Walking upright and using our hands for other things is a tremendous advantage over four-legged life. But for the female of the species, it compromises carrying a baby.

In four-legged animals, the tummy hangs down, the spine is relatively straight, and balance isn't affected as the baby grows. Have you ever thought about how, in humans, the woman must lean back in order not to fall over as her abdomen enlarges? Women who struggle with back pain in pregnancy sure have!

New research published in the scientific journal Nature sheds some light on how the spine has adapted to the challenges of vertical life. In human males two backbones allow the spine to arch back, while females can arch across three spine segments.

And in females these bones are shaped to permit even more extension, with larger surfaces and a wedge-like shape, allowing more lordosis, or arching. This same adaptation was found in female fossils of Australopithecus, our biped ancestor from two millions years ago. Males don't have these special spinal bones, nor do females from species that walk on all fours.

These skeletal adaptations take some stress off the muscles of the back, as the woman leans back to compensate for her growing abdomen. If you picture the life of humans when we lived in caves, getting food and escaping predators were the most important activities.

So we evolved some adaptations to walking upright that allowed us to move quickly and comfortably when pregnant. You may think it is hard to move at the end of pregnancy, and find it hard to imagine living the life of a cavewoman while carrying a nine-month fetus, but the back pain and difficulty moving that we feel as we get big would be worse without this amazing spinal adaptation.