Men almost wiped out the human race with their tendency to get into fights and kill one another, a new study has suggested.
Between 7000 and 5000 years ago there was a genetic "bottleneck", in which there was only about one man capable of reproducing for every 17 or so women, the Stanford University researchers say.
Previously, the lack of diversity was put down to the "founder effect" - small numbers of individuals keeping new settlements alive as they spread across the prehistoric world.
But the new study has a much more brutal hypothesis its authors call "patrilineal kin group competition" - in other words, groups of men competing for women's attention fought so much, there were few of them left over to repopulate the Earth.
"The presence of such groups results in violent intergroup competition preferentially taking place between members of male descent groups, instead of between unrelated individuals," the study claims.
"Casualties from intergroup competition then tend to cluster among related males and group extinction is effectively the extinction of lineages."
In plain English, when a group of men - often related - lost a battle, they were usually wiped out completely, and their women would be seized by the victors.
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The researchers say this is just a theory. An alternative hypothesis could be that a disease spread around the world, wiping men out or affecting the male-female birth ratio.
However, such effects are small and cannot account for the 1:17 disparity between male and female population sizes, they say.
The study was published in journal Nature.