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, S P E C I E S 3 1
leaving human ancestors on the dry, eastern side. The French paleon-tologist 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 3 2 G E N O M E
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 our close cousinship with chimps if we could not read S P E C I E S 3 3
genes, so we would never have been aware of the existence of our many and closer australopithecine cousins if we had not found fossils (by 'we', I mean principally the Leakey family, Donald Johanson and others). Despite their robust name (which refers only to their heavy jaws), robust australopithecines were little creatures, smaller than chimps and stupider, but erect of posture and heavy of face: equipped with massive jaws supported by giant muscles. They were into chewing - probably grasses and other tough plants. They had lost their canine teeth the better to chew from side to side. Eventually, they became extinct, some time around a million years ago. We may never know much more about them. Perhaps we ate them.
After all, by then our ancestors were bigger animals, as big as modern people, maybe slightly bigger: strapping lads who would grow to nearly six foot, like the famous skeleton of the Nariokotome boy of 1.6 million years ago described by Alan Walker and Richard Leakey.7 They had begun to use stone tools as substitutes for tough teeth. Perfectly capable of killing and eating a defenceless robust australopithecine — in the animal world, cousins are not safe: lions kill leopards and wolves kill coyotes - these thugs had thick craniums and stone weapons (the two probably go together). Some competitive impulse was now marching the species towards its future explosive success, though nobody directed it - the brain just kept getting bigger and bigger. Some mathematical masochist has calculated that the brain was adding 150 million brain cells every hundred thousand years, the sort of useless statistic beloved of a tourist guide.
Big brains, meat eating, slow development, the 'neotenised' retention into adulthood of childhood characters (bare skin, small jaws and a domed cranium) - all these went together. Without the meat, the protein-hungry brain was an expensive luxury. Without the neotenised skull, there was no cranial space for the brain. Without the slow development, there was no time for learning to maximise the advantages of big brains.
Driving the whole process, perhaps, was sexual selection. Besides the changes to brains, another remarkable change was going on.
Females were getting big relative to males. Whereas in modern 3 4 G E N O M E
