“And when those barriers to isolation are removed, the new species finds itself in competition with its predecessors. If it is better adapted to the prevailing conditions, it will survive by out-competing the parent stock. If not, it declines.

“When our scientists believed there was only one Earth, they suspected the evolution of humanity had been the consequence of a number of speciation steps. The ape-like bipedal Australopithecines gave rise to tool users, who in turn produced erect hairless creatures capable of walking on the open plain, who in turn gave rise to various species of Homo sapiens — the family that includes myself. It is believed that at some points in history there were many hominid species, all derived from the base Australopithecine stock, living together on the Earth. But my kind — Homo sapiens sapiens — proved the fittest of them all. By out-competition, the variant species were removed.

“Presumably, each speciation episode was instigated by the isolation of a group of the parent stock. We had generally assumed that the key isolating events were caused by climate changes: rising or falling sea levels, the birth or death of forests, the coming and going of glaciation. It was a plausible picture. Before we knew of the Red Moon.”

“And now your radical hypothesis—”

Nemoto tapped her papers. “What if the vagaries of the Red Moon were involved, in all this? Look here. This central column sketches the history of the Earth.”

“Your Earth.”

Nemoto smiled, her small naked face pinched. “Assume that the base Australopithecine stock evolved on Earth. Imagine that the Red Moon with its blue Wheel portals scooped up handfuls of undifferentiated Australopithecines and, perhaps some generations later, deposited them on a variety of subtly different Earths.”

“It is hard to imagine a more complete isolation.”

“Yes. And the environments in which they were placed might have had no resemblance to those from which they were taken. In that case our Australopithecines would have had to adapt or die. Perhaps one group was stranded on a world of savannah and open desert—”

“Ah. You are suggesting that the hairless, long-legged Runners might have evolved on such a world.”

“Homo erectus — yes. Other worlds produced different results. And later, the Red Moon returned and swept up samples of those new populations, and handed them on to other Earths — or perhaps returned them where they had come from, to compete with the parent stock, successfully or otherwise.

“My species shares a comparatively recent common ancestor with creatures like the Hams — which are of the type we call Neandertals, I think. Perhaps a group of that ancestral stock was taken to the world the Hams call the Grey World, where they evolved the robust form we see now. And, later, a sample of Hams was returned to the Earth. Later still, groups of Homo sapiens sapiens — that is, my kind — were swept here from the Earths of the groups called the English and the Zealots, and no doubt others.” She gazed at her diagrams. “Perhaps even my own kind evolved on some other Earth, and were brought back by the Moon in some ancient accident.”

Manekato picked her nose thoughtfully. “Very well. And my Earth — which you have labelled ‘Banded Earth’ ?”

Somewhat hesitantly, Nemoto said, “It seems that your Earth may have been seeded by Australopithecine stock from my Earth. You seem to have much in common, morphologically, with the robust variant of Australopithecines to be seen in the forests here, called Nutcrackers.”

Manekato lay back and sighed, her mind racing pleasurably. “You fear you have offended me by delegating my world to a mere off-shoot. You have not. And your scheme is consistent with the somewhat mysterious appearance of my forebears on Earth — my Earth.” She glanced at Nemoto’s sketches. “It is a promising suggestion. This strange Moon might prove to be the crucible of our evolution; certainly it is unlikely that hominid forms could not have evolved independently on so many diverse Earths. But such is the depth of time involved, and such is the complexity of the mixing achieved by our wandering Moon, the full picture is surely more complicated than your sketch — and it is hard to believe that your Earth just happens to be the primary home of the lineage… And how is it that so many of these other Earths share, not just hominid cousins, but a shared history, even shared languages? Your own divergence from the Zealot type must be quite ancient — their peculiar tails attest to that — and yet your history evidently shares much in common with them.”

Nemoto frowned, her small face comically serious. “That is a difficulty. Perhaps there is such a thing as historical convergence. Or perhaps the wandering of the Moon has induced mixing even in historical times. Cultural, linguistic transmission—”

It was a simplistic suggestion, but Manekato did not want to discourage her. “Perhaps. But the truth may be more subtle. Perhaps the manifold of universes is larger than you suppose. If it were arbitrarily large, then there would be an arbitrarily close match to any given universe.”

Nemoto puzzled through that. “Just as I would find my identical twin, in a large enough population of people.”

“That’s the idea. The closer the match you seek, the more unlikely it would be, and the larger the population of, umm, candidate twins you would need to search.”

“But the degree of convergence between, say, the Zealot universe and my own language, culture, even historical figures — is so unlikely that the manifold of possibilities would have to be very large indeed.”

“Infinite,” said Mane gently. “We must consider the possibility that the manifold of universes through which we wander is in fact infinite.”

Nemoto considered that for a while. Then she said, “But no matter how large the manifold, I still have to understand why this apparatus of a reality-wandering Moon should have been devised in the first place — and who by.”

Manekato studied Nemoto, wishing she could read the hominid’s small face better. “Why show me your schema now?”

“Because,” Nemoto said, “I believe all of this, this grand evolutionary saga, is now under threat.”

Manekato frowned. “Because of the failure of the world engines?”

“No,” Nemoto said. “Because of you. And Renemenagota of Rano.”

A shadow fell over Manekato’s face. “Your ape may be right, Mane. You should listen to it.”

It was Without-Name. She stepped forward, carelessly scattering Nemoto’s spidery diagrams.

Emma Stoney:

Emma lifted her head. “Hall-oo! Hall-oo!” Her call, though pitched higher than that of the men who mostly ventured outside the stockade, was, she was sure, a pretty accurate imitation of the soft cries of returning hunters.

Within a couple of minutes she heard an answering grunt, and the rattle of heavy wooden bolts being slid back.

All or nothing, she thought. Malenfant — or death.

When the heavy gate started to creak open, she yelled and threw herself at it. Her flimsy mass made no difference. But the Hams immediately copied her, making a sound like a car ramming a tree. The splintering gate was smashed back, and she heard a howl of pain.

The Hams surged forward. There were people in the compound, women and children. As three immense Hams came roaring in amongst them, they ran screaming.

Emma glanced around quickly. She saw a litter of crude adobe huts, that one substantial chapel-like building at the centre, a floor of dust stamped flat by feet and stained with dung and waste. She smelled shit, stale piss.

Now the door to one of the buildings flew open. Men boiled out, pulling on clothing. Inside the building’s smoky darkness Emma glimpsed naked Runner women, some of them wearing mockeries of dresses, others on beds and tables, on their backs or their bellies, legs splayed, scarred ankles strapped down.