National Geographic : 2006 Jul
* I anE o* U. 140 NATIONAL GEOGRAPHIC *JULY 2006 Upside-down and ready to deliver, a human fetal skull facing rear-in position for birth-points out perhaps the greatest downside of being upright: a difficult passage through the bony canal of a female pelvis. So tight is the fit that the human fetus must rotate as it moves down the canal; in some cases its cranial bones must squeeze together and overlap, compressing the skull a few millimeters so the infant can exit. The challenging nature of human birth reflects a history of com promise between the demands of bipedalism and a big brain. As our pelvis narrowed and our brain expanded, more than doubling in size from two mil lion years to 100,000 years ago, the fit grew even tighter. The solution: Humans are born with their brains only a quarter of adult size.