Mr joseph vempeny Posted on : 10/02/2008 06:21:35

LIFE - ORIGIN AND EVOLUTION

LIFE

Miss Universe

Imagine an explorer from outer space entering our solar system for the first time and observing each planet, first from afar, then after landing. The most noticeable thing for this explorer will be that, out of the nine planets of varying sizes, and a host of satellites, one stands out as the most beautiful heavenly body. One might even say that only one of these is really beautiful. For all we know, this could be, and most probably is, the most beautiful of the heavenly bodies in our entire galaxy or even the entire universe. We all can be proud of the fact that this singular beauty, this Miss Universe is none other than our dear Mother Earth - that the most beautiful planet is our home. The bright color of the sky and of the ocean, the pattern of clouds that blankets the ‘big blue marble’, the forests and grasslands, the coral reefs and lagoons, none of these are to be found on the others. Any explorer, terrestrial or extra terrestrial, cannot help but be enchanted by the beauty of the plants and trees, the leaves and flowers, the birds whose feathers can be found in all imaginable colours, the variety of the butterflies, the numerous species of both wild and domestic animals as well as the topographic beauty of the land section of the earth. If one considers these things on land beautiful, what is found underwater, the ocean floor or the coral reefs will appear even more beautiful.

What makes the earth different from others of her kind, what makes our home so beautiful – the secret of her beauty - can be described in one word: LIFE. It is the presence of life that makes the earth unique. Without any living things on it, the earth would be just another vast desert; no grass, no forest, no coral reef; just a hot or cold, dry or wet, desert - a dead planet. Life makes all the difference and makes the earth beautiful.

What is life? Not easy to answer. We may say that life is something that makes a dog different from its chain; that makes a tree different from the soil on which it stands, that makes a man different from his corpse. That is life, as I understand it. A scientist may define it rather differently. According to Gerald Joyce, one of the current authorities in the field of life sciences: “Life is a self sustained chemical system capable of undergoing Darwinian Evolution”. Here Evolution means ability for reproduction, mutations and natural selection. The whole surface of the earth, both land and water, are full of innumerable varieties of living beings, of all sizes and shapes, falling under three general categories; animals, plants and microbes. The one trait common to all these life forms is their ability to reproduce, some by the simple process of self-replication, others by sexual reproduction. All the higher forms of life are made of tissues with special functions and each of these tissues are made up of tiny units of life called cells.

To understand life is to understand a cell. Easier said than done. Whether from a tissue in the human body, whether from a fruit fly or an ant, a cell is a cell and is not very different from the unicellular organisms like the amoeba or bacteria. To say that a cell is very complex shall be the understatement of the century. A detailed diagram of a cell from a biology textbook may resemble the map of a city. A cell resembles a city in the way things are organized, specialized, transported, utilized, consumed, waste products removed, security checks installed and so on and so forth.

The Book of Life

Of all the myriad parts in a cell, the one part that carries in it the secret of life is the DNA (Dioxy ribonucleic acid). This is the part of the cell that holds the key to cell division or the art of self-replicating, the art of reproduction and survival, the art of life. The DNA also contains the genes that carry the hereditary traits from parents to offspring. The human genome contains about 30000 genes while the simplest single celled organism contains around 500 genes. The double helix structure of the DNA is basically made of various combinations of the four bases, now simply called A, T, C, G (adenine, thymine, cytosine, guanine), making the rungs of the ladder while the guardrails are made of a nucleotide pair of sugar and phosphate. If each of these chemical units is taken for a letter of the alphabet, the DNA for the human being, the human genome, is a book containing three billion letters. It seems more apt to call it an encyclopedia of 1000 volumes each of 1000 pages, each page containing 3000 letters. This “book of life” is the instruction manual for assembling the human body. One spelling mistake, one printing error, among these 3 billion letters can cause untold miseries; chronic illnesses, deformities, Parkinson’s disease and what have you. Even the cell of a simple organism like a fruit fly has a similarly complex DNA in which almost 50% of the genes are the same as in the human one. Most of the cells in our body contain all this DNA material in 46 chromosomes. The exceptions are the reproductive cells containing 23 chromosomes in each the male sperm and the female ovum. The red blood cells also do not possess the entire DNA as in other cells.

To say that our DNA is the instruction manual for assembling the human body may at first seem preposterous or at least a bad joke. Not if you understand the function of DNA in protein synthesis. Genes, units of heredity passed from parent to offspring, are pieces of DNA encoding the information for the synthesis of a specific protein. There is the danger of overlooking the significance of the phrase, “synthesis of protein”. Only recently did this significance dawn on me, even though I had known for a long time that protein synthesis is one of the functions of the DNA. Let us see how the DNA punch card or instruction manual is employed in assembling this human body.

When the sperm fertilizes the egg, life begins in the form of a single-celled zygote, which has a full complement of the entire DNA, half from the mother, half from the father. The DNA then gets busy directing the rest of the cell in the production of essential proteins, first for growth and then for cell division. During the cell division the DNA replicates itself into two full sets and then two cells are formed but do not separate. This process continues, two becoming four, eight, sixteen and so on. As the size of the embryo increases the number and variety of proteins needed for further growth also increases. The DNA continues to give clear instructions resulting in the synthesis of proteins needed for the tissues of muscle, bone, blood, brain and nerve cells as well as for various organs. The miracle of the growth of a zygote into an embryo, into a foetus, into an infant, into a child, into an adult, is made possible by the ability of the DNA to direct the rest of the cell to manufacture the right kinds of proteins at the right time and place. One has to be really naïve to think or believe that by the chance combination of some molecules non-living matter got the miraculous faculty called life, the extraordinary power to replicate itself.

Most of the cells as we know today contain, RNA molecules along with DNA. RNA, though not as complex as DNA, also shares the self-replicating capacity of the DNA and has important role to play in protein synthesis. One of these two is essential for life. Both are found in most cells of the higher animals and plants as well as in the single celled organisms, with the general exception of viruses which have only the DNA or the RNA, not both.

The Origin of Life.

Now comes the million-dollar question: “How did it all start?” How, when and in what form did life appear on the earth? It is certain that life must have started in its simplest form. But how simple is the simplest form of life? Fifty years ago many scientists believed that they knew all about the cell. But today they are not so sure. Craig Venter is the head of Celera International, one of the two firms that decoded human genome, one who studied in detail the genes present in the human DNA. In 1996 Venter entrusted a team under Scot Peterson with a 10 year mandate to find answers to two questions: “How many genes does a cell need to live? And which genes are they?” After 5 years the team is still working on the project, on an organism called Mycoplasma Genitalia, without a clear answer in sight. This particular organism was chosen because this was the one with the lowest number of genes, 470 only. Compared to this the bacterium Haemophilus Influenzae has 1700 genes and the human being has around 30000. Venter himself had hoped to “find out how a cell works” when he was sequencing the genome of the bacterium Haemophilus Influenzae, but he found it too complex. This is what Venter had to say: “I naively thought that we could have a molecular definition of life, come up with a set of genes that would minimically define life. Nature just refuses to be so easily quantified.” In short, we have to conclude that the simplest form of life is not all that simple.

Since 1859, when Darwin Published “On the Origin of Species”, scientists have been trying to create life from the basic chemicals in the lab. They have not succeeded so far. In 1950 Stanley Miller subjected a mixture of methane, ammonia and carbon dioxide to electric sparks and succeeded in getting some amino acids, simple organic compounds that are the building blocks of protein. He had assumed that the atmosphere of the ancient earth consisted of these gases and the electric sparks did the job of lightning. Now we know that ammonia was not present in the atmosphere of the earth at any time in its history. Many other theories on the origin of life on earth by chance combination of molecules had to be given up due to lack of sufficient evidence to support these theories.50 years after Miller’s attempt, scientists still have not produced anything that resembles a protein or the RNA or the DNA. That is why Craig Venter said in 2001: “Right now the only way to get life is from life itself.” Here he is referring to the attempts of his team to assemble the simplest form of DNA that has the self-replicating property from the genes of simple bacteria like the Mycoplasma Genitalium.

“No one knows how life arose on the desolate young earth, but one thing is certain: life’s origin was a chemical event,” writes Robert Hazen of Carnegie Institution of Washington, in a lengthy article on the possible role minerals might have played in the chemical reactions that led to the formation of the first living entity. “Scientists are far from creating life in the laboratory, and it may never be possible to prove exactly what chemical transformations gave rise to life on earth” he says elsewhere in the same article (Scientific American April 2001).

One might ask: “Can’t we give a rational explanation of the origin of life in terms of chemical reactions instead of bringing in the idea of God, which idea is very difficult to explain in scientific terms?” Is it not reasonable to assume that with the energy of the sun or the lightning or that of the volcanic vents on the ocean floor, certain compounds would have by chance combined together forming the first simple, self-replicating molecules? Many are still trying hard to find experimental evidence to such a hypothesis. But the probability of the molecules in air or water combining ‘by chance’ to form life is less than the probability of a typhoon passing through a junkyard making the various pieces of junk to join together by chance to form a Jumbo Jet ready to take off. Compared to the assembling of chemicals to form a DNA or RNA molecule or a cell, the work of assembling a Jumbo Jet is rather easy or at least possible. Man has done it. But he has failed to make life from lifeless atoms and molecules.

Seeing the futility of getting a rational explanation to the origin of life on earth as a result of chemical reactions, some have suggested that life did come to the earth from outer space on some of the meteors or comets. This seems even more improbable because the heat of the impact should have destroyed any form of life. Even if life came from outer space how did it originate there? Certainly not by mere chance. God should have created it somewhere out there. Whichever way we look at it, life is too complex to have just come into being by mere chance. Even though we assume that evolution is guided and directed by God, creation of life stands out as a separate act even as the creation of the cosmos or the creation of the human soul.

It is not as if we bring in the idea of creation and of God when everything else fails. The various attempts by scientists were mentioned to show that mere chance does not cause a storm or a lightning to assemble a luxury car or a jumbo jet or a DNA molecule. When we say God created life we do not mean that God came down with a test tube and reagents and made the first form of life. Nor did he stop his ‘activities’ once life was created. He definitely directs and controls the progress of life from organic molecules to single cell to complex organisms and to the human being, the process we call biological evolution.

Before a proper unicellular living organism was created the chemicals necessary for these were created. These include amino acids necessary for the formation of protein as well as sugars and bases needed for the complex nucleic acids. This could be considered part of the chemical evolution – from quarks to ions to atoms to simple molecules to organic molecules. But the transition from the chemical to the biological, from non-living matter to a living entity - the appearance of “life”- was not just another reaction. To the system of molecules so assembled, God gave a special something, the power to reproduce and multiply, the life force, that something which distinguished life forms from inanimate matter and made it “alive”. This is Creation of Life.

Evolution of Life

The creation of life is supposed to have taken place some 4 billion years ago when the earth was still in the infant phase, which is to say, only about 600 million years old. According to some scientists the first living entity could have been just an RNA molecule along with some proteins and a membrane to enclose these. DNA could have come later. The unicellular organisms that we know today, the amoeba, bacteria, diatoms, and planktons, all are much more sophisticated compared to those early forms. They were more like viruses, which have only DNA or RNA and have no nucleus. The first cells, with both RNA and DNA and cell membrane still had no nucleus.

The first great revolution in the story of evolution was the ability of some of the cells to perform the great miracle of life, what we call photosynthesis, wherein the cells manufactured their own food and started producing oxygen. These were still anaerobic, not needing oxygen for energy. These were probably like the cyanobacteria. The cells still lacked nucleus. Even after this, the pace of evolution was very slow. The oxygen produced by these cells never came out of water to enter the atmosphere. The iron present in the water used up all the oxygen produced to form iron oxide. It took nearly one billion years to convert all the iron of the ocean to its oxide, and to have sufficient oxygen dissolved in the water to support the next generation of aquatic life forms, those that use oxygen to produce energy. After this only did oxygen enter the atmosphere.

Nearly 2 billion years passed after the first appearance of life before advanced unicellular organisms, complete with nucleus evolved. Though the pace of evolution was still very slow, the existing varieties of life were multiplying fast and life was becoming abundant in the oceans. But a real explosive increase in the number and variety of life forms began with the next revolution of biological evolution, the exchange of genes by the process of sexual reproduction. This took place some 1.2 billion yeas ago. Still there were only single-celled organisms and only in water. First fossil evidence of multicellular organisms such as sponges and worms belong to a period some 600 million years ago. This was followed by a profusion of aquatic life forms, trilobites, clams and other invertebrates. In the plant kingdom, other than plankton and diatoms there were algae of a few varieties. Sometime along this long route to progress, somewhere about 500 million years ago, some plant organisms had found their way into fresh water rivers, lakes and swamps.

Out of the Oceans

The oceans had nurtured life for more than 3 billion years before life appeared on land or in fresh water. How could we explain this? Why this delay? The answer is quite simple. Fresh water lakes and rivers are found on landmasses or continents. There were no such landmasses for the first 2 or 3 billion years. There were only some archipelagos, groups of tiny islands, which were the peaks of sub-oceanic mountains formed by the volcanic activity. The first super continent – Pangea, to the geomorphologist – formed around the South Pole about 600 million years ago. By now the atmosphere took in the present composition and the earth became more hospitable to life. Rains and seasons became decisive factors in the spread of life. Aquatic life forms began to spread in fresh water streams, rivers and swamps on this super continent.

Still the dry land was desolate. And then, some algae and lichen that was on the edges of swamps slowly found their way onto wetland near the swamps and gradually onto dry land. This, the start of the colonization of the land took place some 500 million years ago. Till a few years ago the general idea among the scientific circles was that life forms from the ocean came to the land. But recently it was confirmed that it was from fresh water that life came to the land. From the simple plants like moss and lichens higher forms like ferns and cycads evolved. Even much later the land became host to the first true trees. But these were still, of the non-flowering variety. Not even grass was there. The first forests were of the coniferous trees like pines and firs.

Vertebrates

When these were happening on land some equally important stage in the history of biological evolution was being enacted in the oceans. This was the appearance of fish, the first vertebrate organisms, those with internal skeleton with backbone as the mainstay. We still have no idea as to when and how the first animals made the appearance on land. But we know that the earliest of these were invertebrates like spiders and dragonflies. One of the most interesting chapters in the story of evolution of life that I have come across is the story of how the fish came to walk on land. Like many other evolutionary stories this one also has its origins in the fossil remains and fossil footprints. Here the fossil and the fossil footprints belonged to what the scientists call a tetrapod, a fish that had adapted four of its fins modified to act as feet. This was discovered on the shores of Ireland and dated some 450 million years old. It is said of palaeontologists, that they make a mountain of theories out of a molehill of evidence. Yet, often these theories hold water and help us to a better understanding of the world around us. I personally liked the story built around the fossil of the tetrapod as it gives us the most likely scenario of how vertebrates came to land. The story goes like this.

A certain type of fish that made a swamp or a marshy place its home found it hard to swim around in the thick soup of marsh with the aid of the fins. It was easier to get along and find its food by holding on to the floor and to the reeds and branches in the swamp. For this purpose it adapted its fins for gripping and thus slowly the fins turned to four legs. The tail fin stayed as a fin as long as it was in water or marshy places. The shortage of oxygen in the marsh made the fish to replace the gills by lungs. Soon it found that it could find its food from the shores of the swamp as well. Then one fine day it said good-bye to the water and came to the land and became the first vertebrate to occupy land. It couldn’t have been a mammal, most likely a reptile, as mammals were still higher up in the scale of life. But it seems that in appearance this first creature looked like something between a dog and a lizard. A very surprising fact is that the link between this tetrapod and the fish still exists today. In the marshes near the Congo River you find a fish without fins. It is called the long fish. This has, in place of the fins four long wire-like appendages. They use these limbs effectively to move about in the thick muddy swamps. The tail fin tells you that this is at least in part a fish. This species has developed some mechanism for breathing atmospheric oxygen in cases of drought, along with dissolved oxygen. This species has not undergone any evolutionary change during the last 400 million years. This is one of what we call a living fossil.

A certain type of fish that made a swamp or a marshy place its home found it hard to swim around in the thick soup of marsh with the aid of the fins. It was easier to get along and find its food by holding on to the floor and to the reeds and branches in the swamp. For this purpose it adapted its fins for gripping and thus slowly the fins turned to four legs. The tail fin stayed as a fin as long as it was in water or marshy places. The shortage of oxygen in the marsh made the fish to replace the gills by lungs. Soon it found that it could find its food from the shores of the swamp as well. Then one fine day it said good-bye to the water and came to the land and became the first vertebrate to occupy land. It couldn’t have been a mammal, most likely a reptile, as mammals were still higher up in the scale of life. But it seems that in appearance this first creature looked like something between a dog and a lizard. A very surprising fact is that the link between this tetrapod and the fish still exists today. In the marshes near the Congo River you find a fish without fins. It is called the long fish. This has, in place of the fins four long wire-like appendages. They use these limbs effectively to move about in the thick muddy swamps. The tail fin tells you that this is at least in part a fish. This species has developed some mechanism for breathing atmospheric oxygen in cases of drought, along with dissolved oxygen. This species has not undergone any evolutionary change during the last 400 million years. This is one of what we call a living fossil.

The descendants of this reptile and probably some forms of amphibians were populating the dry land and multiplying fast when catastrophe struck about 250 million years ago. This caused the extinction of 90% of the life on earth, forests, animals, as well as aquatic life. The decaying forests caused an abnormal increase in the propagation of fungi. The presence of excessive quantities of the remnants of fungi in the rock layers of this period is what gave away the clue to this catastrophe. What did cause this super-massive natural disaster? Was it meteorites or abnormally high volcanic action or a deluge? Or was it a combination of all these? Probably a combined attack. The impact, at close intervals of two or more large meteors could have melted the crust of the earth and unleashed volcanic activity of extremely high magnitude. The temperature rose too high and the dust and fumes that filled the skies blocked out the sun for a very long period of time. It was just providence that this catastrophe did not wipe out life entirely from the face of the earth.

This event could have also kick-started the process that led to the break up of the single landmass into continents, the process of plate tectonics. Nor was this the first catastrophe of its kind. It was rather a common occurrence. Though very destructive in nature these so-called catastrophes had helped in shaping the earth and life on it into what it is today. The disaster that destroyed 90% of life seems to have at the same time made the earth more fertile and habitable. In a couple of million year’s time, when the dust and cloud had settled after the devastation, there was an unprecedented and accelerated growth of life forms on land. From this debris grew immense forests of conifers, cacti and giant ferns. The reptilian population that survived this disaster began to grow like nobody’s business. This was the Jurassic age of dinosaurs. Animals of that size and mass had not been found on earth before or after this period.

Mammals

The age of the dinosaurs lasted roughly 150 million years. This was the period when the first birds evolved from the reptiles and invented the new and fast mode of transport - flight. This period also saw the appearance of the first mammals, probably tiny rodents. In the plant kingdom also there were new developments, such as the appearance of flowering plants like grass followed by trees. The reign of the reptilian giants on land ended some 65 million years ago as a result of another catastrophe, though not as devastating as the one before that. The most probable cause in this case also is impact of meteors. Whatever the physical cause of the extinction of the dinosaurs, the real cause in my opinion is that the earth could not bear them any more. They had grown too big and too destructive and the One “up there” decided it is time to put an end to their tyranny. Another way of putting it is that nature has its own ways of safeguarding the overall interest of life on earth and will not allow any one species to have its own way at the expense of others. Humans have destroyed more forests than the dinosaurs but man has replaced most of them with some other form of vegetation, crops, farms or plantations. Yet today scientists are concerned about the damage to ecology that we are doing and are planning measures to counteract them. Hence all the frenzy of conservation.

Though all the huge forms of reptiles became extinct in this process, the lesser reptiles did survive. The crocodile, Komodo dragon and monitor lizard can be considered the living fossils of the Jurassic age. The mammals that survived began to evolve fast to take over the leading role left by the dinosaurs. We see an explosive increase in the families and species of mammals during the last 90 million years. The ancestor of primates - of monkeys, apes and humans - came on the scene about 70 million years ago. These were lower primates, like lemurs or bush babies. Scientists call them Prosimiens, meaning ‘before the apes’. Higher primates or anthropoids branched off from this line around 45 million years ago. This is the group that gave rise to monkeys, apes and humans. During the last 7 million years the chimpanzees and guerrillas have not evolved much but the other branch that led to us underwent unbelievable changes in that relatively short span of time. But then, that is another story.