Sunday, October 9, 2011 - 0 comments

The Evolution of Human Birth : 4. Walking on Two Legs

By : Karen R. Rosenberg and Wenda R. Trevathan

IN MODERN HUMANS, both bipedalism and enlarged brains constrain birth in important ways, but the first fundamental shift away from a nonhuman primate way of birth came about because of bipedalism alone. This unique way of walking appeared in early human ancestors of the genus Australopithecus at least four million years ago [see “Evolution of Human Walking,” by C. Owen Lovejoy; Scientific American, November
1988]. Despite their upright posture, australopithecines typically stood no more than four feet tall, and their
brains were not much bigger than those of living chimpanzees. Recent evidence has called into question which of the several australopithecine species were part of the lineage that led to Homo. Understanding the way any of them gave birth is still important, however, because walking on two legs would have constricted the maximum size of the pelvis and birth canal in similar ways among related species.

The anatomy of the female pelvis from this time period is well known from two complete fossils. Anthropologists unearthed the first (known as Sts 14 and presumed to be 2.5 million years old) in Sterkfontein, a site in the Transvaal region of South Africa. The second is best known as Lucy, a fossil discovered in the Hadar region of Ethiopia and dated at just over three million years old. Based on these specimens and on estimates of newborns’ head size, C. Owen Lovejoy of Kent State University and Robert G. Tague of Louisiana State University concluded in the mid-1980s that birth in early hominids was unlike that
known for any living species of primate.

The shape of the australopithecine birth canal is a flattened oval with the greatest dimension from side to side at both the entrance and exit. This shape appears to require a birth pattern different from that of monkeys, apes or modern humans. The head would not have rotated within the birth canal, but we think that in order for the shoulders to fit through, the baby might have had to turn its head once it emerged. In other words,
if the baby’s head entered the birth canal facing the side of the mother’s body, its shoulders would have been oriented in a line from the mother’s belly to her back. This starting position would have meant that the shoulders probably also had to turn sideways to squeeze through the birth canal.

This simple rotation could have introduced a kind of difficulty in australopithecine deliveries that no other known primate species had ever experienced. Depending on which way the baby’s shoulders turned, its head could have exited the birth canal facing either forward or backward relative to the mother. Because the australopithecine birth canal is
a symmetrical opening of unchanging shape, the baby could have just as easily turned its shoulders toward the front or back of its body, giving it about a 50–50 chance of emerging in the easier, face-forward position. If the infant were born facing backward, the australopithecine mother—like modern human mothers—may well have benefited from some kind of assistance.

Next : Childbirth across Cultures


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