It is 3-4 million years ago. Our ancestors are down from trees and are standing on their feet by now. It is a sunny, pleasant day. A male human ancestor is cautiously scoping the savannah for that big cat he saw the other day, but instead, he sees a female human ancestor. His loins catch fire, he starts running after her like a madman. She notices and tries to escape, but he tackles her and throws her to the ground. But he knows, he knows there are innumerable dangers all around him. That big cat he was on the lookout for could materialize at any moment and eat him before he sows his seed. Perhaps a larger and stronger male ancestor will spot them, crack his skull and inseminate the female himself. This, that and the other...danger is everywhere. So, the original male ancestor instinctively knows he has to be as quick as he can and empty his semen as fast as he can. The faster he can come, literally the higher his chances of survival -- survival in the present and survival into the future through projection of his genes, which, I thus posit, was an evolutionary advantage. Relevant tangent: I also feel this is the reason why there are so many men who ejaculate very fast today. We have been programmed for millions of years to do just that.