Evolution

The cooling was not a uniform trend. The planet tipped and bobbed in its endless dance around the sun, subtly shifting its degree of tilt, its inclination, and the fine-tuning of its orbit. And with each cycle the ice came and went, came and went; ocean levels fluctuated like the pumping of a heart. Even the land, compressed under kilometers of ice or released by its melting, rose and fell like a rocky tide.

Sometimes the climate shifts could be savage. Within a single year the amount of snowfall in an area could double, the average temperature fall by ten degrees. Faced with such chaotic oscillations, living things moved, or died.

Even the forests marched. Spruce proved a fast migrant, followed by pine, capable of marching at a kilometer every two years. The great chestnuts, massive trees with heavy seeds, could manage a pace of a hundred meters a year. Before the Ice Age the animals of the middle latitudes of the northern hemisphere had been a rich mixture of fleet grazers like deer and horses, giant herbivores like rhinos, and fast-running carnivores like lions and wolves. Now the animals were driven south in search of warmth. Populations of animals from different climatic zones were mixed up and forced to compete in fast-changing ecological arenas.

But some creatures began to adapt to the cold, to exploit the food supplies that still existed at the feet of the ice sheets. Many animals grew thick fur and layers of fat — large animals, like rhinos, and smaller animals, like foxes and horses and cats. Others began to take advantage of the huge temperature swings between the seasons. They migrated, moving north in the summer and south in the winter; the plains became a huge tidal wash of life, great mobile communities patiently stalked by predators.

There had been a catastrophe of mixing in the Americas. The two continents, north and south, had been separated since the shattering of Pangaea some one hundred and fifty million years ago. The fauna of South America had evolved in isolation, and was dominated by marsupial mammals and ungulates. There were marsupial "wolves" and saber-toothed "cats"; there were ungulate "camels" and trunked "elephants," and giant ground sloths that could weigh three tons and stood six meters tall when they stood up to browse on palm leaves. There were still glyptodonts, not so dissimilar from the huge armored beast that had terrified Roamer, and the top predators were giant flightless birds, just as in archaic times. This exotic assemblage had been left alone to develop — though it was supplemented from time to time by waifs, brought by rafting or temporary bridges, like Roamer and her hapless companions, whose children had populated the South American jungles with monkeys.

But when the Panama land bridge was closed there was a massive migration from north to south of insectivores, rabbits, squirrels, mice, and later dogs, bears, weasels, and cats. The natives of South America failed to compete with these new arrivals. The extinctions took millions of years, but the empire of the marsupials was done.

For all the difficulty and dying, this time of fast and savage changes was, perversely, a time of opportunity. In the entire four-billion-year history of the Earth there had been few times more propitious for diversification and evolutionary innovation. Amid much extinction there was wild speciation.

And right at the center of this ecological cauldron were the children of Capo.

The next morning dawned brightly, with a washed-out blue sky. But the air was very dry and smelled oddly sharp, and the heat was soon stifling. The animals of the savannah seemed subdued. Even the birds were quiet; the carrion eaters clung to their tree roosts like ugly black fruit.

With their bare, sweating skin, the people were as well equipped for this hot, open dryness as any other species here. But they too began their day listlessly. They milled about their island of rock, picking at what was left of yesterday’s food.

This wasn’t a particularly rich area. The people didn’t discuss their plans — they never did, and anyhow they had no real plans — but it was obvious they shouldn’t stay here. Before long some of the men started to set off toward the water course to continue their walk to the south.

But the Brat’s condition had worsened overnight. The soles of his feet were cracked and oozing a watery pus, and when he tried to put his weight on them he cried out in pain. He wouldn’t be walking anywhere today.

Calm, Far’s grandmother, and most of the other women stayed close to the Brat. As for the men, the women just ignored their antics as they impatiently paced up and down the trail they had begun toward the south.