Giving birth is more difficult - and dangerous - for modern humans than for any other primate. Not only do human mothers have to push out babies with unusually big heads, but infants also have to rotate to fit their heads through the narrow birth canal. Now, a new virtual reconstruction of the pelvis of a Neanderthal woman suggests that Neanderthal mothers also had a tough time giving birth to their big-headed infants - but the babies, at least, didn't have to rotate to get out.
     Paleoanthropologist Timothy Weaver of the University of California, Davis, thought the shift to this more complicated rotational birth predated the split between modern humans and Neanderthals. That's because Neanderthals, which lived until 30,000 years ago in Europe, also had big heads and, presumably, used the same evolutionary strategy to deliver their big-brained babies. But it has been difficult to test this idea. The only known female pelvis of a Neanderthal, discovered in Tabun (Israel), is fragmentary. Collaborating with Jean-Jacques Hublin at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, Weaver got permission to make computed tomography-scans of the pelvis. The two researchers were able to refit the pieces of the pubis, ischium, and ilium together in a three-dimensional, virtual reconstruction. They also used landmarks on the pelvic fragments to compare the pelvis to those of modern humans - and to predict the size and shape of the missing pieces, such as the sacrum and dimensions of the pelvic outlet.
     The reconstruction suggests that the pelvis of the Tabun Neanderthal was widest from side to side all the way down the birth canal, more like that of Homo erectus or australopithecines than modern humans. And that means that although Neanderthal mothers still had difficult births because of their babies' large heads, their babies did not rotate in the womb, the team reported in the Proceedings of the National Academy of Sciences. So why would our ancestors evolve such a complicated birth in the first place? Other research shows that they had to balance pressures to adapt to the hot climate in equatorial Africa - and tall, slender-hipped humans thermoregulate in the heat better than short, stocky humans (whose physiology retains heat better in the frigid latitudes). By evolving a birth canal that is wide front to back, our ancestors were able to accommodate both narrower pelvises and the delivery of big-brained babies, suggests Weaver.
     But it will take more than a virtual pelvis to convince other researchers. "I don't know if I believe the reconstruction," says paleoanthropologist Karen Rosenberg of the University of Delaware, Newark. She and others have questions about the accuracy of the reconstruction of the missing parts of the pelvis, which are critical for proving there was no rotation. "Given the poor preservation of the Tabun pelvis, ...this is a bold claim," says anthropologist Marcia Ponce de León of the University of Zurich in Switzerland. She does agree, however, with one conclusion: "Birth was equally difficult in Neanderthals as in modern humans," with or without a twist.

Source: ScienceNOW (20 April 2009)