So the farmers train devoted bands of orchard guardians to risk their lives against the stone, copper and bronze of their attackers. And, eventually, the elders whose ancestors appointed these valiant defenders find them at their doorstep—with sticks and stones, and a particularly nasty sort of beaked mace fashioned like an eagle’s head. They say: We take the risks, we reap the rewards. We rule now. After a few generations, any village that isn’t run by a military aristocracy has gone the way of the dodo.

Now every surviving city state is a military dictatorship, where warriors are lionized and whose leaders are celebrated in song and story. The martial cities feed themselves on tales of how their ancestors were “destined,” and how they are not just more skilled at wielding the beaky mace, but superior. Everyone else is fit only to work the fields and clean the dung pits. Or, when you get right down to it, to be driven out of their villages by the bronze-mailed phalanxes of the warriors. It gets to the point where reputation steals a march on reality, but hearts and minds are half the battle. If your enemies keep running away, it’s easy to maintain a winning streak.

The leading city states can still be found in a vast fertile basin in Africa. They change over the centuries, but what doesn’t change is the creed. Every state is insistent that its bloodline is the purest, that diluting it with others would be heresy. Enough generations of this, and each city state’s inhabitants develop a distinct look, only accentuating the us and them.

The lemurs manage to go on for a surprisingly long time, despite the cultural stagnation and inevitable population shrinkage that their isolation brings. They fight, and one city or another becomes the big dog. Sometimes things become particularly fraught and one state goes on a genocidal spree against a neighbour. And always there are less of them, even though the world is milder now and the harsh times exist only in myths.

The real leveller is disease. Every generation sees plagues decimate one city state or another, laying such waste to the population that often there is nobody left to explain to passers-by that here were the chosen of the gods, the anointed bloodline that kept themselves pure. Disease approves of a restricted gene pool, which limits a population’s ability to resist infection. And when plague arrives, the lemurs sicken in their thousands, inbred and insular as they are.