Long time ago, long before the Arabs showed up, long before the Europeans arrived, African societies were incredibly nonviolent. Tribes settled disputes without going to war. Then something happened. The upshot was that Africans fell to warring with each other. What happened?
A clue came, I think, from the experience of an African-American woman who wanted more than anything to learn the identity of her tribe before her ancestors were sold into slavery and shipped to the American colonies.
So when the service became available to the public, she paid to have her DNA taken and traced. While this couldn't necessarily nail down a specific tribe, it could give her an idea of the region in Africa her ancestors had originally come from.
Her experience, which was filmed for a TV program, could be summed in the old warning, "Be careful what you wish for."
The DNA test results, once analyzed by an ethnologist specializing in African tribes, showed that her ancestors came from so many African regions that she had to face the awful inference about life in Africa prior to the arrival of the foreign slave traders. It was surely one of constant raids by tribes on each other to capture people and take them as slaves.
The realization devastated the woman, who'd idealized life in long-ago Africa.
If I could talk to that woman today, I might persuade her to take a more phlegmatic view of human nature's failings.
I remember a French woman who'd worked most of her life in Ivory Coast telling me 40 years ago that on rare occasions when a man and woman from different tribes fell in love and wanted to marry, everyone in both tribes said it wouldn't work -- and made so much trouble for the prospective couple that rarely did the relationship progress to marriage.
I think severe inbreeding is what happened. With no war, and with reasonably prosperous societies that lived a settled existence away from trade routes, tribes tended to avoid intermarriage because there was no need for it. But severe inbreeding produces many offspring that have severe mental and physical handicaps.
The connection between these afflictions and inbreeding probably wasn't noticed at first, but as the instances piled up, and as tribes shared horror stories about the physical and mental degeneration amongst their people, someone would have put two and two together.
However, just because intermarriage was frowned on, some genius got the idea of stealing fresh breeding stock from another tribe. The rest would have been blood-soaked history.
Just a guess, and it's possible scientists who specialize in such research have already made this guess. But if it's in the ballpark think of what it implies. The terrible deal for humanity is that sooner or later one must sacrifice paradise to stay strong.