxygen-deprived conditions.

THE EMERGENCE

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Primitive microorganisms produced food as green plants of today do

Chemical studies of the extraterrestrial regions have also unequivocally demonstrated that the type of chemical reactions leading to the generation of complex organic biomolecules do occur even under those extreme conditions. They too, thus, go in favour of the theory of chemical evolution. A

tanley MILLER (b.1930) was a young graduate student in 1951 about to launch his research work aimed at acquiring a doctorate degree. Being at the University of Chicago, he had come to learn about his mentor Harold Urey's theory that the primitive Earth had a reducing atmosphere which was essential for the synthesis of complex organic molecules from the gaseous environment. He saw in this an opportunity to carry out a highly significant piece of scientific investigation. He, therefore, proposed to Urey a scheme whereby his postulate could be experimentally tested.

S Cooking the Soup

So, Miller took a big five-litre flask containing water to simulate the ocean. The water was kept boiling. The steam thus generated was passed through a tube into another big flask kept at a higher level. This second flask contained a mixture of methane, ammonia and hydrogen, a representative specimen of the primitive atmosphere. A pair of electrodes were inserted into this upper vessel, so that an electric arc could discharge electricity through the mixture. The energy needed to get a chemical reaction going was thus provided even as it simu-

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Miller and his apparatus

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lated to the extent possible the conditions that were envisaged to have prevailed at the dawn of the Earth's life. The bottom of this flask containing the 'artificial atmosphere' was connected to a condenser, so that the products formed in that gaseous chamber were washed down back into the 'artificial ocean' below. One could call it an attempt to recreate the 'rains' that must have showered down on the 'virgin' Earth. The closed system represented the early hydrosphere in which the Earth was ensconced. Miller kept the cauldrons bubbling for a full week. Anyone watching the experiment would surely have experienced an eerie feeling with the 'ocean' madly boiling, the 'lightning' discharge furiously flashing and the 'rain drops' continuously falling into the sea. But the results would have left him in even greater awe. For they were far beyond the initial expectations. The lower flask which had contained only water at the beginning of the experiment, barely a week earlier, was now full of organic molecules like urea, formic acid, acetic acid and propionic acid. More importantly, a number of amino acids like glycine, alanine, aspartic acid and glutamic acid were present. All of these molecules are essential to life. Formic, acetic and propionic acids are simple forms of the constituents of fats. Amino acids link together to form proteins. Urea, rich in nitrogen, plays an important role in many biological processes. Was Miller merely lucky in getting these molecules synthesized in the experiments conducted by him? Were the results of his experiments just a flash in the pan? Certainly not. Any new scientific evidence is not accepted as representing the real life situation until several other scientists test it by conducting their own experiments under either the same or specifically altered conditions. This is exactly what happened when Miller announced his findings. Many other

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scientists from different parts of the world took a cue from him and repeated the experiments. Philip ABELSON of the Carnegie Institute in Washington employed different gaseous mixtures in a series of experiments. Whenever, the composition of these gas clouds was such that it did not contain oxygen, he was able to obtain organic compounds similar to those generated in Miller's experiments. But, if oxygen was present in the atmosphere, no amino acid was synthesized. This constituted the proof that the primitive Earth had no free oxygen. It also made it clear in no uncertain manner that if oxygen, now so crucial to the very existence of life, was present at the very beginning, no life form could have emerged in this universe. Similar experiments carried out by numerous scientists in many countries all resulted in obtaining organic molecules which could be considered as forerunners of primitive life STATUTORY WARNING

ORIGIN] OF I LIFE I LAB

OXYGEN IS INJURIOUS TO LIFE

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Ponnamperuma (left) and Calvin (right) "The ocean' was enriched at the end of their experiment

forms. Thus broadly speaking, Oparin as well as Haldane can be said to have been vindicated. There were other variations on the theme too. Some had speculated that energy was provided not by lightning but by intense radiation. Cyril Ponnamperuma and Melvin CALVIN (b.1911) set up an experiment in which the gaseous mixture was bombarded with high energy electrons generated in a linear accelerator. The gaseous environment was held in a horizontal tube. A bulb attached to it contained the 'ocean' and a cold finger above it at the top of the tube allowed condensation of the vapours. Once again the 'ocean' was seen to be enriched at the end of the experiment with life-supporting organic molecules including nucleic acid bases like Adenine and Guanine.

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Several other scientists simulated other scenarios. Different mixtures of gases in varying concentrations were used to recreate different types of atmospheres. Besides electric discharge and high energy electrons, ultraviolet rays, heat and ultrasonic waves, as in the shock waves generated as a result of the impact of a meteor, have been employed as sources of energy. In almost every situation where the atmosphere was of a reducing nature and the amount of available energy was adequate, several bio-organic molecules including even the constituents of nucleic acids were formed. Thus, the basic tenets of the theory of chemical evolution were substantiated with experimental evidence. That the precursors of the building blocks of life could be synthesized essentially due to chemical interactions taking place naturally under the primitive environment was proven beyond doubt. What remained to be seen was whether these small, albeit highly important, organic molecules could come together to generate the more complex biomolecules. These complex macromolecules, particularly the proteins and nucleic acids, are essentially long repetitive chains of a few simple constituents. Amino acids join hands to form polypeptide chains. One or more such chains may now link up to form a protein. Likewise, a sugar molecule, either of the ribose or deoxyribose kind, would get bound to a phosphoric acid and one of the nitrogenous bases to constitute a nucleotide. Such nucleotides would then join together to form one strand of a nucleic acid. It may remain in that state or embrace a complementary strand to make up the nucleic acid. All amino acids have a negatively charged carboxyl group at one end and a positively charged amino group at the other. A link between two different amino acid molecules can easily be forged when an acid group interacts with the amino group. The strong bond that is formed is called peptide bond. In the process of forming this bond, a molecule of water is squeezed out. Likewise, when a nitrogenous base like Thymine reacts

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The acid group reacts with the amino group to form a peptide bond

with a sugar, say deoxyribose, again a water molecule is eased out in order to generate a strong covalent linkage between the two. The story repeats itself when a sugar moiety joins hands with a phosphoric acid molecule to form a compound of sugar phosphate. It is not just coincidence that in every single case it is the elimination of a molecule of water that helps establish a strong bond between the simple constituents to give rise to the large, complex molecules so essential to life. It is, perhaps, a measure of the simplicity with which natural processes have inevitably participated in the evolutionary process. It is a reminder that one does not have to invoke any non-physical, supernatural force for the emergence of life. Nonetheless, the question still arises as to how the water molecule came to be eliminated, so that in the natural course of events the life-supporting molecules were formed. To resolve this a number of experiments in various laboratories have been carried out.

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An easy way to remove water is of course to evaporate it by heating. Sidney FOX (b.1912), therefore, took a mixture of amino acids especially concentrated with glutamic acid and aspartic acid. A current of nitrogen was passed over the mixture which was then heated to a temperature of 180-200 degrees celsius. At the end of the experiment long chain polymeric molecules with typical high molecular weights were obtained. This may have happened under laboratory conditions. But did the latter simulate what happened at those early times? John BERNAL (1901-1971) has suggested that the lagoons dried up under continuous and intense bombardment of Sun's rays coming through in full force since the protective shield of ozone was not yet in place. In the circumstances, the small molecules came together to form the polymers. With rain water washing the lagoons these molecules were then pushed into the oceans. Such a process could indeed, logically, have taken place. Still, if it could be shown that even inside the oceans with water everywhere a molecule of water can be eliminated to forge a sturdy link between two small molecules, it would ring more true. It would be easy to accept that these simple chemical reactions were responsible for progressive developments leading to the emergence of life. Several scientists have, therefore, tried to find out if polymers can be synthesized out of amino acids in the presence of water. They have succeeded in demonstrating that such an apparently impossible feat can be achieved. A water molecule can really be expelled even as plenty of water is present in the surrounding. However, this requires the action of another agent called the condensing agent. Hargobind KHORANA (b.1922), the Noble laureate, had succeeded in synthesizing polynucleotides in his laboratory. For this purpose he had employed as condensing agents a family of compounds which were derivatives of cyanides.

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Hargobind Khorana synthesized the first artificial gene

The simplest of the latter were cynamide and dicyandiamide. Calvin and also Ponnamperuma employed these to obtain polymers from mixtures of amino acids. Even a tetramer of hydrogen cyanide was found to be an efficient condensing agent by Leslie ORGEL (b.1927), James FERRIS (b. 1932) and others. When one recalls that hydrogen cyanide is a molecule present in the molecular clouds of interstellar space, it is easy to understand how the large, life-supporting organic molecules could have formed under the primitive environment.

ife, as we see around us, is fascinatingly diverse. There is a variety of size, shape, structural features and functional attributes. If one is reviled by the grossly unpresentable appearance of a hippopotamus or a rhinoceros, one is equally captivated by the stunning beauty of a deer or a peacock. If there is an elephant today that is very large in size or a now extinct dinosaur which was even larger by several times, there is an ant or a fruitfly which is so very tiny. And then there are micro-organisms like bacteria and viruses which are even smaller. One cannot see them without the help of instruments like a microscope. The thorns of a porcupine are sharp and pointed like a needle; so are those of some cacti. But then these very prickly plants sport some of the most ravishingly colourful flowers when in bloom.

L

The First Steps

Yet, underlying this seeming diversity, there is a common feature. All life forms, howsoever complex, are made up of cells. A cell is the basic constituent of all animals and plants. Some living organisms, like the human beings or large trees are constituted of millions of cells. Others, like

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bacteria, algae or fungi, are single-celled life forms. In more complex organisms, like higher animals and trees, the constituent cells assume different shapes and sizes. The cells that come together to form a heart adopt a different shape and size from those that constitute the blood vessels that carry the vital fluid to or from the heart. The cells of a leaf, likewise, have a different structural appearance and functional responsibility than those of the trunk. Even so, all cells have almost the same basic structure. All living organisms can be divided into two categories depending on the nature of their constituent cells. Cells of life forms that are made up of more than one cell have a well organized nucleus which is the controlling centre of all cellular activities. Organisms that have such well nucleated cells are called eukaryotes. On the other hand, cells of more primitive organisms like bacteria lack the nucleus. These organisms are known as prokaryotes. No matter what type a cell is, it is an instrument of energy transformation. It has to convert energy, obtained from food and nutrients, to a form essential for maintaining life. Towards this objective it represents a well organized entity made up of several constituent units. But by themselves these constituent units, composed of several organic molecules, cannot sustain life. Only by coming together in an organized and definite pattern inside a cell can they make up a living entity. It is, therefore, logical to believe that organized life must have arrived on the Earth in the form of a cell. Lynn MARGULIS (b.1938), the renowned biologist and science writer, has pointed out other clues to ancient history of the organized living world. She suggests that since "all insects have six legs and hard outer skeletons it is likely that the common ancestor of all insects also had such features. Similarly, because dogs, cats, pandas, chimps and even human beings have four limbs with five fingers or toes on each paw or hand, four-legged,

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The arrival of nucleus brought order

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Cell is the commor ancestor of all life forms

five-toed common ancestors can be inferred for these mammals". Even so, if one starts looking for the common ancestor of all life forms what would one find? Certain physical features, like a heart for example, which appear to be so very essential to certain living beings, turn out to be only specialized structures not common to all. If, therefore, one starts looking for those features which are common to each and every one of

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the multitude of life forms on the Earth it will be seen that the common ancestor will have to be a cell. These may seem to be mere theoretical conjectures. But practical evidence, too, supports these speculations. Palaeon-

Morphological features of an organism are well preserved in a fossil

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tologists study the types of living organisms, both plants and animals, from different times in the past. Fossils, which are remnants of prehistoric life form buried deep within the Earth in the layers of sedimentary rocks, constitute their experimental material. A fossil is formed when the organic matter inside a living organism is eroded and is replaced by inorganic silica. Thus, the general morphological features of an organism are usually well preserved in a fossil. Examination of such fossil records has shown that among animals, mammals first appeared only at the beginning of the tertiary period about 65 million years ago. Modern man arrived even later, around 3-5 million years ago. In the mesozoic period, lasting for 185 million years prior to the tertiary period, the dominant animals were reptiles. Fish and other marine organisms alone were present during the palaeozoic period before that which had lasted for some 320 million years. Elso BARGHOORN (1915-1984) and James SCHOPF (b.1941) of the Harvard University in the USA were trying to find out in the sixties, if there was any evidence of life in the precambrian period which preceded the palaeozoic. Painstaking investigations by them provided proof that life in its simplest manifestation had already begun when the atmosphere of the Earth was changing from a reducing one to that where oxygen was becoming relatively abundant. These two scientists discovered that in the mud of Bitter Springs, Australia, well preserved microfossils of some 30 species of microorganisms were present. There were some fossils of blue-green algae and fungi which were 900 million years old. This discovery of ancient fossils opened an exciting field. Several subsequent findings not only substantiated this original discovery but showed existence of primitive life forms at even older times. For example, microfossils found at the Gunflint iron formations along the shores of Lake Supe-

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Microfossils from mud of Bitter Springs, Australia

rior in Ontario, Canada, were seen to be 1.8-2.1 billion years old. Some of these were rod-shaped bacteria. In the Dharwar sediments of the Sandur district in Karnataka, microfossils more than 2 billion years old were unearthed. A still older deposit (approximately 2.8 billion years) was found in the cherts of limestones of stromatolitic frotescue fossils of Austraila. By far the oldest microfossils date back to 3.5 billion years ago. These were discovered by Schopf and co-workers from the Western Warrawoona group of rocks in Western Australia. These fossil deposits provide strong indication that organized life existed at least 3.5 billion years ago. And in all probability microorganisms were the earliest life forms.

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Look! sphere's the Grandac, of us all

These simple microbes must then be the common ancestors of all the multifarious living beings one encounters today. All the individual constituents in the form of organic molecules had evolved as a result of physical and chemical processes taking place naturally. Obviously, the question arises whether the same physicochemical laws could have driven the processes one stage further, so that these constituents get organized. And could this organized unit have evolved a cover for itself in the form of a membrane? Aleksandr Oparin, Sidney Fox, Cyril Ponnamperuma and many other scientists studying the phenomenon of origin of life answer these questions with an unequivocal 'YES'. Several possible mechanisms by which different organic

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Water molecules are either included or excluded from the cluster

chemicals could have come together and graduated to a biological cell have been proposed by these men of science. Bungenberg DE JONG was a Dutch physical chemist who had studied extensively the properties of colloidal suspensions. He had observed that if one started with dilute or weak solutions electrical charges on the colloidal particles drove them to form large aggregates. Initially, a small cluster is formed. The molecules within the cluster interact, leading to formation of a big cluster. Consequently, water molecules are either enclosed within the cluster or excluded altogether to remain outside the cluster. Slowly an organized ensemble is formed with a boundary of its own. However, the boundary wall is not a totally closed unit forever severing all connec-

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Oparin and his coascervate droplet

tions between the inside and outside. On the contrary it is permeable, allowing materials to flow from inside to outside or vice versa, but in a highly selective fashion. Impressed by these findings Oparin and his associates took polymeric materials., like gum arabic, and heated them. They were then dissolved in boiling water and allowed to cool slowly. When at the end of the experiment tiny spheres of diameter about two thousandths of a millimetre were formed, their joy knew no bounds. These Russian scientists then tried the same experiment using different polymeric

THE FIRST STEPS

Protenoid microspheres and Sidney Fox

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compounds. Every time it led to the formation of the coascervate droplets. In some, even some activity resembling that of enzymes, the ubiquitous biochemical catalysts so essential to sustaining the highly evolved life of today, was noted. Oparin called these coascervates precells or protocells, suggesting that this was the mechanism by which the primitive cells were born. Sidney Fox has come up with an alternative pathway by which protocells could have arisen. He was influenced by the experiments carried out in the 1930s by Alphonse HERRERA (1868-1942), a little known Mexican scientist. Using such simple organic molecules like formaldehyde and ammonium thiocyanate he was able to produce microstructures that looked remarkably like a cell. Since some of the substances like formaldehyde that he had used were found abundantly in the interstellar matter, Herrera's experiments suggested to Fox a possible mechanism by which protocell could have arrived on the Earth. Herrera was, perhaps, ahead of his time. That is probably why his experimental demonstrations did not attract much attention. However, Fox revived Herrera's concepts by repeating those experiments. He, therefore, took some amino acids and heated them to a temperature of 180 degrees Celsius. They were then dissolved in boiling water and allowed to cool. This resulted in the emergence of a large number of protenoid microspheres. These spherules were highly uniform in size and shape. Each one of these was about two microns in diameter, approximately the same size as that of many bacteria. A single milligram of the constituent organic chemicals could yield as many as 100 million of these microspheres. Apart from the size and shape which were similar to those of single-celled microorganisms, there was another reason why these microspheres were thought to resemble the protocells. When they were washed with salt and cut open,

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Protocells

their outer walls were seen to have a double layer much like that of the membranes of cells. Many other mechanisms by which the biological entity of a protocell could have arisen from a mixture of organic molecules have been proposed. Cyril Ponnamperuma has pointed out that formation of a membrane to enclose the constituent molecules is a very important prerequisite for the formation of a protocell. He, therefore, finds the occurrence of biphasic vesicles at boundaries of oceans very appealing. If a film of fatty substances were to be present on the surface of the ancient oceans, then merely the force of action of winds could have broken the film resulting in the formation of small globules. The membrane would be formed out of the film. This would enclose the materials in the ocean during the formation of spherical structures. Several theories exist today to explain how a biological entity could have emerged naturally out of the abundance of organic chemicals. Most of these are also supported by some

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experimental evidence. However, it is not yet clear which of these several hypotheses is the right one. What is agreed by all is that primitive life in the form of a protocell arose as a result of natural physicochemical interactions between the different organic chemicals present at the time. In addition, it is also possible that the environment that time would have played a role. But the precise manner in which the metamorphosis of organic chemicals into a simple yet organized biological entity took place is still a subject of intense scientific study.

oots, the block buster novel by Alex HAILEY published in the 1970s, has been quite a literary phenomenon of recent times. It has reportedly sold millions of copies in hardcover and paperback editions. Later, it was transformed into a television serial. When that was being telecast as many as 80 percent of the Americans sat glued to the small screen.

R

The Family Tree

Hailey's powerful prose, no doubt, contributed largely to the success. But no less important, and perhaps more so, was the theme of the novel. Hailey had recounted the saga of the efforts to trace his ancestors. Starting from his home in USA the trail had led him to a village in West Africa. He stopped once he had discovered the identity of the Negro tribesman who was taken as a slave across the Atlantic to the new world. The reason that the tale captured the imagination of such a large audience was that the desire to unravel one's ancestral history and go to the roots of the family tree is widespread. The same desire has driven palaeontologists to dig out the evolutionary history of the living world. And their efforts to construct this evolutionary tree, start-

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96 LIFE IN THE UNIVERSE

Palaeontologists unravel the evolutionary history

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A certain trait is inherited from a common ancestor

ing from the primitive protocell to the bewilderingly diverse spectrum of life today, make no less thrilling a story than any mystery yarn. Different scientific disciplines have lent different tools to help decipher the phylogenetic history of life on this planet. Phylogeny is the term used by scientists for the evolution of any species. The word has a Greek origin, phylo means race and geny means birth. Palaeontologists depend mainly on the examination of fossil records to establish phylogeny. On the other hand, cytologist would look for external appearances of the cell and microscopically visible structural features. But the biochemists would like to study the structure of constituent biomolecules and their functional characteristics.

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In this endeavour, a simple tenet which constitutes the cornerstone of the evolutionary theory helps scientists. If several organisms are found to share a certain trait, then that attribute was, in all probability, inherited from a common ancestor. It is unlikely that the particular feature evolved independently along several separate paths. Advances in organic geochemistry and biochemistry have made it possible to determine such shared features at the molecular level. Organic compounds, even if they are available only in nanogram quantities, can now be properly characterized. So, from the nature of these organic compounds inferences can be drawn about the type of life forms which contained such chemicals. Unfortunately, the older a rock sample harbouring these chemicals, the greater is the likelihood of its having got modified as a result of the various physicochemical environments to which it was subjected over eons. Even so, highly useful information can always be obtained by meticulous and systematic study. For this purpose, after a suitable rock or sediment sample is selected, it is macerated and ground to a fine powder. Molecular species from the powder are then extracted by appropriate chemical treatment. Inorganic material, if any, present in the sample is eliminated by treating the solution with hydrochloric or hydrofluoric acid. The resultant residue is now ready for being analysed with techniques like gas chromatography or mass spectrometry. Such analyses have yielded an important clue about the time at which a major change in the Earth's environment took place. It is now generally agreed that in the beginning the Earth's atmosphere did not contain oxygen to any appreciable extent. In contrast, today the gas is so abundant that life is crucially dependent on its availability. Obviously, sometime during the geological history of the planet a gradual transformation in the composition of its environment must have taken place.

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Let me also wear a mask; there's oxygen around S

Organisms capable of carrying out photosynthesis use carbon dioxide, water and light energy to produce the carbohydrates they need for sustenance. In the process, oxygen is given out as an unwanted byproduct. Such organisms existed in the initial stages and the oxygen released by them kept on accumulating in the atmosphere. An important component of the photosynthetic apparatus of these organisms is the molecule of chloipphyll. It is a complex organic molecule with side chains of isoprenoid compounds like phytanes which contain 20 carbon atoms

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and pristanes containing 18 carbon atoms. When old rocks (approximately two billion years old) were examined by geochemical techniques, the presence of the isoprenoid side chains were revealed. Obviously, photosynthetic systems must have been present at those ancient times. Such and other studies help understand the chronology of the emergence of different plant and animal species. However, their inter-relationship can be deduced by studying similarities and differences between the constituent molecules. All life forms, for instance, contain one set of amino acids, the building blocks of proteins. Likewise, they also contain one set of nucleotides which represent the constituent units of the nucleic acids, RNA and DNA. Further, the essential features of the complex system which ensures that the genetic information resident in the DNA is handed down faithfully from one generation to the next are also common to all organisms. The sequence of nucleotides in the DNA molecule contains the information that decides the sequence of amino acids in the proteins. If the genetic information in the DNA molecules is passed on from the parent to the progeny with high fidelity, then the sequence of nucleotides of the two would be identical. One can arrive at the same conclusion indirectly by looking at the sequence of amino acids in a protein. Thus, comparison of such sequences between organisms helps to establish the inter-relationship of different organisms, indeed different species. Such elegant biochemical phylogenetic studies have now become possible thanks to some remarkable developments in molecular biology. Comparison of the sequences of bases in a DNA molecule for determining the extent of similarity between them can now be done in several ways. One technique preferred by many scientists is called restriction mapping. The underlying principle is deceptively simple. Restriction enzymes which

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f

are a special type of chemical scissors can cut the DNA molecule at specific sites. Each enzyme recognizes a particular sequence and nicks the nucleotide chain at that point. Thus, well defined fragments of the molecule can be obtained. The pieces are then made to move along a gel under the influence of an electric field. The distances to which they move yield information about the lengths and weights of

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these pieces. Each piece thus leaves on the gel an imprint of its signature. Several restriction enzymes can be used to obtain different pieces and the tell tale imprint of the fragments can be obtained to construct a restriction map of the DNA. Now, the maps of two different organisms can be compared. If the organisms are closely related, the sequences of nucleotides in their DNA would be very similar. Consequently, the restriction fragments too would not be much different. Hence, there would be a very good match between the maps. Similar techniques are available for comparing the sequences of proteins from different organisms. The degree of difference or diversity among the sequences helps in estimating the order, and perhaps the relative times of divergence of species from ancestral relatives. Initially, the members of any new species are few in number. Almost all of them would be closely related. Therefore, the degree of divergence in the sequences of their proteins or DNA would be small. As time passes and the species grows the family tree develops newer branches. Different members of the species drift apart. Consequently, the variations in the protein or DNA sequence of different members become large. It, thus, becomes possible to determine how old a particular family of organisms is merely by finding out the extent of variation of molecular sequences within its members. Margaret DAYHOFF (1925-1983) and colleagues in USA, and John BARNABAS (b.1929) and coworkers at the National Chemical Laboratory in Pune, among others, have carried out such detailed analysis in an attempt to construct the phylogenetic tree, particularly of the bacterial species. These biochemical investigations have revealed that the first organisms to colonise the Earth were prokaryotes, single celled bacteria that could live in the absence of oxygen. These obtained the necessary nutrition and energy from the environment. Gradually, photosynthetic organisms like the

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THE FAMILY TREE 103

A phylogenetic tree

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blue-green algae and purple bacteria emerged. Harnessing light energy they also released oxygen, thereby bringing about a major change in the Earth's atmosphere. True to Darwin's theory of evolution new organisms which could not only survive in the oxygen rich atmosphere but could use oxygen for their needs came to be developed. These aerobic species prospered and proliferated. Then, around 1.5-2.0 billion years ago the first eukaryotes made their appearance. There are different opinions regarding the specific mechanisms by which this transformation was brought about. According to some biologists, the prokaryotic cells initially started developing internal

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membranes. These enveloped the major macromolecules like DNA and RNA, thus establishing a rudimentary nucleus. Subsequently, some of the nuclear genes 'escaped' and became engulfed by other membranes. This led to the formation of organelles like mitochondria and plastids. If such is the case, then one would expect that the DNA in the mitochondria would not be different from that in the nucleus. This, however, is not the case. That is why many other biologists including Lynn Margulis have sought to explain the emergence of eukaryotes in another way. According to their theory, independent, freeliving micro-organisms started living together in clusters. Initially, the association was random and casual. However, as time passed, symbiotic relationships were seen to provide certain advantages. These, then became more common and came to be preferred. Gradually, the partnerships became more intimate and some were joined permanently. The organelles were formed from independent prokaryotes which is why their DNA is different. The eukaryotic cells thus formed were not only more sturdy but capable of further development. Different mechanisms of replication also evolved along with their emergence. The benefits of sexual reproduction in generating different species were realised. The evolutionary process gathered momentum. And, as the atmosphere too became more conducive, life form climbed on to the land from water. From that moment there was no looking back. Life had taken firm control of the Earth.

t was a pleasant summer evening in London a little over a

I

We are not alone!

hundred years ago. Thomas HUXLEY (1825-1895), the renowned and highly respected biologist, was walking down a South Kensington street. Accompanying him was an eighteenyear-old student of his, eagerly listening to the master. Suddenly, the youngster stopped in his tracks. Pointing a finger at the faint reddish dot of Mars coming up over the horizon he asked his mentor, "Sir, do you think there's life out there?" "Nobody knows", replied Huxley. But that did not deter the student, Herbert WELLS. He let his imagination soar and came up, a few years later, with the classic The War of the Worlds in which the Earth was invaded by Martians. That, of course, was a piece of fiction. But the question he had asked earlier is not. "Is there life anywhere else in the universe? Are we alone?" These questions have been plaguing mankind at least for the last hundred years. There are excellent reasons for raising these questions. Not the least of these is the fact that man is a social animal. Therefore, the

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desire to find others of the same kind and to establish communication with them is innate to him. Having realized that his species is the most evolved and has taken firm control of this planet it was but natural that he should start looking for his ilk in the neighbourhood and beyond. But apart from this natural urge, there are other, more logical justifications to believe that the Earth may not be the only place in this vast universe where life has emerged and evolved and continues to thrive. To consider that we are alone in this universe would imply that emergence and evolution of life on this Earth is a unique phenomenon. In turn, it would suggest that the Earth is a special planet that is only one of its kind. There is no evidence to substantiate such a conjecture. In fact, Nicholaus Copernicus had enunciated a principle now known after him. He stressed this while declaring that the planetary system is heliocentric and not geocentric. He had argued that there is no mitigating feature to suggest that the Earth should have the privileged position at the centre of the universe. In fact, the Earth is like any other planet. Nor was the emergence of life on the Earth an accident. It was but a logical and inevitable consequence of the process of planetary formation. The latter itself is guided by the physical and chemical laws which direct and govern the

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The heliocentric (left) and the geocentric (right) planetary system

universe. Thus, wherever stars with their own planetary retinue exist in the universe, there is a possibility of life having emerged and evolved. In 1960, the American astrophysicist Frank DRAKE (b.1930) approached this problem theoretically in a novel way. He argued that the number of technologically advanced civilizations representing intelligent life in the whole universe depended on several factors. The most important

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On Earth Ott Mercury

On Mars

among these was the rate at which new stars were forming in the universe. Only a fraction of these stars would evolve in a way to support a planetary system. Unless a planet emerged, there is no possibility of life evolving. Therefore, this probability of planetary development around a star was also an important factor. Life on the Earth is crucially dependent upon availability of oxygen in the air, water on the surface of the planet as well as a range of temperatures conducive for its sustenance and proper nurturing. If the planet was, on the average, more cold than Earth like Mars, Jupiter or other outer planets or more hot like Mercury, life, as is seen on the Earth, cannot be sustained. Likewise, the availability of other elements like carbon, nitrogen, hydrogen, phosphorus is also important to maintaining life on the Earth. So, the possibility of having, among all those planets around a star, one with the appropriate environment for emergence and maintenance of life would be an important factor. It is, of course, not neces-


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I think some \ one's trying toj contact us /

sary that lite elsewhere in the universe be a carbon copy of the one that has prospered on the Earth. But if such limiting assumption is made, then one would only get an estimate which would be lower than the true number. Drake has suggested other equally important parameters that would have a bearing on the possibility of an intelligent life elsewhere. He contends that only a fraction of such ecologically suitable planets would actually support life. And only a fraction of these, would have intelligent life. If such extraterrestrial life is to be discovered or if human species have to have a chance of making contact with it, then that it too should become technologically advanced. Further, it should have a sufficiently large lifetime, so that it can harness

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its advanced technological status to communicate with other similarly equipped civilization. The universe abounds in stars. In our galaxy, the Milky Way, alone there are hundred billion stars. Its age is estimated to be 10 billion years. That would imply that on an average 10 new stars get formed every year. One may assume that to be the average for the entire universe. Thus, one gets a value for the most important factor propounded by Drake. How many of these new born stars would carry an entourage of planets with them? Until about 50 years ago, most scientists believed that planetary systems were extremely rare. However, with the advent of radio astronomy, a number of powerful techniques have been developed which have allowed a more accurate investigation of the vast space. If a planet revolves around a star, the gravitational pull of the former is felt by the star. The magnitude, no doubt, depends upon the relative sizes of the two bodies. But powerful instruments help detect existence of such an attraction of even a very small magnitude. In 1989, for example, astronomers from the Mauna Kea Observatory reported observing the existence of such gravitational tug in seven nearby stars. These and similar observations have now made scientists change their minds. They are convinced that existence of planetary systems around a star is more a rule rather than an exception. Therefore, 50 percent of all the stars can be reasonably expected to have planetary companions. It would also be not too unreasonable to expect that at least one of the planets revolving around a star would possess the necessary ecological attributes that would lead to the emergence and subsequent support of life forms. This, however, does not imply that life would get firmly established on each of such ecologically suitable planet. That number could be but a fraction of this. There is no way of accurately assessing what this fraction would be. But, scientists estimate it to be 10 percent.

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A star and a planet experience the gravitational pull of each o t h e r

Not all life forms that emerged would evolve to a stage where intelligence would become a discernibly significant trait. Again, only some of the intelligent life forms would develop to acquire technological capability necessary for space travel or radio communication. The distances between stars are large. Radio signals emanating from a planet orbiting a star would, therefore, take a long time to reach another planet where an intelligent, technologically advanced civilization capable of receiving and deciphering that signal exists. For example, the star

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nearest to us on the Earth is Proxima Centauri which is four and a half light years away. One is not sure if any planet keeps company with it. But studies of Barnard's Star, six light years away, have indicated the possibility of a Jupiter-like planet revolving around it. Still, it is not at all certain if life would exist on that as yet unobserved planet. It is, thus, more likely that the radio signals moving with a velocity of light would have to travel for thousands of years before encountering a receptive listener. It is, therefore, essential that the life span of the civilization sending, or receiving, such a signal should be much longer than the time it would take to traverse the distance if it has to establish meaningful communication with a like-minded counterpart. A reasonable estimate for this is considered to be a million years. If one now multiplies all these factors, as Drake has suggested, one gets the figure of 125,000. In other words, there can be, in this vast universe, 125,000 planets on which a technologically advanced intelligent life form exists. This is by no means an accurate measurement of this number. Scientists are fully aware that only the estimate of the rate of formation of new stars is reasonably accurate. Estimated values for all other factors are highly speculative. However, even if this number is wrong by an order of magnitude the resultant figure still would be large enough to provide hope that we are not alone. But, if that is so, how can we call any one of them ? Alternatively, can we find out if any one is trying to catch our attention? These are the questions which prompted mankind's search for extraterrestrial intelligence, SETI. In 1959, Philip MORRISON (b.1915) was listening to a concert of chamber music in the auditorium of Cornell University. As the strains of a Bach concerto wafted up to him, he had a stroke of inspiration. He suddenly thought that radiowaves may be a good medium for communicating with

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SETI programme attempts to listen to a possible radio signal from an alien civilization

an alien civilisation. They can go across the astronomical distances between stars. They can be easily transmitted. More importantly, they can carry substantial amounts of information with them. It is obvious that if someone out there was trying to reach us, he too would employ radiowaves to make contact. These ideas led to the establishment of SETI programme under which attempts are being made to listen to a possible radio signal loaded with information emanating from an alien civilisation. There was one major question that those supervising this project had to solve. Radiowaves come in a relatively large range of frequencies. They occupy that part of the

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electromagnetic spectrum with frequencies between 10 kilo Hertz (kHz) to 100 billion or 100 giga Hertz (gHz). Which particular frequency would be used by a civilisation to transmit a message? Giuseppe COCCONI along with Morrison provided an answer. They pointed out that hydrogen is the most abundant element in the universe. Any technologically advanced civilisation capable of transmitting a signal across thousands of light years would be equipped with that information. It would also know that hydrogen emits radiation at a frequency which is its special characteristic. That frequency is 1.42 gHz equivalent to a wavelength of 21 centimetres. If a galactic life species is trying to talk to us, it would most probably choose that frequency to do so. Ever since, for the last 30 odd years, scientists have been carefully listening to all transmission on that frequency. Recently, in 1993, D.G. Blair of Western Australia University, chose another frequency for listening to such a broadcast. This is 4.462336275 gHz. He arrived at this very specific number by multiplying the hydrogen frequency by the number Pi. His argument in favour of that choice is an extension of that proferred by Cocconi and Morrison. If the sender of this extraterrestrial message is a technologically advanced intelligent being, he would readily know the relationship between the radius of a circle and its circumference. This is constant throughout and is equal to the number Pi. The latter has been measured accurately to several decimal places. Blair, therefore, multiplied the two universal constants to come up with a new frequency to listen to. He felt the need of a new frequency since despite careful attempts for more than a quarter of a century with ears glued to the hydrogen frequency, mankind had drawn a blank. We, of course, do not have to wait for some one to get in touch with us. We too can send a message in the hope that it

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will be received by our counterpart similarly lending an ear to a signal coming across from a distant planet. In 1974, Drake, Carl SAGAN (b.1934) and colleagues devised a message providing basic information about the human species. It was a coded message, a kind of terrestrial telegram, loaded onto a carrier radiowave. It was beamed at the Great Cluster M13, a congregation of a million stars, in the constellation Hercules which is 25,000 light years away. The message is in the binary language used in computers. It consists of a total of 1679 bits, strings of ones and zeros. The number, like Blair's frequency, is a product of two prime numbers 73 and 23. Thus, if the receiver succeeds in deciphering this simple mathematical code, then the bits can be arranged in a matrix of 73 rows and 23 columns. If now a zero is represented by a dark square and a one by a bright square, a unique picture would emerge which would give the receiver a good deal of The terrestrial telegram basic information about Homo sapiens and the planet the species inhabits. The top of the picture depicts, in the binary form, the numbers one to ten. Below them are listed the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorus,

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WE ARE NOT ALONE!

The Arecibo Observatory

the five elements crucial to life on the Earth. Further down the chemical formula of DNA, the molecule of life, is given. Its helical three dimensional structure is also shown. This is followed by a stick like figure representing the human species and information about total human population of the Earth. The average height of man, expressed in multiples of the wavelength of the carrier radiowaves, the Sun and the nine planets with a clue that the message originates from the third planet are also included in the diagram. The message ends with information about the diameter of the Arecibo radiotelescope used for the transmission. This long distance extraterrestrial call has not yet been answered in the last 20 years. But, it would have been a

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surprise had anyone done so. Because the star cluster at which it is aimed is expected to receive it 25,000 years from now. That is why the failure to detect a signal from outer space for the last 30 years also has not deterred scientists. They are prepared to wait. They have enormous patience. Modifying slightly Bhavabhuti's precept they are apt to say, "Kalohyayam Nirazvadhih Vipulashcha Viswah". Time is eternal and the universe infinite.

Glossary Asteroid - Small rocky bodies in orbit around the Sun between Mars and Jupiter. Biosphere - The part of the globe in which life can exist. It includes parts of the Earth's crust, oceans and the atmosphere. Carbonaceous chondrites - A kind of stony meteorite containing large amounts of carbon. Colloidal suspension - A non-separable mixture of immiscible substances one of which is of particulate nature dispersed in the other. A common example is milk which is a colloidal suspension of fat globules in a watery solution. Comet - A celestial body made up of ice and dust some of which are seen to move around the Sun in highly elongated orbits. They often develop a long 'tail' of gas and dust as they come near the Sun. Electronic configuration - The arrangement of electrons around the nucleus of an atom. Extraterrestrial region - Region of space outside the Earth's atmosphere. Galaxy - A large system of stars, dust and gas held together by gravity. Geocentric - A system in which the Earth is in the centre. The earlier theory of the universe which considered the Earth to be fixed with the Sun, planets and stars going round it. Heliocentric - A system in which the Sun is in the centre. The currently accepted theory of the solar system in which the planets go round the Sun. Ion - An electrically charged form of an atom, produced by the gain or loss of an electron. Isotopes - Different forms of an element produced by variations in the number of neutrons in the nucleus and, consequently, in the atomic weight of the element.

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Mesozoic period - The geological era dating from about 270-230 million years ago. Meteor -Tiny particles of dust left over from comets which produce streaks of light as they enter the Earth's atmosphere with great speed and burn out. Meteorite - A stony or metallic object that has fallen to the Earth from outer space. These are often fragments of asteroids. Mutation - A random change in the genetic material of a cell. Nucleic acid - A large chain-like organic molecule, found mostly in the nucleus of living cells. It can be of two types called DNA and RNA and play an important role in passing on hereditary characteristics from parents to offspring. Palaeozoic period - The geological era which extended from about 570 million years to about 225 million year before present. Polymer - A compound made up of large molecules formed by joining together of a large number of smaller units. Polynucleotides - Long chains of nucleic acids. Primordial soup - A soup-like broth of chemicals formed in oceans of the early Earth from which living cells are believed to have appeared. Radioactive - Unstable elements which spontaneously emit particles or radiation as they change into other elements. Reducing atmosphere - An atmosphere where excess of hydrogen is present. Spectral lines - Bright lines produced in a spectrum by a luminous source such as a star. Supernova - Enormous stellar explosions in which all but the inner core of a massive star is blown off into interstellar space. Tertiary period - The gelogical era that followed the cretaceous period about 65 million years before present and ended about two million years before present.

EXOTIC

myths about the origin of life have existed for centuries. The riddle - when and how did life begin - has occupied man's mind since he appeared on this Earth. Scientists have pooled together their knowledge and proposed several theories to solve this riddle and strip it bare of myths. The fascinating story of life in the universe from the beginning has been told in a very absorbing manner in this lavishly illustrated book, specially targeted at students and the lay public. After describing in detail the origin and evolution of life forms in a most gripping style, it poses the mindboggling question : Is there extra-terrestrial life in the universe?

About the Authors After serving Bhabha Atomic Research Centre for 30 years, Dr. M.S. Chadha retired as Director of the the Centre's Biochemcial Group in 1988. He is currently a senior scientist of the Indian National Science Academy. Dr. Chadha is a fellow and member of several academies and societies. He is the author of over 120 research publications and six books and proceedings of seminars/symposia. Life in the Universe is his first popular science book. With the NCSTC National Award and INSA Indira Gandhi Award for the best S&T coverage in his bag, Dr. Phondke is a prolific popular science writer. He served a 23-year stint as a research scientist in BARC during which he published about 100 research papers. From research he switched over to science communication and was the Editor of Science Today and Science Editor of the Times of India group of newspapers. He is currently the Director of Publications & Information Directorate.

ISBN : 81-7236-084-3

Life in the Universe

Life in the Universe

Life in the Universe