Death by Agriculture
Modern humans have experienced a number of severe bottlenecks and occasionally long lasting population bottlenecks in our prehistory, with numbers reduced to the low thousands or even less. The most recent of these occurred four to eight thousand years ago and is peculiar in that it seems to have affected only males. The result is that we are descended from many fewer males than females.
A plausible reason for this restriction was the invention of agriculture and property. A farm has limited divisibility, and consequently is usually inherited by only one child - a son. Other sons have to go off to war or some other occupation unlikely to leave progeny. Note that this property goes hand in hand with polygamy. Modern polygamous societies also result in excess males. In the case of the polygamous Mormon offshoots, most of the young males are typically driven off while the old guys "marry" all the girls. Something similar happens in some Muslim groups, giving rise to large numbers of young men with little hope of marriage, and every incentive for warlike activity.
64% of European Men are Descended from just Three Bronze Age Warlords between 3,500 to 7,300 years ago