Scientists believe that fighting as a result of periods of population booms helped humans evolve at such a rapid pace:

During the onset of an Ice Age or a drought, human populations would have contracted, driving the remaining small, isolated human groups to diversify – developing different biological traits from one another.

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When conditions improved again, populations that had developed different characteristics would have come into contact again. This might have increased the likelihood of competition between distinct groups, but also allowed genetic novelties to spread that would not have arisen without the initial fragmentation.

via Adam Orbit