from Our Babies, Ourselves by Meredith F. Small (pp 8-12)

Unlike apes, who hang from the trees with long arms and walk along the ground balancing their weight on the surface of their knuckles, individuals in the human line stood up on their back legs. … [H]umans, and our ancestors, have used bipedal walking as the primary form of locomotion. That switch to bipedalism eventually brought us the pain of childbirth.

…

The sacrum, the lower back, is wider and thicker and it angles into the pelvic opening to help support the internal organs in a creature with a shifted center of gravity. And so, right in the middle of what should be a clear hole for infants to pass through, there is a point of the sacrum coming radically close the the pubic bones of the front of the pelvis–a detour sign for emerging babies that arose when humans stood up on two legs.

…

And so the infant enters the birth canal at a side angle, twists to accommodate the midsection, flexes the head down to bypass the squeeze at the sacrum, and ends up coming out face down. Since the outlet faces more backward than downward, the baby also has to bend a bit and thus comes out at an angle facing the mother’s back. The shoulders follow a similar path, dipping and twisting; but since the head is then already out, this requires the execution of a head-body twist…

January 26th, 2010 | Category: book | Subscribe to comments | Leave a comment | Trackback URL