On top of what other people said: humans as a selection pressure.

Lots of predators wait for the young,sick, small, or wounded to be separated from the herd. A pack of lions rarely goes for the head bull, because that would be more risk, and use up a lot of energy.

But humans, although if we're not physically good hunters, have great brains. So, instead of pouncing on the runt of the litter that fell behind, and basically living on scraps, we were able to use different strategies like ambush, tools, traps, fire, and advanced teamwork to pick off the larger members of a herd. For thousands of years, we've been able to go after the large, fat, alpha-mammoth instead of the end of the herd's babies.