Among all the primates, we humans are the only ones that stand on two legs. You may have seen other animals stand on two legs briefly, such as a cat or a dog. But they always return to their original posture. Even apes, who have arms and hands much like humans, still put them down to walk.

How did it come about that we do every day what nobody else does? Well, one theory says that standing up is the result of reaching up.

Think of it this way. You may have seen a cat go up on its hind legs in order to pull food down off a table. Suppose those same conditions remained in place for a hundred thousand generations. The only food was always on tables, and only accessible to cats who could reach for it. In time, the cats who couldn't manage that trick would die out. The ones who could manage it would survive and reproduce. A cycle begins.

It is thought that this kind of scenario may have led the predecessors to human beings to go from being four-footed to being upright creatures. In the forests and savannas where our species got its start, being able to reach up and pluck fruit trees was a good thing. We see chimpanzees do this even today, standing temporarily on two feet while grabbing food with their hands.

Given a few million years, these conditions may have given rise to an animal that can stand up all the time, not just briefly. That animal is us.

So, what comes next? Next time.