It started with population. Not having a fixed breeding season was among the reasons why mankind achieved dominance; it kept our numbers topped up at an explosive rate. Past certain stage restrictive process set in: male libido is reduced or diverted into nonfertile channels, female ovulation is irregularized and sometimes fails completely. But long before we reach that point we find the company of our fellow creatures so unbearable we resort to war, or a tribal match. Kill one another or ourselves.