At length Capo reached the ground, fur bristling, panting hard. He was on the edge of a small clearing centered on a clogged, marshy pond. He still wasn’t done with his display. He threw himself back and forth around the line of trees, drumming with open palms on tree trunks, ripping off thin branches and shaking them so their leaves cascaded around him, screeching and hooting the while.

Finger had picked himself up from where he had plummeted. Limping slightly, he crawled into the shade of a low palm and cowered away from his master’s display. Other males hopped and hooted in backside-kissing support. One or two females were already up. They kept out of Capo’s way, but otherwise maintained their own morning routines.

As he finished his display, Capo spotted Howl, a female with a peculiarly high-pitched way of calling. She was crouching at the base of an acacia, picking lumps off a morel and popping them into her mouth. Howl was not yet pubescent, but she wasn’t far off. Spying the tight pucker of her genitals, Capo immediately hardened.

Still bristling, panting a little, he swaggered over to Howl, lifted her hips, and smoothly entered her. Her passage was pleasingly tight, and Capo’s supporters hooted and growled, drumming on the ground, urging on their hero. Howl did not resist, adjusting her posture to accommodate him. But as he thrust, she continued to pluck bits of morel, not much interested.

Capo withdrew from Howl before he ejaculated: too early in the day for that. But as a coup de grâce he turned his back on his cowering subordinates, bent over, and ejected a spray of shit that showered over them. Then he threw himself flat on the grass, arms akimbo, and allowed a few of his more favored subordinates to come close and begin the day’s grooming.

Thus the great boss, the capo di capo of this troop — the progenitor of mankind, the ancestor of Socrates and Newton and Napoleon — had begun his day in suitable splendor.

The next priority was to fill his belly.

Capo selected one of his subordinates — Frond, a tall, sinewy, nervous creature — and, hooting loudly, delivered a series of slaps and blows to the cowering creature’s head.

Frond quickly got the message. His assignment was to lead the troop’s daily forage in search of food and water. He selected a direction — east, as it happened, into the light of the rising sun — and, his gait a mixture of clumsy knuckle-walking and upright sprinting, he ran back and forth along a trail that led that way, glancing back at Capo for approval.

Capo had no reason not to choose that direction. With a swagger, his big knuckles punching into the soft ground, he set out after Frond. The rest of the troop quickly formed up behind him, males and females alike, infants clinging to their mothers’ bellies.

The troop worked its way through the trees at the forest fringe, foraging systematically, after their fashion. Mostly they sought fruit, though they were prepared to take insects and even meat if it was available. The males noisily postured and competed, but the females moved more calmly. The smallest infants stayed with their mothers, though older youngsters rolled and wrestled.

As they worked their endless way through the forest, the friendships of the females quietly endured. The truth of Capo’s society was that the females were its foundation. The females stuck to their kinship groups and shared the food they found — a practice that made good genetic sense, as your aunt and nieces and sisters shared your own heritage. As for the males, they just went where the females went, their dominance battles a kind of showy superstructure, signifying very little of true importance for the troop.

With a moist dick and pleasantly aching fists, and the prospect of a belly soon to be filled, Capo ought to have been as happy as he could be. Life was good, here in the forest. For Capo, top of the heap, it could hardly get any better. But still that bit of unease lingered.

Unfortunately for Capo’s mood, the pickings that morning were poor. They were forced to keep moving.

They came across other animals, here in the forest. There were okapi — short-necked giraffes — and pygmy hippos and dwarf forest proboscideans. It was an ancient fauna clinging to the conservatism of forest ways. And there were other primates too. They passed a pair of giants: huge, broad-shouldered, silver-haired creatures who sat massively on the ground, feeding on the leaves they plucked from the trees.

They were like the potbellies of Roamer’s day. Capo’s forebears had developed a new kind of teeth, the better to cope with their fruit diet: Capo had large incisors for biting, necessary for fruit, whereas his molars were small. These leaf eaters’ teeth were the other way around; leaves didn’t need much biting but took a lot of chewing. Closely related to the gigantopithecines of Asia, these great beasts, weighing a quarter of a ton each, were among the largest primates who would ever live. But the giants were rare in Africa now.

They were not in direct competition with Capo’s troop, who, lacking the giants’ immense multiple fermenting stomachs, could not feed on leaves. Still, it bothered Capo to have to divert his course to avoid these silent, patient, statuesque creatures. Not wishing to lose face, Capo knuckle-walked up to the larger of the giants — a male — and displayed, fur bristling, running in circles, drumming on the ground. The leaf eater watched, impassive and incurious. Even sitting down he towered over Capo.

Honor satisfied, Capo skirted the giants and moved on.

It wasn’t long before the morning march came to an end, as the troop ran out of trees.

Here was the root of Capo’s unease. This shrinking, half-flooded patch of forest was not as abundant a home as it used to be. It was just an island, in fact, in a greater, more open world.

Peering out of the trees, he glimpsed that world, still emerging from a misty dawn.

This scrap of forest lay in the palm of an extensive, glimmering plain. The land was like a park, a mix of open green plains and patches of forest. Much of the forest was palms and acacias, but there was some mixed woodland, both conifers and deciduous trees — walnut, oak, elm, birch, juniper.

What would most have surprised Roamer, Capo’s distant great-aunt, was the nature of the ground cover that stretched over those open green areas. It was grass: hardy, resistant, now spreading with slow, unheralded triumph across the world.

And on the plain there were many, many lakes, ponds, marshes. Mist rose everywhere, the sun’s early heat filling the air with moisture. A great river, having spilled from the southern highland, curled lazily over the plain. Around its banks stretched extensive floodplains, some of them marshy or sheets of open water. The land was like a full sponge, brimming with water. Some of the trees were dying, their roots in some cases actually standing in shallow water. The forest remnants, already shrunken by the world’s continuing cooling and drying, were being drowned.

This soggy plain stretched to the north as far as Capo’s eyes could see. But off to the south the land climbed to an immense wall notched by the outflow of that mighty river. Before that great ridge was a more barren area littered with wide, bone-white sheets of salt, on some of which stood small, stagnant-looking lakes.

There was a bellow from the north, and Capo turned back that way. The animals of the plain were going about their business. In the distance Capo could see what looked like a herd of wild, overgrown pigs rooting in the long grasses. Their low-slung gray-brown bodies made them look like huge slugs. They were not pigs or hippos; they were anthracotheres, a holdover from much more ancient times.

Two huge chalicotheres worked their way slowly across the plain, plucking at shrubs with their huge paws. They picked only fresh shoots, and put them into their mouths, delicate as pandas. The taller, the male, was nearly three meters high at the shoulder. They had bulky bodies and stocky hind legs, but their forelegs were long and surprisingly graceful. But, because of their long claws, they could not put their front feet on the ground, and walked on their knuckles. In their bodies they looked a little like huge, short-haired gorillas, but they had long equine heads. These ancient animals were cousins of the horses. Once they had been widespread, but now the shrubs on which they depended were becoming scarce; this species was the last of the chalicothere kind.