Our larger-than-average buttocks.

Because having big asses enabled us to walk fully upright, which in turn left our hands free, which enabled us to pick up sticks and rocks and pummel our neighbors with them, which then in turn led to the development of more complex brains so that our neighbors could come up with a good way of pummelling us back and then we could come up with a better way of pummelling them again . . . and then keep doing that for a few thousand years 'til we get to nuclear and chemical weaponry, all because humankind's great big backsides enabled our ancestors to stand up and thwock their neighbors over the head with a tree branch.

The ass, fo shizzle.