Closer to hand, the apes could hear a steady, noisy rustling. Hesitant, they peered out. A family of a kind of elephant was working at the trees at the forest clump’s edge, using their trunks to pull away branches and cram foliage into their mouths. These were gomphotheres, massive creatures. Each had four tusks, a pair protruding from both upper and lower jaws, giving its face the look of a forklift.

This was the heyday of the proboscideans. The very successful elephantine body plan had spun off a whole range of species across the world. In North America the mastodons would survive until humans arrived. Another family was the shovel-tuskers like these gomphotheres, with their hugely expanded and flattened lower tusks. And, walking through Africa and southern Asia, there were the stegodons, with long, straight tusks. They were the ancestors of the true elephants and the mammoths, who had yet to appear.

The sound of the gomphotheres’ calls, carrying far in the cold morning air and echoing deep into the infrasonic, was eerie. These particular proboscideans were omnivorous. They were scarcely fleet-footed hunters. But on the whole a meat-eating elephant was best avoided.

That was when Frond, the spindly male, unexpectedly knuckle-walked out of the forest’s shade and into grass tall enough to come up to his shoulders. The grass waved around him, stirred by a breeze, languid waves crossing the empty acres.

Hesitantly Frond got to his hind legs. For a heartbeat he stood upright, peering out into a world beyond the primates’ reach, out into the green emptiness where animals walked, the antelopes, elephants, and chalicotheres grazing the abundant grass.

Then he dropped back to all fours and scuttled back into the forest’s shadows, his nerve gone.

Capo gave him a sound beating about the head for taking such a risk. Then he led his troop back into the deeper forest.

Capo hauled himself up an acacia tree, seeking fruit and flowers. Capo climbed steadily. He used a kind of shimmying style, pulling himself up with his arms while gripping the tree trunk with his feet to provide a platform.

It was a feat Roamer could not have achieved — or indeed any monkey. Capo’s apes had flat chests, short legs, and long arms. They had achieved greater flexibility by moving their shoulder blades to the backs of their bodies, which enabled Capo to reach up above his head. All this was equipment for hauling oneself up a tree trunk. Where Roamer had spent much of her life running along branches, Capo was a climber.

And this re-engineering for climbing had had another side effect, easily visible in Capo’s long, narrow body. Working vertically, with a new bone structure and system of balance, Capo was already preadapted to walking on two feet. Sometimes he did this in the trees, holding on to branches for balance, trying to reach the highest fruit — and sometimes his kind would stand up out in the open, as Frond had demonstrated.

As their bodies had been redesigned, the apes had become smarter.

In these tropical climes fruit trees rarely fruited simultaneously. Even when you found a fruiting tree, you might have a long way to travel to the next. So the apes needed to spend much of each day searching for patchy resources, foraging alone or in small groups, collecting together again to sleep in the treetop refuges. This basic architecture of food gathering had shaped their social lives. For one thing they needed to understand their environment very well if they were to find the food they needed.

And, given the way they lived their lives, their bonding was loose. They could split and recombine, forming special relationships with other members of the community, even though they might not see them for weeks at a time. Keeping track of a multileveled, fissile social complexity required increasing smartness. As the apes juggled their relationships, it was as if they were living through a soap opera — but it was a social maelstrom that honed their developing minds.

In the first years after the great split of the archaic anthropoid stock into apes and monkeys, the apes had become the Old World’s dominant primates. Though shrinking climate belts restricted them to the middle latitudes, there was plenty of room for them in a continuous band of forest that had spanned the whole of Africa and stretched across Eurasia from China to Spain. Following this green corridor the apes had walked out of Africa and spread through the Old World forests. In fact, they had migrated alongside the proboscideans.

At their peak there were more than sixty ape species. They had ranged from cat-sized to the size of a young elephant. The largest, like the giants, were leaf eaters, the midsized — those the size of Capo — took fruit, but the smallest, weighing under a kilogram or so, were insectivores, like their remote ancestors. The smaller the animal, the faster its metabolism and the higher the quality of the food it demanded. But there was room for everybody. It had been an age of apes, a mighty anthropoid empire.

Sadly for them it hadn’t lasted.

As the world continued to cool and dry, the great forest belts had shriveled into isolated islands, like this one. The vanishing of forest connections between Africa and Eurasia had isolated the Asian ape populations, which would develop independently of events in Africa, into the orangutan and its relatives. With the reduced ranges had come a dwindling of numbers. Most ape species had, in fact, already long gone extinct.

And then had come the rise of a new competitor.

Capo reached a clump of foliage where, he knew, this particular acacia had an especially productive patch of flowers. But he found the spiny branches already stripped. When he pried them aside he was met by a small, startled black face, fringed by white fur and a gray topknot. It was a monkey — like a vervet — and juice dribbled from its small mouth. It peered into Capo’s eyes, squealed, and shot out of sight before he could do anything about it.

Capo rested for a while, scratching his cheek thoughtfully.

Monkeys were a pest. Their great advantage was that they were able to eat unripe fruit. Their bodies manufactured an enzyme to neutralize the toxic chemicals used by the trees to protect their fruit until their seeds were ready to germinate. The apes could not match this. So the monkeys were able to strip the trees before the apes even arrived. They were even moving out into the grasslands, feeding off the nutlike seeds to be found there. To the apes, the monkeys were as tough a competition as the rodents had always been.

High over Capo’s head, a slim form moved, swinging gracefully and purposefully. It was a gibbon. It raced through its forest canopy at extraordinary speed. It used its body as a pendulum to gain momentum, and, like a child on a fairground swing, it pumped its legs up and down to build up its speed.

The gibbon’s body was a kind of extreme version of the apes’ long-armed, flat-chested design. The ball-and-socket joints in its shoulders and wrists had been freed up so that the gibbon could hang from its arms and twist its body through a full circle. With its low weight and extreme flexibility, the gibbon could hang from the outermost branches of the highest trees, and it was able to reach the fruits that grew at the end of the thinnest branches, safe from even tree-climbing predators. And, able to hang upside down from branches, it could reach goodies out of the grasp of other apes, who were too heavy to climb so high, and even the monkeys, who ran along the tops of the branches.

Capo peered up at the gibbon with a kind of envy for a grace, speed, and skill he could not match. But, magnificent though it was, the gibbon was not a triumph for the apes but a relic, forced by the competition it had lost to the monkeys to eke out its living on the ecological margins.