Why Did Most Male Lineages Disappear 7,000 Years Ago?

David Geary says most male lineages disappeared around 7,000 years ago because organized groups of related men killed the men in the groups they expanded into. After early agriculture, male lineage diversity collapsed. At one point the record shows 17 female relatives for every one male relative, a drop he places at about 5,000 to 7,000 years ago, in various places across the world. The signal is on the Y chromosome, so it has to be male kin groups. Related men in an organized group carry the same or very similar Y chromosomes. When they expand and the other group's men are killed, the other Y signature is erased. Women were usually kept. The archaeology varies, and sometimes women were killed too, but if anyone is removed, he says, it is the men.

Disease or famine cannot explain a drop that hits only men. Disease and famine would have hit women about as hard. A pattern specific to men, he says, is other men killing. For almost all of human history the male-female split stayed relatively even, and then the sex ratio changed. Reproductive power sat with the few men still there. Among primates, when males compete hard, males are bigger and more aggressive than females, and the more dominant males reproduce out of proportion to their numbers.