Genome: The Autobiography of a Species in 23 Chapters. Matt Ridley
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 chimp genome.5
So let us picture this isolated group of animals on an island, real or virtual. Becoming inbred, flirting with extinction, exposed to the forces of the genetic founder effect (by which small populations can have large genetic changes thanks to chance), this little band of apes shares a large mutation: two of their chromosomes have become fused. Henceforth they can breed only with their own kind, even when the ‘island’ rejoins the ‘mainland’. Hybrids between them and their mainland cousins are infertile. (I’m guessing again – but scientists show remarkably little curiosity about the reproductive isolation of our species: can we breed with chimps or not?)
By now other startling changes have begun to come about. The shape of the skeleton has changed to allow an upright posture and a bipedal method of walking, which is well suited to long distances in even terrain; the knuckle-walking of other apes is better suited to shorter distances over rougher terrain. The skin has changed, too. It is becoming less hairy and, unusually for an ape, it sweats profusely in the heat. These features, together with a mat of hair to shade the head and a radiator-shunt of veins in the scalp, suggest that our ancestors were no longer in a cloudy and shaded forest; they were walking in the open, in the hot equatorial sun.6
Speculate as much as you like about the ecology that selected such a dramatic change in our ancestral skeleton. Few suggestions can be ruled out or in. But by far the most plausible cause of these changes is the isolation of our ancestors in a relatively dry, open grassland environment. The habitat had come to us, not vice versa: in many parts of Africa the savannah replaced the forest about this time. Some time later, about 3.6 million years ago, on freshly wetted volcanic ash recently blown from the Sadiman volcano in what is now Tanzania, three hominids walked purposefully from south to north, the larger one in the lead, the middle-sized one stepping in the leader’s footsteps and the small one, striding out to keep up, just a little to the left of the others. After a while, they paused and turned to the west briefly, then walked on, as upright as you or me. The Laetoli fossilised footprints tell as plain a tale of our ancestors’ upright walking as we could wish for.
Yet we still know too little. Were the Laetoli ape-people a male, a female and a child or a male and two females? What did they eat? What habitat did they prefer? Eastern Africa was certainly growing drier as the Rift Valley interrupted the circulation of moist winds from the west, but that does not mean they sought dry places. Indeed, our need for water, our tendency to sweat, our peculiar adaptation to a diet rich in the oils and fats of fish and other factors (even our love of beaches and water sports) hint at something of an aquatic preference. We are really rather good at swimming. Were we at first to be found in riverine forests or at the edges of lakes?
In due time, human beings would turn dramatically carnivorous. A whole new species of ape-man, indeed several species, would appear before that, descendants of Laetoli-like creatures, but not ancestors of people, and probably dedicated vegetarians. They are called the robust australopithecines. The genes cannot help us here, because the robusts were dead ends. Just as we would never have known about