As the natural systems of the planet broke down, humans would discover conclusively that they were still… — 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.

In the last centuries of the empire, educational standards and literacy had fallen. In the dulled heads of the masses, distracted by cheap food and the barbaric spectacles of the coliseums, the values on which Rome had been founded and the ancient rationalism of the Greeks had been replaced by mysticism and superstition. It was—Honorius had explained to his pupil—as if a whole culture was losing its mind. People were forgetting how to think, and soon they would forget they had forgotten. And, to Honorius’s thinking, Christianity only exacerbated that problem.

“You know, Augustine warned us that belief in the old myths was fading—even a century and a half ago, as the dogma of the Christians took root. And with the loss of the myths, so vanishes the learning of a thousand years, which are codified in those myths, and the monolithic dogmas of the Church will snuff out rational inquiry for ten more centuries. The light is fading, Athalric.”

For the genes it made sense, of course. Otherwise it would not have happened.