Evolution

The new grass-eating herbivores developed specialized ruminant guts able to digest the grassy fodder over long periods and hence extract the maximum nutrient from it, and teeth able to withstand the abrasive effect of silica grains in grass blades. Many herbivores learned to migrate, because of the seasonality of the rainfall. These new mammals were larger than their archaic ancestors, lean and long-legged with specialized feet and a reduced number of toes to help them walk and run long distances and at speed. And meanwhile there was a sharp rise in the types of rodents, like voles and field mice, able to eat grass seeds.

New carnivores rose, too, equipped to feast on the new herds of large herbivores. But the rules of the ancient game had changed. In the sparse cover of a grassland, predators could see prey from long distances — and vice versa. So predators and prey began a metabolic arms race, with the emphasis on speed and endurance; they developed long legs and quick reactions.

A new kind of landscape began to spread — especially on the eastern side of the continents that were sheltered from the predominantly westerly winds and the rain they carried: open, grass-covered plains marked by scattered scraps of bush and woodland. And in turn animals who adapted to the new vegetation were rewarded with a guaranteed food source that could spread across hundreds of kilometers.

But their specializations, and the stability of the grasslands, would lock in the grazers to the grasses, the predators to their prey, establishing a close codependency. In this period the deer, cows, pigs, dogs, and rabbits looked little different from their equivalents of human times five million years later — although many of them would have looked surprisingly large; they would later be outcompeted by their smaller, faster cousins.

Meanwhile the opening up of land bridges, caused by the falling sea levels, led to a great crisscross migration of animals. Three kinds of elephants — high-browsing deinotheres, omnivorous gomphotheres, and browsing mastodonts — crossed from Africa to Asia. Along with them traveled the apes, cousins of Capo. And in the other direction came rodents and insectivores, cats, rhinos, mouse deer, pigs, and primitive types of giraffe and antelope.

There were some exotica, especially on the islands and the separated continents. In South America the largest rodents that ever lived were flourishing; there was a kind of guinea pig as large as a hippo. In Australia, the first kangaroos appeared. And what would later be considered tropical animals could be found in North America, Europe, and Asia: In England, the Thames was broad and swampy, and hippos and elephants basked on its floodplain. The world had cooled greatly since Noth’s time, but it still wasn’t cold; the deepest chill would afflict later ages.

But still the drying continued. Soon the older mosaic of grassland and woodland able to support a wide variety of animals lingered only in the equator-straddling Africa; elsewhere the grasslands opened up into arid plains, the savannah, steppe, and pampas. In these coarser, simplified conditions, many species fell away.

This intense evolutionary drama was driven by the endless shifts in Earth’s climate — and the animals and plants were as helpless as bits of flux on a great terrestrial forge.

The next morning there was no luxurious ball scratching. As soon as he woke, Capo sat up, hooted softly at the pain of yesterday’s lesions and bruises, and voided his bladder and bowels in a fast, efficient movement, ignoring the chitters of protest from below.

He vaulted from his nest and began to shimmy down the tree. Just as yesterday he roused the troop by crashing into their nests, hooting, kicking, and slapping. But today Capo wasn’t interested in displaying; this morning his purpose was not dominance but leadership.

His determination was still strong in his mind. The troop had to move. Where they should go wasn’t part of his unsophisticated decision making yet. But what was very clear in his head was the pressure of yesterday, his competition with Boulder, what he had sensed of the overcrowding of this little patch of forest.

The troop gathered together on the ground, more than forty of them, including infants clinging to their mothers’ bellies or backs. They were sleepy, wary, scratching themselves and stretching. No sooner had Capo gotten them gathered, of course, than they were drifting apart again, plucking at bits of grass and moss on the ground, reaching for low-growing figs and other fruit. Even among the males he saw reserve, rivalry, resentment; they might resist him just to make their own points in the endless plays for dominance. And as for the females, they were a law unto themselves, for all of Capo’s noise and violence.

How was he going to be able to lead this lot anywhere?