But as the climate oscillated, the sea level rose again, and Atlantic waters broke through the Gibraltar barrier. Now, the ocean was refilling. But Capo had nothing to fear of giant waves cascading from the west, for even a thousand Niagaras could not refill an ocean overnight. The Gibraltar waters suffused the great basin more gradually, creating great rivers. The old seafloor turned slowly into sodden marshland, where the vegetation slowly died, before the waters rose so high they covered over the ground altogether.

But after each refilling the global ocean levels would drop again, and once again the Mediterranean would evaporate. This would happen as many as fifteen times over the million years bracketing Capo’s brief life. The Mediterranean would be left with a complex seabed geology, with layers of silt sandwiching salt pans laid down in the successive dryings.

But this trapped ocean’s dryings were having a profound effect on the area Capo lived in — and on Capo’s kind. Before the great dryings, the Sahara region had been densely forested and well watered, and home to many species of apes. But with the climatic pump of the dryings, and in the lengthening rain shadow cast by the more remote Himalayas, the Sahara was becoming increasingly arid. The old forests were breaking up. And with them the communities of apes were splintering, each fragmentary population embarking on its own journey to a new evolutionary destiny — or extinction.

But the great rumbling, the blurred vision of Gibraltar, was too remote to have any meaning for Capo. He turned away, and stumbled down onto the plain.

At last Capo moved off bare rock on to vegetation. He relished the green softness of the grass under his knuckles as he loped forward. As the others tumbled after him they rolled and sprawled, pulling up the long grass around them, relishing the delicious contrast with the hard lifeless rock.

But they weren’t home yet. A stretch of a few hundred meters of open savannah, studded with thorn bushes, separated them from the nearest forest clump — and the plain was not unoccupied.

A group of hyenas worked at a fallen carcass. Bulky, round, it might have been an infant gomphothere, perhaps felled by a chasma. The hyenas snapped and growled at each other as they worked at the scavenged meat, their heads buried in the creature’s stomach, their sleek bodies writhing industriously.

As Capo cowered in the grass, Frond and Finger came up alongside him. They hooted softly, and gave Capo’s backside a perfunctory groom, picking out bits of dust and rock. The younger males were cursorily acknowledging his authority. But Capo could tell they were impatient. Weary, thirsty, hungry, thoroughly spooked by the trek across the openness, they, like the rest of the troop, longed to reach the shelter and provision of the trees. And that was corroding Capo’s hold on them. The tension between the three males was powerful, toxic.

But it was a confrontation conducted in near silence, as the three of them kept their presence concealed from the hyenas.

While Capo still hesitated, it was Frond who made the move. He took one, two tentative shuffles forward. He received a hefty clout on the back of his head from Capo for his defiance. But Frond just bared his teeth, and moved out of reach.

The tall grass stems waved languorously at Frond’s passing, as if he were swimming through a sea of vegetation. And now Frond got up on to his hind legs, poking his head, shoulders, and upper torso out of the grass so he could see better. He was a slim shadow, upright, like a sapling.

The hyenas were still intent on their baby elephant. Frond ducked back into the grass and continued on his way.

At last he reached the nearest stand of trees. Capo, with a mixture of resentment and relief, saw him climb up a tall palm tree, his legs and arms working in synchrony, like components of a smoothly oiled machine. When Frond had reached the top of the palm he hooted softly, calling the others. Then he began plucking nuts from the palm and throwing them down to the ground.

One by one, led by Finger and the senior female, Leaf, the apes scurried through the grass toward the forest pocket.

They were not troubled by the hyenas, though many of the scavengers scented the vulnerable apes. They were fortunate that in the bloody calculations of the hyenas’ small minds, the lure of the immediately available meat outweighed the attraction of attacking these dusty, ragged-looking primates.

Capo tried to make the best of it. He slapped and punched the other males as they loped along, as if the whole thing had been his idea, as if he were directing them in their short migration. The males submitted to his blows, but he sensed a tension about them, a subtle lack of deference that made him uneasy.

On entering the forest, the apes fanned out.

Capo pushed through a bank of slim young trees to find a marshy lake: flat green-blue water surrounded by the comforting green and brown of forest. He hurried down to the water’s edge, pushed his muzzle into the cool liquid, and began to drink.

As the apes reached the water, some of them waded into it, walking upright until they were waist deep. They used their fingers to strain blue-green algae from the water and gobbled it down: a way of feeding that was another little gift of bipedalism. Several youngsters dove headlong into the water and started scraping the accumulated dust out of their fur; they made a terrific hooting and splashing. A flock of birds had been drifting in peace at the heart of the lake, but now they took fright, and clattered thunderously into the sky.

But some of the younger males had gathered together at the water’s edge, Frond and Finger among them. Frond had found a cobble that might serve as a hammer-stone; he was toying with it experimentally. And every now and then the males cast sly glances toward Capo. Their body language was redolent of conspiracy.

Capo pursed his lips and blew a soft raspberry.

He was very smart at working through social problems. He knew what the younger males were thinking. He had brought them to safety, but that wasn’t good enough: his performance as they had crossed that last grassy barrier had not convinced anyone. To restore his authority he was going to have to do some impressive displaying. He could rip down some branches and start stalking around the water’s edge, for instance; the foliage, the water, and the light would make for a powerful show. And then there would be hard battles to be won.

But perhaps now wasn’t the time.

He watched mothers gently bathing their infants, younger males wrestling almost politely as their limbs and skin recovered from the heat and aridity of the salt pan. Later. Let them get over the trek, before business as usual was resumed.

And besides, truth be told, he didn’t feel up to a great new war right now. His limbs ached, his skin was sore and covered in scrapes and lesions, and his gut, used to a continual flow of food and water, rumbled at the stop-start treatment it had endured. He was tired. He rubbed his eyes and yawned, allowing himself an explosive belch. Time enough later for the hard work of life, of being Capo. For now he needed to rest.

With that excuse lodged in his mind, he turned away from the water and loped into the forest.

He quickly found a kapok tree filled with large ripe fruits. The kapok, though, was armed with long sharp thorns to defend its fruit. So he tore two smooth branches from the tree and placed one under each foot, gripping the branches with his toes. Then, clinging to the branches with his feet, he climbed the tree, marching over the thorns as if they didn’t exist. The action of climbing made his limbs glow with the accustomed pleasure, their ancient design fulfilled; if he never took another step on the ground in his life he would have been content.