Kitabı oku: «Life on Earth»

ONE

The Infinite Variety

It is not difficult to discover an unknown animal. Spend a day in the tropical forest of South America, turning over logs, looking beneath bark, sifting through the moist litter of leaves, followed by an evening shining a mercury lamp on a white screen, and one way or another you will collect hundreds of different kinds of small creatures. Moths, caterpillars, spiders, long-nosed bugs, luminous beetles, harmless butterflies disguised as wasps, wasps shaped like ants, sticks that walk, leaves that open wings and fly – the variety will be enormous and one of these creatures is quite likely to be undescribed by science. The difficulty will be to find specialists who know enough about the groups concerned to be able to single out the new one.

No one can say just how many species of animals there are in these greenhouse-humid dimly lit jungles. They contain the richest and the most varied assemblage of animals and plant life to be found anywhere on earth. Not only are there many major categories of creatures – monkeys, rodents, spiders, hummingbirds, butterflies – but most of those types exist in many different forms. There are over forty different species of parrot, over seventy different monkeys, three hundred hummingbirds and tens of thousands of butterflies. If you are not careful, you can even be bitten by a hundred different kinds of mosquito.

Marine iguana (Amblyrhynchus cristatus) underwater, Fernandina Island, Galapagos Islands, Ecuador.

In 1832 a young Englishman, Charles Darwin, twenty-four years old and naturalist on HMS Beagle, a brig sent by the Admiralty in London on a surveying voyage round the world, came to such a forest outside Rio de Janeiro. In one day, in one small area, he collected sixty-eight different species of small beetle. That there should be such a variety of species of one kind of creature astounded him. He had not been searching specially for them so that, as he wrote in his journal, ‘It is sufficient to disturb the composure of an entomologist’s mind to look forward to the future dimensions of a complete catalogue’. The conventional view of his time was that all species were immutable and that each had been individually and separately created by God. At the time, Darwin was far from being an atheist – he had, after all, taken a degree in divinity at Cambridge University – but he was deeply puzzled by this enormous multiplicity of forms.

During the next three years, the Beagle sailed down the east coast of South America, rounded Cape Horn and came north again up the coast of Chile. The expedition then sailed out into the Pacific until, 1,000 kilometres from the mainland, they came to the lonely archipelago of the Galapagos. Here Darwin’s questions about the creation of species recurred, for in these islands he found fresh variety. He was fascinated to discover that the Galapagos animals bore a general resemblance to those he had seen on the mainland, but differed from them in detail. There were cormorants, black, long-necked diving birds like those that fly low along Brazilian rivers, but here in the Galapagos, their wings were so small and with such stunted feathers that they had lost the power of flight. There were iguanas, large lizards with a crest of scales along their backs. Those on the continent climbed trees and ate leaves. Here on the islands, where there was little vegetation, one species fed on seaweed and clung to rocks among the surging waves with unusually long and powerful claws. There were tortoises, very similar to the mainland forms except that these were many times bigger, giants that a man could ride. The British Vice-Governor of the Galapagos told Darwin that even within the archipelago, there was variety: the tortoises on each island were slightly different, so that it was possible to tell which island they came from. Those that lived on relatively well watered islands where there was ground vegetation to be cropped, had a gently curving front edge to their shells just above the neck. But those that came from arid islands and had to crane their necks in order to reach branches of cactus or leaves of trees, had much longer necks and a high peak to the front of their shells that enabled them to stretch their necks almost vertically upwards.

The suspicion grew in Darwin’s mind that species were not fixed forever. Perhaps one could change into another. Maybe, thousands of years ago, birds and reptiles from continental South America had reached the Galapagos, unintentional passengers on the rafts of vegetation that float down the rivers and out to sea. Once there, they had changed, as generation succeeded generation, to suit their new homes until they became their present species.

The differences between them and their mainland cousins were only small, but if such changes had taken place, was it not possible that over many millions of years, the cumulative effects on a dynasty of animals could be so great that they could bring about major transformations? Maybe fish had developed muscular fins and crawled on to land to become amphibians; maybe amphibians in their turn had developed watertight skins and become reptiles; maybe, even, some ape-like creatures had stood upright and become the ancestors of man.

In truth the idea was not a wholly new one. Many others before Darwin had suggested that all life on earth was interrelated. Darwin’s revolutionary insight was to perceive the mechanism that brought these changes about. By doing so he replaced a philosophical speculation with a detailed description of a process, supported by an abundance of evidence, that could be tested and verified; and the reality of evolution could no longer be denied.

Put briefly, his argument was this. All individuals of the same species are not identical. In one clutch of eggs from, for example, a giant tortoise, there will be some hatchlings which, because of their genetic constitution, will develop slightly longer necks than others. In times of drought they will be able to reach leaves and so survive. Their brothers and sisters, with shorter necks, will starve and die. So those best fitted to their surroundings will be selected and be able to transmit their characteristics to their offspring. After a great number of generations, tortoises on the arid islands will have longer necks than those on the watered islands – one species will have given rise to another.

This concept did not become clear in Darwin’s mind until long after he had left the Galapagos. For twenty-five years he painstakingly amassed evidence to support it. Not until 1859, when he was forty-eight years old, did he publish it, and even then he was driven to do so only because another younger naturalist, Alfred Wallace, working in Southeast Asia, had formulated the same idea. He called the book in which he set out his theory in detail, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.

Since that time, the theory of natural selection has been debated and tested, refined, qualified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and behaviour have given it new dimensions. It remains the key to our understanding of the natural world and it enables us to recognise that life has a long and continuous history during which organisms, both plant and animal, have changed, generation by generation, as they colonised all parts of the world.

There are now two direct sources of evidence for this history. One can be found in the genetic material in the cells of every living organism. The other lies in the archives of the earth, the sedimentary rocks. The vast majority of animals leave no trace of their existence after their passing. Their flesh decays, their shells and their bones become scattered and turn to powder. But very occasionally, one or two individuals out of a population of many thousands have a different fate. A reptile becomes stuck in a swamp and dies. Its body rots but its bones settle into the mud. Dead vegetation drifts to the bottom and covers them. As the centuries pass and more vegetation accumulates, the deposit turns to peat. Changes in sea level may cause the swamp to be flooded and layers of sand to be deposited on top of the peat. Over great periods of time, the peat is compressed and turned to coal. The reptile’s bones still remain within it. The great pressure of the overlying sediments and the mineral-rich solutions that circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone, but they retain the outward shape that they had in life, albeit sometimes distorted. On occasion, even their detailed cellular structure is preserved so that you can look at sections of them through the microscope and plot the shape of the blood vessels and the nerves that once surrounded them. In rare cases, even the colour of skin or feathers can be detected.

Saddleback Galapagos tortoise (Chelonoidis nigra hoodensis) in defensive posture, Espanola Island, Galapagos Islands.

Fossil ammonites (Arnioceras semicostatum), in a sample of rock from the lower Jurassic period (195 to 172 million years ago), Robin Hood’s Bay, Yorkshire, UK.

The most suitable places for fossilisation are in seas and lakes where sedimentary deposits that will become sandstones and limestones are slowly accumulating. On land, where for the most part rocks are not built up by deposition but broken down by erosion, deposits such as sand dunes are only very rarely created and preserved. In consequence, the only land-living organisms likely to be fossilised are those that happen to fall into water. Since this is an exceptional fate for most of them, we are never likely to know from fossil evidence anything approaching the complete range of land-living animals and plants that has existed in the past. Water-living animals, such as fish, molluscs, sea urchins and corals, are much more promising candidates for preservation. Even so, very few of these perished in the exact physical and chemical conditions necessary for fossilisation. Of those that did, only a tiny proportion happen to lie in the rocks that outcrop on the surface of the ground today; and of these few, most will be eroded away and destroyed before they are discovered by fossil hunters. The astonishment is that, in the face of these adverse odds, the fossils that have been collected are so numerous and the record they provide so detailed and coherent.

How can we date them? Since the discovery of radioactivity scientists have realised that rocks have a geological clock within them. Several chemical elements decay with age, producing radioactivity in the process. Potassium turns into argon, uranium into lead, rubidium into strontium. The rate at which this happens can be estimated. So if the proportion of the secondary element to the primary one in a rock is measured, the time at which the original mineral was formed can be calculated. Since there are several such pairs of elements decaying at different speeds, it is possible to make cross-checks.

This technique, which requires extremely sophisticated methods of analysis, will always remain the province of the specialist. But anyone can date many rocks in a relative way by simple logic. If rocks lie in layers, and are not grossly disturbed, then the lower layer must be older than the upper. So we can follow the history of life through the strata and trace the lineages of animals back to their beginnings by going deeper and deeper into the earth’s crust.

Near horizontal layers of sedimentary rock, cut through by the Colorado River, forming the Grand Canyon, USA.

The deepest cleft that exists in the earth’s surface is the Grand Canyon in the western United States. The rocks through which the Colorado River has cut its way still lie roughly horizontally, layer upon layer, red, brown and yellow, sometimes pink in early light, sometimes blue in the shadowed distance. The land is so dry that only isolated juniper trees and low scrub freckle the surface of the cliffs, and the rock strata, some soft, some hard, are clear and stark. Most of them are sandstones or limestones that were laid down at the bottom of the shallow seas that once covered this part of North America. When they are examined closely, breaks in the succession can be detected. These represent times when the land rose, the seas drained away and the seabed became dry so that the deposits that had accumulated on it were eroded away. Subsequently, the land sank again, seas flooded back and deposition restarted. In spite of these gaps, the broad lines of the fossil story remain clear.

A mule will carry you in an easy day’s ride from the rim to the very bottom of the Canyon. The first rocks you pass are already some 200 million years old. There are no remains of mammals or birds in them, but there are traces of reptiles. Close by the side of the trail, you can see a line of tracks crossing the face of a sandstone boulder. They were made by a small four-footed creature, almost certainly a lizard-like reptile, running across a beach. Other rocks, at the same level elsewhere, contain impressions of fern leaves and the wings of insects.

Halfway down the Canyon, you come to 400-million-year-old limestones. There are no signs of reptiles to be found here, but there are the bones of strange armoured fish. An hour or so later – and a hundred million years earlier – the rocks contain no sign of backboned animals of any kind. There are a few shells and worms that have left behind a tracery of trails in what was the muddy seafloor. Three-quarters of the way down, you are still descending through layers of limestone, but now there is no sign of fossilised life whatever. By the late afternoon, you ride at last into the lower gorge where the Colorado River runs green between high rock walls. You are now well over a vertical kilometre below the rim, and the surrounding rocks have been dated to the immense age of 2,000 million years. Here you might hope to find evidence for the very beginnings of life. But there are no organic remains of any kind. The dark fine-grained rocks lie not in horizontal layers like all those above, but are twisted and buckled and riven with veins of pink granite.

Are signs of life absent because these rocks and the limestones directly above are so extremely ancient that all such traces have been crushed from them? Could it be that the first creatures to leave any sign of their existence were as complex as worms and molluscs? For many years these questions puzzled geologists. All over the world, rocks of this antiquity were carefully searched for organic remains. One or two odd shapes were found, but most authorities dismissed these as patterns produced by the physical processes of rock formation that had nothing whatever to do with living organisms. Then during the 1950s, the searchers began to use high-powered microscopes on some particularly enigmatic rocks.

Around 1,600 kilometres northeast of the Grand Canyon, ancient rocks of about the same age as those beside the Colorado River outcrop on the shores of Lake Superior. Some of them contain seams of a fine-grained flint-like substance called chert. This was well known during the nineteenth century because the pioneers used it in their flintlock guns. Here and there, it contains strange white concentric rings a metre or so across. Were these merely eddies in the mud on the bottom of the primeval seas, or could they have been formed by living organisms? No one could be sure and the shapes were given the noncommittal name of stromatolite, a word derived from Greek meaning no more than ‘stony carpet’. But when researchers cut sections of these rings, ground them down into slices so thin that they were translucent and examined them through the microscope, they found, preserved in the chert, the shapes of simple organisms, each no more than one or two hundredths of a millimetre across. Some resembled filaments of algae; others, while they were unmistakably organic, had no parallels with living organisms; and some looked to be identical with the simplest form of life existing today: bacteria.

It seemed almost impossible to many people that such tiny things as microorganisms could have been fossilised at all. That relics of them should have survived for such a vast period of time seemed even more difficult to believe. The solution of silica which had saturated the dead organisms and solidified into chert was clearly as fine-grained and durable a preservative as exists. The discovery of the fossils in the Gunflint Chert stimulated further searches not only in North America but all over the world, and other microfossils were found in cherts in Africa and Australia. Some of these, astonishingly, pre-dated the Gunflint specimens by a billion years, and some scientists now claim to have found fossils from around 4 billion years ago, not long after the formation of the earth. But if we want to consider how life arose, fossils cannot help us, for the origin of life involved the interaction of molecules, which leave no fossil traces. To understand what scientists think happened we have to look back beyond even the earliest microfossils, to a time when the earth was completely lifeless.

In many ways the planet then was radically different from the one we live on today. There were seas, but the way the land masses lay bore no resemblance in either form or distribution to modern continents. Volcanoes were abundant, spewing noxious gases, ash and lava. The atmosphere consisted of swirling clouds of hydrogen, carbon monoxide, ammonia and methane. There was little or no oxygen. This unbreathable mixture allowed ultraviolet rays from the sun to bathe the earth’s surface with an intensity that would be lethal to modern animal life. Electrical storms raged in the clouds, bombarding the land and the sea with lightning.

Laboratory experiments were made in the 1950s to discover what might happen to these particular chemical constituents under such conditions. Such gases, mixed with water vapour, were subjected to electrical discharge and ultraviolet light. After only a week of this treatment complex molecules were found to have formed in the mixture, including sugars, nucleic acids and amino acids, the building blocks of proteins. We now know that such simple organic molecules can be found throughout the universe, including on interstellar bodies such as comets. But amino acids are not life, nor are they even necessary for life to exist. The experiment proved little about the origin of life.