It is of course a truism that all logically possible universes must exist. - Stephen Baxter

Relentless Pleistocene selection was shaping everything that would make up humanity. Even love was a by-product of evolution. Love, and the pain of loss.

After a time it became unbearable, the lesson blinding in its cruelty: but if the universe didn’t get you, other sentient beings would.

No," Morrow said. "I can't accept that. I don't always agree with the Planners. But they aren't killers."
"You think not?" Uvarov laughed again. "The survivalists—your 'Planners'—are psychotic. Of course. As I am. And you. We are a fundamentally flawed species. Most of humanity, for most of its history, has been driven by a series of mass psychotic delusions. The labels changed, but the nature of the delusions barely varied...