During the last weeks of pregnancy, many women become so large in the middle that they look as though they might tip over. But through evolutionary processes, nature has devised a way to keep them upright on their sometimes swollen feet.
In a new study, US researchers show how the lower spine in females has evolved to support the obstetrical load experienced by species that stand upright on two feet, so that the centre of balance is repositioned over the hips.
"Most previous studies on the evolution of how human mothers have accommodated pregnancy have focused on the pelvis and the problems of birthing; this is the first study to look at how mothers cope with the considerable challenges of stabilisation while they are pregnant," senior author Dr Daniel E Lieberman, from Harvard University in Cambridge, Massachusetts, told Reuters Health.
The normal curvature of the lower spine helps position the trunk above the hips in humans, thereby stabilising the upper body over the legs, according to the report in the issue of Nature. Pregnancy, however, complicates matters as the weight shifts forward.
In their study, Lieberman's team shows how over time the lower vertebrae in human females have become reinforced to allow the exaggerated curved position of the spine assumed by pregnant women when they are standing upright. This position allows the trunk's center mass to remain above the hips.
By studying fossils of Australopithecus, the researchers found that these vertebral changes actually preceded the evolution of Homo sapiens.
"For me, the biggest surprise was that we can see this maternal adaptation in the fossil record so far back," Lieberman said. "It makes sense that evolution would have favoured mothers who were better at coping with these demands, but I didn't expect the fossil record would be good enough to yield such evidence."
In future studies, he added, "I think we need to look more at the costs of this adaptation and how it relates to the back problems that so many of us regularly experience."
SOURCE: Nature, December 13, 2007.