It was even worth trying to cheat, as Scar-face had done. It didn’t work very often — women quickly learned they had to see the most impressive ax being made before them — but occasionally it paid off, and the liar got a chance to pass on his genes at very low cost.

This mixing up of toolmaking with sexual courtship would have a profound effect on the future. As no male could afford not to make axes just as his forefathers had done, it was a recipe for stultifying conservatism. These people would make the same tool to the same plan, over and over, across several continents, despite several glacial cycles, for a million years. Even the different species who followed them would continue with the same technology. It was a continuity and a consistency that no institution, no religion would ever match. Only sex had a strong enough hold on the human mind to have achieved such a vast freezing.

When he worked on his tools, Ax had to think, to some extent, like a human. Unlike the pithecine stone-slammer who would take whatever shape and size of flake his cobble offered him, Ax had to have an image of the final artifact in his mind. He had to select the raw materials and hammer-stones to match that vision, and he had to work systematically toward his goal. But his mind was divided as no human’s could be. Ax made his tools like a human, but he attracted mates like a peacock or a bower bird.

When Ax was done, he turned the tool he had made over and over in his hands, showing her its fine faces, its smoothly finished edge. It was magnificent, if impractical.

Far, brought up in a subtly different culture, had no clear idea of what he was doing — and she was just as baffled by Scar-face’s attempt to cheat. But she did sense Ax’s interest in her, and a warmth in her belly spread in response. And a more calculating corner of her mind was aware that if she mated with Ax, if she became pregnant, then she would become part of this group, and her future would be secured.

But she had never had sex, not with anybody. Longing, fearful, she sat there at the edge of the streambed, her legs still tucked against her chest. She didn’t know how to respond.

At length he dropped the beautiful ax, among so many others. Baffled, casting backward glances at her, he walked away.

Speciation — the emergence of a new species — was a rare event.

One species did not morph smoothly into another. Rather, speciation relied on a group of animals being isolated from the larger population and put under pressure to survive. The isolation could be physical — say, if a group of elephants was cut off by a flood — or it could be behavioral, if, for instance, one group of hominids that had adopted a particular way of scavenging was shunned by another group that hadn’t.

Variation was implicit in the genome of every species. It was as if every species, at any given moment, was contained in a field, fenced off by the habitable limits of its environment. Every viable variation would come into play, to fill up every available corner of the field. An isolated group was stuck in a fenced-off corner of the field. But perhaps a little of the outer fence came down, opening up a new and empty field, into which they began, slowly, to diffuse. More variation might be necessary to fill the newly available space — and if the necessary variation wasn’t available in the genome, perhaps it could be generated by mutation.

In the end, those who reached the furthest corner of the new range might have gone a great distance, genetically, from those who had remained in the old field. If the distance became too great for the old and new kinds to crossbreed, a new species was born. Later, when the isolating barriers came down, the evolved kind might interact with the parent type — perhaps to supplant them.

Some three hundred thousand years earlier, in another part of Africa, a group of nondescript forest-fringe pithecines had found themselves cut off from their home range by a lava flow, cast out of their forest once and for all.

There were many challenges to be met. The old pithecine habits of forest-fringe hunting had been a start, something to build on. But out on the savannah the food supply was very different from that in the forest. Whereas the forest had provided a steady supply of fruit, the main savannah food was meat. Meat was high-quality nutrition, but it came in packages scattered sparsely over an arid, inhospitable landscape, packages you had to be smart to spot, get hold of, and use. And stranded out on the savannah, away from the trees’ shelter, a new kind of body was needed to cope with the aridity and the heat, new kinds of behavior needed to extract the resources needed from the new environment — and to survive in predator hell.

Within a mere few dozen generations Far’s ancestors had adapted drastically.

The ancient primate body plan had been rebuilt, stretched tall almost to human proportions. Far’s body was much bulkier than that of the ancestral apes. She was twice as heavy as an adult gracile pithecine. That bulk was an adaptation for openness: a larger body was more efficient at storing water, a key advantage on a plain where there could be many hours’ walking between water sources.

And her metabolism had become efficient at creating and storing subcutaneous fat, for fat was a key fuel reserve. Ten kilograms of fat would be sufficient to see her through forty days without food, enough to ride out all but the most severe seasonal fluctuations. The fat had fleshed out her body, giving her swollen breasts, buttocks, and thighs, a much more human shape than the pithecines’ chimplike slackness. But Far was not a round ball; instead she was tall and thin, so that her body was also an efficient radiator of waste heat, and when the sun beat down from above, comparatively little of her skin was directly exposed to its radiation.

More heat adaptations: Apart from her head, with its grooming patch of hair, her skin was all but bare. And she sweated, unlike Capo, unlike any other ape outside her species family, for bare, sweating skin was a better temperature regulator than hair for creatures destined to spend their lives in open tropical sunlight. Sweating was a paradox, for it meant Far lost water. So she had to be smart enough to find water sources to make up for that, and, unlike some of the true savannah creatures, her kind would always be tied to some extent to water courses and the coasts.

The most apelike characteristics of the pithecines — their grasping feet, long arms, and stooping gait — had soon been abandoned. Far’s feet were best fit for running and walking, not climbing: her big toe was now a toe, not a thumb. But Far’s rib cage was a little high, her shoulders a little narrow: even now her body still carried with it traces of its vanishing adaptation to the trees — as would modern humans’, as would Joan Useb’s.

Meanwhile her brain had grown to more than three times the mass of a pithecine’s, the better to handle the puzzles of a difficult landscape and the intricacies of still more complex societies of large groups of savannah foragers. That big brain was very energy-hungry, but Far’s diet was much richer than any pithecine’s, with plenty of high-protein foods like meat and nuts, which in turn required greater intelligence to gather. Thus her smartness had been driven by a virtuous circle of development.

All these changes were drastic, and yet they had been achieved by an evolutionary strategy of remarkable economy. It had been heterochrony — different timing. Walker infants looked much as their more apelike ancestors had — as would human babies — with relatively large skulls, small faces and jaws. If you wanted to become Capo, you grew your jaw large and kept your brain relatively small. But Far’s brain had grown large while her jaw had stayed small. Even the much larger size of her body had been achieved by stretching out growing phases: her body had something like the relative dimensions of a fetal Capo, inflated to adult size.