an ape. At that point, ten million years before the present, there probably lived at least two species of ape in Africa, though there may have been more. One was the ancestor of the gorilla, the other the common ancestor of the chimpanzee and the human being. The gorilla’s ancestor had probably taken to the montane forests of a string of central African volcanoes, cutting itself off from the genes of other apes. Some time over the next five million years the other species gave rise to two different descendant species in the split that led to human beings and to chimpanzees.
The reason we know this is that the story is written in the genes. As recently as 1950 the great anatomist J. Z. Young could write that it was still not certain whether human beings descended from a common ancestor with apes, or from an entirely different group of primates separated from the ape lineage more than sixty million years ago. Others still thought the orang utan might prove our closest cousin.2 Yet we now know not only that chimpanzees separated from the human line after gorillas did, but that the chimp–human split occurred not much more than ten, possibly even less than five, million years ago. The rate at which genes randomly accumulate spelling changes gives a firm indication of relationships between species. The spelling differences between gorilla and chimp are greater than the spelling differences between chimp and human being – in every gene, protein sequence or random stretch of DNA that you care to look at. At its most prosaic this means that a hybrid of human and chimpanzee DNA separates into its constituent strands at a higher temperature than do hybrids of chimp and gorilla DNA, or of gorilla and human DNA.
Calibrating the molecular clock to give an actual date in years is much more difficult. Because apes are long-lived and breed at a comparatively advanced age, their molecular clocks tick rather slowly (the spelling mistakes are picked up mostly at the moment of replication, at the creation of an egg or sperm). But it is not clear exactly how much to correct the clock for this factor; nor do all genes agree. Some stretches of DNA seem to imply an ancient split between chimps and human beings; others, such as the mitochondria, suggest a more recent date. The generally accepted range is five to ten million years.3
Apart from the fusion of chromosome 2, visible differences between chimp and human chromosomes are few and tiny. In thirteen chromosomes no visible differences of any kind exist. If you select at random any ‘paragraph’ in the chimp genome and compare it with the comparable ‘paragraph’ in the human genome, you will find very few ‘letters’ are different: on average, less than two in every hundred. We are, to a ninety-eight per cent approximation, chimpanzees, and they are, with ninety-eight per cent confidence limits, human beings. If that does not dent your self-esteem, consider that chimpanzees are only ninety-seven per cent gorillas; and humans are also ninety-seven per cent gorillas. In other words we are more chimpanzee-like than gorillas are.
How can this be? The differences between me and a chimp are immense. It is hairier, it has a different shaped head, a different shaped body, different limbs, makes different noises. There is nothing about chimpanzees that looks ninety-eight per cent like me. Oh really? Compared with what? If you took two Plasticene models of a mouse and tried to turn one into a chimpanzee, the other into a human being, most of the changes you would make would be the same. If you took two Plasticene amoebae and turned one into a chimpanzee, the other into a human being, almost all the changes you would make would be the same. Both would need thirty-two teeth, five fingers, two eyes, four limbs and a liver. Both would need hair, dry skin, a spinal column and three little bones in the middle ear. From the perspective of an amoeba, or for that matter a fertilised egg, chimps and human beings are ninety-eight per cent the same. There is no bone in the chimpanzee body that I do not share. There is no known chemical in the chimpanzee brain that cannot be found in the human brain. There is no known part of the immune system, the digestive system, the vascular system, the lymph system or the nervous system that we have and chimpanzees do not, or vice versa.
There is not even a brain lobe in the chimpanzee brain that we do not share. In a last, desperate defence of his species against the theory of descent from the apes, the Victorian anatomist Sir Richard Owen once claimed that the hippocampus minor was a brain lobe unique to human brains, so it must be the seat of the soul and the proof of divine creation. He could not find the hippocampus minor in the freshly pickled brains of gorillas brought back from the Congo by the adventurer Paul du Chaillu. Thomas Henry Huxley furiously responded that the hippocampus minor was there in ape brains. ‘No, it wasn’t’, said Owen. ‘Was, too’, said Huxley. Briefly, in 1861, the ‘hippocampus question’ was all the rage in Victorian London and found itself satirised in Punch and Charles Kingsley’s novel The water babies. Huxley’s point – of which there are loud modern echoes – was more than just anatomy:4 ‘It is not I who seek to base Man’s dignity upon his great toe, or insinuate that we are lost if an Ape has a hippocampus minor. On the contrary, I have done my best to sweep away this vanity.’ Huxley, by the way, was right.
After all, it is less than 300,000 human generations since the common ancestor of both species lived in central Africa. If you held hands with your mother, and she held hands with hers, and she with hers, the line would stretch only from New York to Washington before you were holding hands with the ‘missing link’ – the common ancestor with chimpanzees. Five million years is a long time, but evolution works not in years but in generations. Bacteria can pack in that many generations in just twenty-five years.
What did the missing link look like? By scratching back through the fossil record of direct human ancestors, scientists are getting remarkably close to knowing. The closest they have come is probably a little ape-man skeleton called Ardipithecus from just over four million years ago. Although a few scientists have speculated that Ardipithecus predates the missing link, it seems unlikely: the creature had a pelvis designed chiefly for upright walking; to modify that back to the gorilla-like pelvis design in the chimpanzee’s lineage would have been drastically improbable. We need to find a fossil several million years older to be sure we are looking at a common ancestor of us and chimps. But we can guess, from Ardipithecus, what the missing link looked like: its brain was probably smaller than a modern chimp’s. Its body was at least as agile on two legs as a modern chimp’s. Its diet, too, was probably like a modern chimp’s: mostly fruit and vegetation. Males were considerably bigger than females. It is hard, from the perspective of human beings, not to think of the missing link as more chimp-like than human-like. Chimps might disagree, of course, but none the less it seems as if our lineage has seen grosser changes than theirs.
Like every ape that had ever lived, the missing link was probably a forest creature: a model, modern, Pliocene ape at home among the trees. At some point, its population became split in half. We know this because the separation of two parts of a population is often the event that sparks speciation: the two daughter populations gradually diverge in genetic make-up. Perhaps it was a mountain range, or a river (the Congo river today divides the chimpanzee from its sister species, the bonobo), or the creation of the western Rift Valley itself about five million years ago that caused the split, leaving human ancestors on the dry, eastern side. The French paleontologist Yves Coppens has called this latter theory ‘East Side Story’. Perhaps, and the theories are getting more far-fetched now, it was the newly formed Sahara desert that isolated our ancestor in North Africa, while the chimp’s ancestor remained to the south. Perhaps the sudden flooding, five million years ago, of the then-dry Mediterranean basin by a gigantic marine cataract at Gibraltar, a cataract one thousand times the volume of Niagara, suddenly isolated a small population of missing links on some large Mediterranean island, where they took to a life of wading in the water after fish and shellfish. This ‘aquatic hypothesis’ has all sorts of things going for it except hard evidence.
Whatever the mechanism, we can guess that our ancestors were a small, isolated band, while those of the chimpanzees were the main race. We can guess this because we know from the genes that human beings went through a much tighter genetic bottleneck (i.e., a small population size) than chimpanzees ever did: there is much less random variability in the human genome than the
