I think that someone pointed it out, but a regular culling of the flock was probably an end result of the constant warfare that humanity seems to have written into it's DNA...
The less fit, able, agile and generally intelligent were almost certain to die first in the battles and the smarter would be able to adapt to ensure that they were safe from risk of death from the fight...
Those weaker physically would be dispatched very quickly and as long as the women were protected for later reproduction, the surviving men would allow for a replenishment of warriors...
