The expansion happened because farmed lands soon became overcrowded. Sons and daughters wanted to own their own slice of the world, to master it as their parents had. This was easily achieved. The farmers’ knowledge was not tied to a particular patch of land, as the hunter-gatherers’ had been. Their thinking was systematic: They knew how to transform the land to make it the way they wanted it — any piece of land. They did not have to accept it as it was. For farmers, colonization was easy.

And so, from the first humble scratched farms in the east of Anatolia, the great expansion began. It was a kind of slow war, waged on the Earth itself, as it was transformed to suit the needs of the growing crowd of human bellies. It became an expansion that would soon outstrip geographically the diffusion of Homo erectus and earlier generations of humans, an expansion that would proceed with astonishing speed.

But the expansion did not occur into a vacuum, but into land already occupied by the ancient hunter-gatherer communities.

It was not possible to share, of course. This was a conflict between two fundamentally different views of the land. The hunters saw their land as a place to which they were attached, like the trees that grew from it. To the farmers, it was a resource to own, to buy, sell, subdivide: Land was property, not a place. There could be only one outcome. The hunter-gatherers were simply outnumbered: Ten malnourished, runtish farmers could always overcome one healthy hunter.

After three days’ traveling, they reached a kind of shantytown, a rough huddle of shelters and lean-tos. Juna peered around, tense, uninterested. “Why have we come here? We should move on before it grows dark—”

Keram placed a kindly hand on her arm. “I thought you would want to stop here. Juna, don’t you recognize this place?”

“You should,” came a woman’s voice, oddly familiar.

Juna turned around. A woman was limping toward her, an ancient piece of skin thrown over her head. Juna’s mind whirled. The words had been strange, yes — because they were in Juna’s birth language, a tongue she had not heard since the day she had followed Cahl out of her village.

Now Juna could see the woman’s face. It was Sion, her older sister. An unidentifiable longing came rushing back. “Oh, Sion—” She stepped forward, arms outstretched.

But Sion drew back. “No! Keep away.” She grimaced. “The sickness did not murder me, as it murdered so many others, but I may carry it yet.”

“Sion. Who—”

“Who died?” Sion barked a bitter laugh. “It would be better for you to ask who survived.”

Juna glanced around. “And is this truly where we lived? Nothing is the same.”

Sion snorted. “The men drink beer and mead. The women labor in the farms of Keer. Nobody hunts now, Juna. The animals have been driven off to make room for fields. We get by. Sometimes we sing the old songs for the farmers. They give us more beer.”

“Who is shaman now?”

“Shamans are not allowed. The last of them drank himself to death, the fat fool.” She shrugged. “It makes no difference. Nothing the shaman could tell us would help us now. It is not the shaman who knows how the wheat grows, nobody but the farmers, and their masters from the city with their bits of string and narrow eyes that peer at the sky.”

The disease, as it happened, had been measles.

Mankind had always been prey to some diseases, of course: leprosy, yaws, and yellow fever were among the most ancient blights. Many of them were caused by microbes that would maintain themselves in the soil, or in animal populations — as yellow fever was carried by African monkeys. But people had had time, evolutionary time, to adapt to most such diseases and parasites.

With the coming of the new, dense communities had come new plagues — crowd diseases, like measles, rubella, smallpox, and influenza. Unlike the older illnesses, the microbes responsible for these diseases could only survive in the bodies of living people. Such diseases could not have evolved in humans until there were sufficiently dense and mobile crowds to allow them to spread.

But, if they infected crowds, they must have come from crowds. And so they had: crowds of animals, the heavily social herd creatures people now lived close to, animals in which the diseases had long been endemic. Tuberculosis, measles, and smallpox crossed to humans from cattle, influenza from pigs, malaria from birds. Meanwhile, with the building of grain stores, the vectors of infectious diseases — rats and mice and fleas and bugs — reached populations of unprecedented density. Still, those who survived developed resistance of some kind, though some of these mechanisms were clumsy, with damaging side effects. The mechanisms of adaptation operated too slowly, compared to the frenzied rate of change of human culture, to iron out the deficiencies.

But the hunter-gatherers at the farms’ expanding borders had no resistance. They were devastated, even as their lands were overwhelmed by their farmer neighbors.

This transition, from the old way of living to the new, was a crucial moment in human history. A mass, unconscious choice was being made between limiting population growth to match the resources available, as the hunter-gatherers of the past had done, or trying to increase food production to feed a growing population. And once that choice had been made, the farmers’ expansion could only accelerate. Henceforth the folk following the older ways would survive only in the most marginal environments, the fringes of deserts, the mountain peaks, the densest jungles — places the farmers could not tame.

It would happen in Africa, where Bantu farmers equipped with iron weapons would spread out of the western Sahara, overwhelming peoples like the pygmies and the Khoisan — ancestors to Joan Useb — at last marching all the way to the east coast of South Africa. It happened in China, where farmers from the north, aided by China’s interconnected geography, would march south to repopulate and homogenize much of tropical southeast Asia, driving existing populations ahead of them in secondary invasions that hit Thailand and Burma.

And the great east-west span of Eurasia proved especially conducive to expansion. Farmers spread easily along lines of latitude, moving into places with a similar climate and length of day to their origin, and so suitable for their crops and beasts. With their cattle and goats and pigs and sheep, their highly productive wheat and barley, and their swelling numbers, the descendants of the farmers of Cata Huuk would build a mighty dominion of wheat and rice. The pyramids of Egypt would be built by workers fed by crops whose ancestors had been native to southwest Asia. They would take their Indo-European language with them, but it would splinter, mutate, and proliferate, generating Latin, German, Sanskrit, Hindi, Russian, Welsh, English, Spanish, French, Gaelic. At last they would colonize a huge east-west band stretching from the Atlantic coast to Turkestan, from Scandinavia to North Africa. One day they would even cross the oceans, in boats of wood and iron.

All across this immense span of cultivated land cities would burgeon, and empires would flourish and decay, like mushrooms. And everywhere the farmers went they carried the great diseases with them, a vicious froth on a tide of language, culture, and war.

Juna said impulsively, “Sister, come with us.”

Sion glanced at Keram and Muti, and laughed. “That will not be possible.” With an expression of anguish, she peered at Juna’s children, who slept in the arms of Muti and Keram. Then she whispered, “Good-bye,” and hurried back to the huts.

Juna made to call good-bye after her, but, she thought, that would be the last word I will ever speak in my own tongue. For I will never come back here. Never.

So, without speaking, she turned her face away and, with her children, resumed her steadfast walk to the west, and the new city on the coast.