Darwin's Island Read online

  Evolutionists are not in the least surprised. They were baffled at some of the decisions made by those who ran the Genome Project. Like Vesalius, James Watson and his colleagues had a Platonic view of existence. Every heart and every human was built on the same plan and to understand one was to understand them all. The first DNA sequencers outPlato-ed Plato for they assumed not just that the essence of humankind could be found within a single person, but that this Mr Average was, in the interests of political correctness, best stitched together from bits of double helix taken from random donors across the globe.

  That was a big mistake. The Platonic approach ignores the vital truth that evolution is a comparative science. Natural selection depends on inherited differences. To understand the past biology needs not just a single genome but many. To map variation from person to person, from place to place, or from species to species shows how, when and where evolution has been at work. So central is diversity to the idea of descent with modification that the first two chapters of The Origin are devoted to the nature and extent of variability in the bodies and habits of plants and animals. Now, genetics has begun to tell the tale in the language of DNA.

  James Watson’s auto-icon disclosed no more than half his secrets for it contained just one of the two versions of the double helix present in each cell. His rival in the race to decipher the secrets of life, the biologist and businessman Craig Venter, was less reticent. He read off both his copies, that received from his father and that from his mother. Venter was happy to reveal its contents: his father died young of a heart attack, and he has himself been bequeathed a variant that predisposes to the disease. He has also inherited genes supposed to increase the wish to seek novelty, to be active in the evening rather than early in the day and to have wet rather than dry ear wax.

  Whatever Venter’s intimate chemistry says about his personality, his bed-time or the exudations of his auditory canal, it has a message for us all for it gave the first hint of the true level of human diversity. Both his parents are white Europeans (and hence represent just a small sample of mankind) but their DNA is distinct at around one site in two hundred along the entire chain - which adds up to tens of millions of differences between them.

  On the global scale, hundreds of millions of sites in the inherited alphabet vary from person to person and the ‘Thousand Genomes Project’, now well under way, has set out to fund out just how many there might be. Unlike its predecessors it will search out rare variants, those carried by fewer than one person in a hundred and present in vast abundance - and given the advances of technology, the project may cost little more than fifty million dollars. Already we know that each of the twenty-three human chromosomes - the physical location of the genes - has millions of single-letter changes aligned along it. The variable sites are so tightly packed together that, over short lengths of the double helix, they almost never separate when the molecule is cut, spliced and reordered, as it always is when sperm or egg is formed. Such long blocks represent sets of chemical letters that travel down the generations together. Rather like surnames, they are excellent clues of relatedness.

  In addition to its single-letter changes, the double helix is marked by duplications of certain pieces and deletions of others. The order of its letters may also be reversed, and great stretches can hop to a new place. A study of three hundred whole genomes has already revealed a thousand and more such differences in the numbers of particular DNA sequences. Some genes are arranged in families - groups of similar structures that descend from a common ancestor and have taken up a series of related jobs. The biggest has eight hundred members. It helps build the senses of taste and smell. Its elements vary in number from person to person and some lucky individuals have fifty more copies of a certain scent receptor than do others.

  Most such changes involve fewer than ten letters, but some are a million base-pairs from end to end. A few people may, because of the gains and losses, have millions more DNA bases and thousands more genes than do others and the potential variation in dose from person to person represents more than the length of the largest human chromosome. Even so, some of the repeated segments have just the same structure in humans as in the coelacanth, which split apart four hundred million years ago.

  DNA is a labile and uncertain molecule. A multiplied sequence often makes mistakes as egg or sperm are formed, to produce longer or shorter versions of what went before. Some bits move or multiply at a rate of one in a hundred each generation rather than the one in a million once assumed to be typical. Age changes us and the double helix is reordered, duplicated and deleted as the years go by (which means that the offspring of older parents inherit more mutations than do those of young).

  Variability beneath the skin is far more extensive than Darwin had ever imagined. Biologists have long known that, with the exception of identical twins, everyone in the world is distinct from everyone else, and from all those who have ever lived, or ever will. That claim is too modest. In fact, every sperm and every egg ever made by all the billions of men and women who have walked the Earth since our species began is unique; a figure unimaginable before the days of molecular biology.

  Such variety links individuals, families and peoples into a shared network of descent. It shows how man is related to chimpanzees, gorillas, orangs and macaques, and for that matter to plants and to bacteria. Evolution - like astronomy - has always looked at the past through the eyes of the present but its new technology - like the star-gazers’ development of giant telescopes - means that it can now see far further and deeper into the universe of life than once it could.

  Even so, biology is not like astronomy. The images that flood from its machines are often blurred and ambivalent. Many statements about ancestry are filled with unproven, and often unstated, assumptions about the rate of change in DNA, the size of ancient populations and the effect - or supposed lack of effect - of mutations on the well-being of those who bear them. The information in the genome is almost limitless, but at present its language remains ambiguous.

  Fortunately, the Earth has some better witnesses to years gone by. Like the remnants of stellar rocks that sometimes strike our planet, they are silent, shattered and few in number but at least they give direct evidence of how the past unfolded. Darwin was well aware of the importance of the fossil record to his case. One page in six of The Origin is devoted to the relics of the rocks, to the record’s imperfections and to the central role it plays as proof of the fact of change. In 1871, no human fossils (with the exception of a skull from Germany now known to come from a Neanderthal) had been recognised. Things have much improved and the primate record is far more complete than it was even a few decades ago. The tale it tells is still fragmented and uncertain, but what it says fits remarkably well with the history revealed by the double helix.

  In the Miocene epoch - from around twenty-three million to five million years ago - the Earth was a true Planet of the Apes. Primates were all over the place, with a hundred or more distinct species of ape, in Africa, in Asia and in Europe. They lived in woodlands, plains, forests and swamps. Some were no bigger than a cat and others larger than a gorilla. For much of the time their capital was in Europe and many of our predecessors have laid their bones there. Then the animals moved on, to set up shop in Africa. A ten-million-year-old fossil from Kenya may be the common ancestor of men, chimps and gorillas. If so, it confirms Darwin’s speculation that it was more probable ‘that our early progenitors lived on the African continent than elsewhere’. He did not, of course, know that continents had broken up and drifted across the world, and that Africa itself did not exist in the earliest days of the evolution of our line.

  One day almost all the players in that ancient drama left the stage. The apogee of the apes was over and their long twilight - now fast turning into night - had begun. The sun began to set on their family well before humans appeared, but, once they did, their nemesis was assured.

  Lucy, the famous fossil of Australopithecus afarensis, was a creature
quite human in appearance, lightly built and little more than a metre tall, with relatively long legs and small teeth. She belonged to a group who lived between three and four million years before the present. Others among her kin left footprints in Tanzanian volcanic ash as proof that they walked upright at a time when their brains were but a third the size of our own. The males were considerably larger than the females. Homo habilis - ‘handy man’ - lived in South and East Africa for about a million years from two and a half million years ago. It had long arms, brow ridges and a larger brain than Lucy, and was quite good at making tools. Similar individuals were found in Africa, and perhaps in Georgia.

  Homo erectus¸ the upright human, the next fossil claimed as a direct (or almost direct) human ancestor, emerged around 1.8 million years ago, and may have split into two species in its homelands in Africa and Asia. Some individuals had brains as large as our own and lived as far north as the South of France. A rather younger European arrived around 1.2 million years before the present, and left a few of his bones in the caves of the Sierra de Atapuerca in northern Spain. That ancient Spaniard has been christened Homo antecessor, and might be the common ancestor of ourselves and the Neanderthals. A later European from around half a million years ago, Heidelberg Man, may have been an antecedent of the Neanderthals rather than ourselves. He too first appeared in Africa. Many - perhaps too many - more supposed members of our close family have been named as distinct species, and the human pedigree has begun to look more like a bush than a tree. As a result, direct lines of descent have become harder to trace than once they were.

  For most of history, our ancestors shared their home with several related species that were much closer to themselves than the chimpanzee is to us. Those days have gone, and nearly all members of man’s ancient household have left no issue today.

  The Neanderthals were once our most immediate kin. They lived in Europe and the Middle East from around a quarter of a million years ago to about thirty thousand. They had bigger skulls - and, perhaps, bigger brains - than modern humans (although they were also beefier in general). They trapped animals in pits, and may have been cannibals (although another view of the carved bones of their fellows is that they represent a ritual burial). Neanderthals lived in small groups in an icy Europe for far longer than our own species has existed, and then disappeared. Like many other apes, they went quickly. Perhaps a cold snap defeated them, for a remnant hung on in the warmth of southern Spain until well after the moderns arrived. The victors had better clothes, which allowed the tropical ape that they were - and we are - to survive in a climate that killed off an animal more used to bad weather but less well clad. Perhaps Homo sapiens murdered the Neanderthals or starved them out, but we do not know. Sex was not on the agenda, for fossil DNA from a Croatian specimen shows that they were quite distinct from our direct ancestors. In addition, today’s Europeans and Middle Easterners retain no ancient lineages that might have come from an extinct relative. DNA suggests that the Neanderthals’ last common ancestor with modern humans lived in Africa more than six hundred thousand years ago, long before Homo sapiens emerged.

  Soon after the loss of his cousin, that species began to spread across the world. Modern humans filled the whole habitable globe no more than a thousand or so years before the present, when at last men and women reached New Zealand and Hawaii. Their ancient journeys can still be read in DNA. The double helix reveals a clear split between Africa and everywhere else, a legacy of the small group of migrants who first stepped out of our native continent into an uninhabited world, together with a second and more ancient split within Africa that separates the Khoi-San - the Bushmen - from all others. Other great genetic trends, such as those across the New World and the Pacific, track the last migrations into a deserted landscape.

  Once, it seemed that modern Europe had a more complicated history than did most of the globe, with several waves of migration superimposed on each other. The genes of local hunters, who arrived long ago, were - perhaps - diluted by those of the first farmers who spread, just a few thousand years before the present, from a population explosion in the Middle East. Some variants do show a trend from south-east to north-west, in a pattern that might indeed reflect a slow wave of inter-communal sex. The archaeology of pots and seeds suggests in contrast that agriculture was taken up at some speed, as soon as people learned about it, with no need for weddings. In Britain, at the western edge of the new technology, carbon dates taken from charred grains suggest that around 4000 BC farming replaced hunting within just a couple of centuries, too fast for any large-scale mixture of populations. There is no real evidence of a flood of lascivious rustics coming from the east. Instead, ancient Europe was more open to ideas than it was to genes. The trends seen today are the remnants of the first grand migration thirty thousand years before the emergence of agriculture, as humans arrived in an empty continent from the south and east. The mitochondrial DNA - the female lineages - found in the remains of a hunter-gatherer group in northern Spain look more or less the same as those of modern Spaniards in the same place, with no sign of mass immigration. Modern Europeans trace most of their heritage to the first wave of hunters. Since then, they - and their DNA - have tended to stay at home.

  As men and women filled the world they killed off many of their kin. The Neanderthals were the first to go, and human habits have not changed since then. Today, just a few remnants of our once extensive clan linger on. In a century or so we will be the single large primate (and almost the only large mammal), to be found outside farms or zoos. Almost all the apes will be gone, some before they are studied by science. That fact is a tragedy both for the creatures involved and for science itself, for each of them says something about our own biological heritage. They contain within their DNA the story of human evolution and, perhaps, more: for some of our own inborn diseases are caused by genes identical to some that function perfectly well in our relatives.

  The physical similarity of primates and humans was noticed by Queen Victoria and, after The Origin, was often used by those anxious to judge the evolutionary status of their fellow men. Charles Kingsley, author of The Water Babies, wrote to his wife about an Irish visit that ‘I am haunted by the human chimpanzees I saw … to see white chimpanzees is dreadful; if they were black, one would not feel it so much.’ Chimpanzees are, indeed, our closest relative. Darwin himself noted that, among their many other affinities to humans, they ‘have a strong taste for tea, coffee, and spirituous liquors: they will also, as I have myself seen, smoke tobacco with pleasure’.

  Whatever our shared vices, chimps are not like us in many ways. They are hairy and bad-tempered and do not show the whites of their eyes. The animals have rather small brains, no ear lobes and cannot walk upright, float, or cry when upset. They give birth with less pain than we do, and the young mature without any obvious period of adolescence. Our kidneys keep salt better in the body than do theirs, and we have more white blood cells. Chimpanzees are in addition safe from the horrors of old age as they tend to die young and even in zoos do not get Alzheimer’s disease. When they are faced with diseases brought on by infection or poor diet, their symptoms often differ from our own, which means that they have not been as useful in medical research as might be hoped.

  Chimp sex life has a definite flavour of its own. Men lack the penile bone found in male chimpanzees, but when it comes to penis size, man stands alone. Women have outer labia, absent in their closest relative. Chimpanzee males have larger testes than we do in relation to their body size and, unlike ourselves, seal up their mates with a sticky plug after sex. Promiscuity is the rule. The creatures copulate with enthusiasm and their close kin the pygmy chimps or bonobos are even more energetic. The females show when they are fertile (unlike women, who conceal all signs of that crucial moment) and the males then indulge in a competitive frenzy to mate with them. Sperm from rhesus macaques, a species known to be highly promiscuous, swim faster and lash harder than those of gorillas, in which a single male more or less m
onopolises the females. Chimpanzee sperm are almost as energetic as those of the macaque while ours lag well behind either. They do, on the other hand, beat the male cells of the gorilla.

  The chimpanzee genome was read off in 2005. Not many of the single letters in the DNA code have changed since the split from our own family line for, on that simple measure, humans and their closest relative are almost 99 per cent the same. At the protein level, too, we are close, with no more than about one amino acid in a hundred having altered.

  Such figures underrate the real divergence of the two species. Changes in the number and position of inserted, repeated or deleted segments mark both lines. There are three times as many alterations of this kind as of single-base changes, which puts the overall difference between men and chimps at around 4 per cent. Primates go in for the gain and loss of genes more than do other mammals and our own lineage is out in front with a rate of change three times faster than average. Homo sapiens has gained seven hundred gene copies since the split with chimps, and the chimp has lost almost the same number. One chromosome has gone even further. Women have two large X chromosomes, men an X matched with a smaller Y. The human and chimp X have diverged by just half a per cent in the single letters of the DNA alphabet while the Y has shifted three times more, as proof that women, with two Xs, are closer to chimpanzees than are men.

  Many of the differences between the two primates have built up because we can modify our environment in a way that other primates cannot. As a result, we depend less on changes in our DNA than once we did and so have lost many once-functional genes. Mankind is feebler than it was. We became shaved monkeys with just a single mutation or a few because a segment of DNA that codes for the hair protein no longer works. It received its fatal blow a quarter of a million years ago. Samson lost his strength with his locks, and so did his ancestors, for the DNA behind certain powerful muscles is out of action in humans compared with their closest living relatives (which is why to wrestle with a chimpanzee is a mistake). A shared déjeuner sur l’herbe is also best avoided, for the animals have enzymes that break down poisons fatal to ourselves. Darwin noted that ‘savages’ ate many foods that were harmful unless cooked; and red kidney beans still fall into that category. Chimpanzees need no kitchens, for they can manage a variety of noxious plants (certain herbs used for medical treatment included) that we cannot. They have also kept many of the talents of taste and smell lost in humans, perhaps because they need to be more careful in their choice of food before they chew it. Many of our own gustatory sensors live on just as battered remnants of once-useful structures.