The big news from the journal Science today is the discovery of the oldest human skeleton—a small-brained, 110-pound (50-kilogram) 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 35 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.

One of the defining attributes of Lucy and all other hominids—members of our evolutionary lineage, including ourselves—is that they walk upright on two legs. While Ardi also walked on two legs on the ground, the species also clambered about on four legs in the trees. Ardi thus offers a fascinating glimpse of an ape caught in the act of becoming human.

The problem is it is doing it in the wrong place at the wrong time—at least according to conventional wisdom, which says our kind first stood up on two legs when they moved out of the forest and onto open savanna grasslands. At the time Ardi lived, her environment was a woodland, much cooler and wetter than the desert there today.

So why did her species become bipedal while it was still living partly in the trees, especially since walking on two legs is a much less efficient way of getting about?

According to Owen Lovejoy of Kent State University, it all comes down to food, and sex.