Go back, over a hundred thousand years, to the time of the last interglacial. Imagine an alien starship designer, a R'il'nian, stranded on Earth when his latest design proves to have some unexpected flaws. He befriends a group of primates, strikingly similar in appearance to himself, and eventually becomes part of their tribe, mating with their women and, totally to his surprise, having a few children. A minority even inherit his lack of aging and esper abilities. 

Over the next few thousand years his genes spread, and he guides the Humans that result into a short-lived civilization capable of building starships to his plans. He himself and some of his descendants return to the stars; others choose to stay on Earth, where their force-grown civilization is soon forgotten as they spread from Africa over the planet.

Those who followed Jarn to the stars create their own civilizations, with some help and protection from Jarn's R'il'nian relatives and led by those of Jarn's descendants expressing most strongly his abilities of conditional precognition and telepathy. The telepathy is important in protecting the Human planets from the Maungs, whose life cycle is totally incompatible with that of Humans.

Now fast-forward to the time when Humans on Earth are rediscovering agriculture. A new disease has arisen among the star-faring Humans. Kharfun syndrome is not serious for them—similar to a mild case of flu. But it is deadly to those leaders with a high fraction of R'il'nian genes, and lethal to the pure R'il'nai. A cure and a method of immunization are found, by Humans and R'il'nai working together, but only a small fraction of the R'il'nai, and almost none of the crossbred R'il'noids, survive. The Human planets are threatened both by the Maungs and by their tendency to war on each other. The surviving R'il'nai are in decline, their already low fertility driven even lower by the aftereffects of the disease and by the immunization, which has the same effect. Eventually a few of the Human leaders petition the R'il'nai to try hybridizing again with the Humans, in hope of producing individuals capable of detecting Maung infestations and stopping wars—duties the R'il'nai have become too few in numbers to continue. 

Thus the Confederation is born. The pure R'il'nai continue to decline, but their place as peacekeepers and guardians against the Maungs is taken by the new hybrids, the R'il'noids. 

The natural increase of the R'il'noids is insufficient to keep up with the growing number of Human-occupied planets in the Confederation. Shortly after the Saxons invade Britain on Earth a R'il'noid, Çeren, develops a laboratory method of increasing the number of hybrids produced. He also develops a measurement, the Çeren index, which is used to rank R'il'noids according to the fraction of active R'il'nian genes.