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.
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