According to Rosenberg, the average pelvic opening in women today is 13 centimeters at its largest diameter and 10 centimeters at its smallest. The average infant head is 10 centimeters from front to back, and the shoulders are 12 centimeters across. And today the birth canal is a twisty tunnel subjecting the infant to a series of complex twists and turns on its way out.
"Until recently, there was a sexism in the study of evolution," Rosenberg says. "Researchers focused on men and the tools they used in hunting, and these things were more difficult to connect to reproductive success and hence to natural selection," she notes.
"With childbirth, as well as many of the other things that happen to women -- pregnancy, nursing, menopause -- it's really easy to see how natural selection works," Rosenberg notes.
Childbirth is just one of a series of examples throughout a woman's life cycle, in which enlisting the help of other women significantly improves reproductive outcomes, according to Rosenberg.
"Women take up the slack for other women when they are pregnant and nursing so that they have the energy to devote to their infants. Cooperative childcare is something in which women help each other out. Often, but not always, these helpers are post-reproductive women who have fewer of those responsibilities of their own, but may be helping out their daughters. All of this puts a great selective premium on a kind of social intelligence that many scientists think partly accounts for the increase in brain size that happened over the last two million years," she notes.
How will women and childbirth continue to evolve? Will the birth canal grow narrower, or wider? Wi
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| Contact: Tracey Bryant tbryant@udel.edu 302-831-8185 University of Delaware Source:Eurekalert |