It's the reason humans do anything.

Did Early Humans Start Walking for Sex?

The big news in the journal Science tomorrow is the discovery of the oldest human skeleton—a small-brained, 110-pound female of the species Ardipithecus ramidus, nicknamed “Ardi.” She lived in what is now Ethiopia 4.4 million years ago, which makes her over a million years older than the famous “Lucy” fossil, found in the same region thirty-five years ago.

Buried among the slew of papers about the new find is one about the creature’s sex life. It makes fascinating reading, especially if you like learning why human females don’t know when they are ovulating, and men lack the clacker-sized testicles and bristly penises sported by chimpanzees.