by Michael Balter on 7 May 2012, 3:20 PM| 2 Comments
It isn't easy being born. Human babies have big heads, which makes their passage through the birth canal a challenge for both them and their mothers. Fortunately, an infant's skull can change shape as it squeezes through because its cranial bones don't entirely fuse together for at least 2 years after birth. A new study shows that this delayed fusion was also a feature of early humans who lived nearly 3 million years ago, even though their heads were much smaller than ours. One possible explanation is that walking upright created new obstetrical challenges even for small-brained human ancestors.
For chimpanzees and other primates, childbirth is relatively easy. The brain of a newborn chimp is roughly 155 cubic centimeters (cc) in volume, less than half that of a newborn human baby, although the overall size of the birth canal is about the same as in humans. Yet there is a big difference in shape between chimp and human pelvises. While the human pelvis has widened considerably over the course of evolution, the demands of upright walking have put constraints on how wide it can be. Bipedalism has also led to a marked vertical shortening of the pelvis, leading to what researchers call the "obstetric dilemma"—the difficult tradeoff between the demands of bipedalism and having babies.
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So why do the skulls of early, small-brained hominins already show the fusion patterns of later, big-brained species? Falk and her colleagues propose three possibilities. First is the "obstetric dilemma," which may have already been a problem for early hominins as they began to walk upright. Thus the brain of an adult A. africanus, while small by modern human standards, is still about 22% larger than that of a chimp. Second, the team suggests, the delayed fusion might have been necessary for accelerated brain growth after birth, which might already have been a feature in early hominins. Finally, changes in the organization of the australopithecine brain, such as an apparent widening of the frontal lobes, might also have made the trip through the birth canal more difficult, even if the overall size of the brain remained relatively small.
Read more here: http://news.sciencemag.org/sciencenow/2012/05/infants-flexible-heads-stretch.html