And this was a time of success for the horses. The descendants of the timid creatures of Noth’s forest world had diversified into many woodland browser types — some of them as large as gazelles, but with tougher teeth than their ancestors had had to take leaves rather than soft fruit — along with longer-legged plains animals slowly adapting to a diet of grass. Most of the horses now had three toes on both their front and back feet, but some plains-living runners were starting to lose their side toes, and were putting all their weight on their central toes. But as the forests shrank this diversity was already falling; soon many of the forest species would disappear. The rodents, too, were diversifying, with the appearance of the first gophers, beavers, dormice and hamsters, a great diversity of squirrels — and the first rats.

But the new conditions were not kind to the primates. Their natural habitat, the tropical forests, had shriveled back to the southern tropics. Many of the primate families had gone extinct. Fruit eaters like Roamer lingered only in the tropical woodlands of Africa and southern Asia, clinging to the year-long food supply these forests still provided. By the time Roamer was born there were no primates left north of the tropics, and — since the rise of the rodents — none in the Americas at all: not a single species.

But that was soon to change.

The sea around Roamer was a sheet of gunmetal gray across which waves rippled, languid as mercury. Roamer was in an utterly baffling place: a sketchy, elemental two-dimensional environment, static yet full of mysterious churning motion, that could not have been more different from the forest.

She felt nervous climbing around on top of the vegetation. She expected some ferocious aerial predator to bite into her skull at any moment. And as she moved she could feel the uneasy raft shift under her, its loosely tangled components rustling with the slow breathing of the sea. It felt as if the whole thing might disintegrate at any moment.

There were just six anthros: three males, two females — including Roamer — and the infant who still clung sleepily to the fur of its mother. These were the only survivors of Whiteblood’s troop.

The anthros sat on a tangle of branches, eyeing one another. It was time to form provisional hierarchies.

For the two females the priorities were clear enough.

The other female, the mother, was a burly individual more than a decade old. This child was her fourth and — though she could not know it — now her only surviving offspring. Her most noticeable characteristic was a fur-free patch of scar tissue on one shoulder where she had once been burned in a forest fire. The infant, clinging to Patch’s chest, was tiny, small even for its age, just a scrap of fur. Patch, the mother, studied Roamer dismissively. Roamer was small, young, and a stranger, not even remote kin. And, as a nursing mother, Patch would always have priority. So she turned her broad back on Roamer and began to stroke her infant, Scrap.

Roamer knew what she had to do. She scuttled over the branches to Patch, and dug her fingers into fur that was still moist and began to comb out tangles and bits of debris. When she probed at Patch’s skin, she found knots of muscle, and places which made Patch wince to be touched.

As Roamer’s strong fingers worked, Patch relaxed slowly. Patch, like all of them, had been battered by her precipitate removal from the forest, and was stressed by her sudden dumping into this extraordinary emptiness and the loss of her family. It was as if she could, for a moment, under the magic of the other’s touch, forget where she was. Even the infant, Scrap, seemed soothed by the contact between the two females.

Roamer herself was calmed by the simple, repetitive actions of the grooming, and by the subtle social bond she was building up with Patch.

The males’ negotiations were more dramatic.

Whiteblood found himself facing two younger males, brothers, in fact. One had a peculiar crest of snow-white hair that stuck up around his eyes, making him look permanently surprised, and the other had a habit of using his left arm predominantly over his right, so much so that the muscles on his left side were much more heavily developed than those on the right, like those of a left-handed tennis player.

Both Crest and Left were smaller and weaker than Whiteblood, and, younger, they had not outranked him back in the forest. But now Whiteblood had lost all of his allies, and together these two might defeat him.

So, without hesitation, he launched into a display. He stood upright, shakily, hooted and shrieked, and threw handfuls of leaves. Then he turned around, spread his backside and blew shit through moist fur.

Left was immediately intimidated. He shrank back, arms folded around himself.

Crest was more defiant, and answered Whiteblood’s display with a shrieking tantrum of his own. But he was outsized by Whiteblood and, without the support of his brother, could not hope to best the older male. When Whiteblood began to cuff him about the head and neck, Crest quickly backed down, tumbling onto his back and spreading his arms and legs like an infant, showing his submission. All of this was halted only when an incautious stamp plunged Whiteblood’s leg through the foliage and into the cold water. He yelped, pulled back his leg, and sat with legs folded beneath him, subdued.

But he had done enough. The brothers approached him now, their heads bent and postures humble. A brief interval of frantic mutual grooming ensured the new hierarchy was reinforced, and the three males started to pick bits of shit out of each other’s fur.

The rough-and-ready communities of Noth had been like street gangs, held together by not much more than brute force and dominance, with each individual aware of little more than her own place in the pecking order. But by now the advantages of social living had driven primate societies to baroque intricacy, and had spurred the development of new types of mind.

Group living required a lot of social knowledge: knowing who was doing what to whom, how your own actions fit in with this, who you had to groom and when, to make your life easier. The larger the group, the greater the number of relationships you had to keep track of, and as those relationships changed constantly, you needed still more computational capacity to handle it all. By allowing their group living to develop to such extremes of complexity, primates continued to get relentlessly smarter.

Not all primates, though.

Through all this the big potbelly had sat on the comfortable branch she had found, methodically stripping it of leaves. She had no interest in the peculiar displays and hairy fiddling of the anthros.

Even among her own kind the potbelly knew little of the society of others. She ignored other females and let herself be bothered by males only when she felt the urge to mate — which, in fact, was on her now. When they were in season anthros like Patch and Roamer showed sexual swellings on their rumps. That would have been of little use to a creature who spent most of her time sitting on her backside, so on the potbelly’s chest pinkish blisters had swollen brightly in an unmistakable hourglass shape. But as there was no male potbelly around, nobody was doing anything about it.

Not that the potbelly cared much. She didn’t understand where she was and what had become of her any more than the anthros did, but it didn’t trouble her. She could see there were plenty of leaves on this fallen tree to last her through the day. She had no real idea that there could be such a thing as a tomorrow different from today, that it might not find her in an endless forest full of nutritious leaves.

Already the anthros were starting to feel hungry; their low-nutrition diet worked through their systems quickly. They broke up their grooming circles and spread out over the branches of the fallen mango. The tree had lost much of its fruit, along with most of its inhabitants, when it fell from the bank. But Crest, one of the brothers, quickly turned up a cluster of fruit that had gotten lodged in an angle of branch and trunk. He hooted to summon the others.