First author

Studies of human evolution have rarely paid attention to the selective forces associated with pregnancy. Past consideration of the skeleton has focused solely on the size and shape of the pelvic opening and its importance in birthing. On page 1075, Katherine Whitcome of Harvard University and her colleagues detail the surprising adaptations found in the human spine — past and present — that allow females to better endure the expanding and shifting pregnancy weight.

What led you to study the role of pregnancy in evolution?

Pregnancy is an obvious selective pressure in evolution. Now that we have a better understanding of how pelvis size and shape affects reproductive success, we've broadened our remit to look at other anatomical features. Women must be able to deliver a baby, but also to carry one as it grows.

How did you seize on the importance of the vertebral column?

In thinking about the upper-body load generated on the vertebral column, I realized that historically most females would have had an additional load — having spent much of their adult lives either pregnant or nursing. So we looked at vertebrae, and found previously unidentified female-specific traits — for example, three wedge-shaped vertebrae in women (compared with only two in men) form a longer curve that better reinforces the lower back. In addition, we found that the joint that buttresses the lower vertebrae is larger, relative to vertebrae size, in women.

Had nobody noticed these male–female vertebrae differences before?

No. The differences are very subtle. If I showed you vertebrae from a woman and a man, it would be hard to distinguish between them, in part because the effect is cumulative over a number of vertebrae. Unless you are using calipers to measure the slight wedging, it is not obvious, but it is significant and biomechanically relevant.

Was this extra lumbar support in women important to the evolution of Homo sapiens?

The big surprise is that these adaptations seem to be fundamental to our bipedal existence. The changes make sense: the challenges faced by modern women during pregnancy, such as balance and stability, were as important, or possibly more so, in early humans and their predecessors, because many — Australopithecines for instance — spent hours foraging, travelling and avoiding predators. Their ability to stay mobile was necessary for survival.