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 monopolises 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.