Stanford researchers recently shared an interesting theory about a catastrophic drop in male genetic diversity due to in fighting for control of clans. The article starts:
“Starting about 7,000 years ago, something weird seems to have happened to men: Over the next two millennia, recent studies suggest, their genetic diversity –specifically, the diversity of their Y chromosomes – collapsed. So extreme was that collapse that it was as if there were only one man left to mate for every 17 women.
“Anthropologists and biologists were perplexed, but Stanford researchers now believe they’ve found a simple – if revealing – explanation. The collapse, they argue, was the result of generations of war between patrilineal clans, whose membership is determined by male ancestors.”
https://news.stanford.edu/press-releases/2018/05/30/war-clan-structubiological-event/.
thanks to Soror Amy for the tip!
That could be apart of it.
Another would be that some men attained vast amounts of wealth and established harems / polygamous marriages and forced most of the other men into involuntary celibacy, castrated their male slaves, etc