Ask any woman who has given birth and she'll be happy to wax lyrical about the bad fit between the modern human pelvis and a baby's big head. Ask any anthropologist and he or she will also be happy to explain, in similar gory detail, that painful labor is the product of an evolutionary compromise to accommodate upright walking in a species that also has an oversized brain. That compromise came, anthropologists used to believe, about 2.4 million years ago when our already bipedal ancestor Homo habilis experienced a huge leap forward in brain size. But a recent announcement in the journal Science of a 1.2 million-year-old Homo erectus pelvis uncovered by University of Indiana paleoanthropologist Sileshi Semaw in the Afar region of Ethiopia in 2001 suggests that painful labor is a relatively modern affliction. The birth canal of that female Homo erectus is, in fact, 30 percent larger than that of the typical modern woman. As a result, Homo erectus birth might have been a relative walk in the park (or on the savanna) compared with today. Those ladies might have simply stopped, crouched down, and pushed. They might have screamed, but surely there was no need for Lamaze, or midwives, or Cesarean sections. The big news, for anthropologists anyway, is that painful labor is much more recent than anyone assumed. How did the anthropologists get this so wrong? Mostly, it was because they didn’t have much material to go on. The only full female pelvis around before this one belonged to Lucy, the Australopithecus afarensis dated at 3.2 million years ago. Lucy's pelvis was clearly made for upright walking, with squat splayed-out blades to accommodate muscles for balance, but the pelvic outlet was ovoid, rather than round. More important, Australopithecines hadn’t really experienced massive brain growth and so researchers have speculated that birth wasn't particularly difficult. But once human brain growth experienced a giant 20 percent leap forward with Homo habilis, ancient human females must have experienced difficult birth because the architecture of the pelvis was surely already constricted like the modern version. It would have had those short blades, a sacrum tipped inward to form a bowl that holds our guts, and the same bony canal that makes birth an obstacle course for babies and a nightmare for mothers. But the new pelvis puts all those assumptions in doubt. Everything below the neck has probably not been basically the same from Homo habilis onward. Instead, body size and proportions, and the shape of the female pelvis, was significantly different. If the current shape of the human pelvis is more recent, then something eventually made the human body taller, leaner, and with a more narrow birth canal. And we can only speculate what those selective forces might have been to bring about all that birth pain. Hopefully, natural selection is now through with this issue. Women have certainly done their bit for making big-brained little humans. If evolution opts for even bigger-brained kids, head growth will have to happen somewhere else than the crowded confines of the womb with its narrow, twisted tunnel that we all had to navigate.
Meredith F. Small is an anthropologist at Cornell University. She is also the author of "Our Babies, Ourselves; How Biology and Culture Shape the Way We Parent" (link) and "The Culture of Our Discontent; Beyond the Medical Model of Mental Illness" (link).