Evolution and humans: big brain birth canal (Evolution)

by David Turell @, Wednesday, April 26, 2017, 21:41 (2495 days ago) @ David Turell

This article is a study of Lucy and how the birth canal might have changed to allows for a larger brain and wider shoulders, and expands on the previous article I presented:


"Modern humans give birth in a way quite different from how their primate relatives do it, according to research described in the book "Human Birth: An Evolutionary Perspective" (1987, Aldine Transaction) by Wanda Trevathan. This is likely because of both the unusually large size of the modern human brain and the way a woman's pelvis is positioned for upright walking,


"In primate babies, skulls are longer from the faces to the backs of the bodies than compared with from the forehead to the chin or from left to right. In most primates, the birth canal is similarly longer in that direction: lengthwise from the front to the back of a female's body. There is often plenty of room for most primate newborns as they exit the birth canal, so most primate mothers do not need help when they give birth.


"In contrast, in modern humans, the width of the birth canal, extending from the right to the left of the body, is bigger than the length. As such, babies enter the birth canal facing sideways. As the baby's head progresses out of the canal, it rotates to face the mother's back so the shoulders can then fit through. Human babies fit very snugly in birth canals, so human mothers generally require at least some assistance during birth,


"DeSilva's team analyzed the fossil pelvis of Lucy and came up with a mathematical model describing how newborns might have made their way through Lucy's birth canal. "What we found with Lucy was very much in between that of chimpanzees and humans,"


"In addition, the researchers said they estimated the width of an A. afarensis baby's shoulders by looking at the relationship between the shoulder widths of adult and newborn primates such as humans, chimps, gorillas, orangutans and gibbons, and by examining the width of an adult A. afarensis' shoulders.


Based on their models, the researchers suggested that, as happens in humans, a baby A. afarensis would have entered the birth canal sideways. However, the researchers also suggested that an infant A. afarensis would have had to tilt only a bit to make way for its shoulders as its head slid down the birth canal, instead of its head rotating 90 degrees as happens with human babies during childbirth.


"These findings suggest that the evolution of rotation during birth may have occurred in two stages, the researchers said. First, after hips designed for upright walking evolved, infants started rotating a bit in the birth canal so it could accommodate the head and shoulders. Then, as brains got bigger in the human lineage, full rotation began happening during childbirth, the study said."

Comment: This article illustrates the problems for evolution according to Darwin. As the baby's head size increases so must the birth canal and because of completely upright posture the pelvis must change its shape. Therefore both the baby's head and the pelvic changes must coordinate exactly or babies die in transit. This must happen with each step in growth of human brain size. The only position in which a woman can deliver by herself is squatting, as described by Pearl Buck in her novels about China before the 20th Century.

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