In light of a (relatively) recently found African fossil, some scientists think that actual bipedalism - rather than the knuckle-walking of apes - may have happened because of sex.
The fossil, 4.4 million years old, was discovered, encased in rock, in the mid-1990s. After years of freeing and reconstructing the bones, it turns out that the creature had a weird combination of monkey and hominid traits. (Hominids, like the famous Lucy, have been thought to be the true ancestors of humans because they walked fully upright.)
But this new specimen, from a time well before Lucy or other hominids, had hips that would let it walk, hands free. It also has enough little bones in its hands to let it carry things as it walked. But its feet retained the out-pointed big toe of apes and monkeys, the better to climb trees, rather than the straight-ahead pointed toe that helps power our (toes-first) walking. It was a mix! (But don't think "missing link." That's a long-since discredited concept.)
Anyway, why would a creature adapt to carrying things and walking upright, at least in the hips, despite an ape-like big toe? Walking still must have been rather difficult.
In the latest issue of National Geographic, author Jamie Shreeve reports that other apes of the time (as now) had long, fierce canine teeth, adapted to fighting off other males to win breeding rights. But this new fossil had much smaller teeth. They didn't fight for females, the story goes, instead they carried food back to their females and children, thus ensuring (they hoped) sexual fidelity.
And that, after all, is how evolution works. Producing children - yours! - that survive. Perpetuating those genes! Ladies, your mothers were not only right, they were right back to 4.4 million years: Men are after "one thing." Even if they have to walk on feet with dumb big toes.