Thursday, March 05, 2009

The Price of Walking Upright

Oh to be a monkey in about 4 months instead of human... things would be so much easier, specifically giving birth.

Labor and delivery is not something I used to think a lot about, outside of the day I spend with my primate behavior classes talking about it.
We discuss the impact of skeletal and pelvic bone arrangement on labor and delivery in my unit on bipedalism. They are often amazed when I tell them other primates give birth in about 30 minutes.

Walking upright is a major adaptation on the part of humankind so it's only fitting to devote a good amount of discussion to it. I think it's a really fascinating
subject, not just because the transition from 'knuckle walking' to walking on two feet required major changes to the arrangement of bones that made human childbirth incredibly dangerous — and painful — but also because no one yet has any convincing and widely accepted explanation for what drove the shift to bipedalism in the first place.

Was it that our hands we free to carry food, tool
s, a baby? Who knows. Some think we stood up because it kept our bodies cooler while walking long distances in the African savanna to find food. There are at least 6 other unique theories out there and whatever the explanation — the cost is significant.

The human pelvis became shorter and broader and the opening in the bones, the birth canal through which a baby passes during delivery, took on a more oval shape. The fossil, Lucy, one of the first skeletons to show signs of upright walking, resembles a modern human pelvis more than a chimpanzee's. The shift meant that babies, including ancestral humans, had to turn sideways during delivery rather than coming straight out like other primates.

When other primates give birth, the babies come out face up, more or less looking right at mom. Mom can see the baby the whole time and guide the little guy out all on her own. Not so for humans. If a human mom were to try to pull her baby out on her own, the angle of the head and neck would be off enough to potentially break. The shape of the human pelvis is great for walking, but very bad for solo delivery. It's believed that humans have always had to have a helper during delivery.

Another unique development that made birth difficult for humans is our large brain. Psychologists and evolutionary anthropologists call this 'encephalization.' Based on fossilized skull remains, our craniums and brains appeared to get significantly larger right around the same time we started walking on two feet.

In most cases, the baby's head is larger than the birth canal and without flexibility, the baby couldn't be delivered successfully, and either baby or mom — or both — would die. Even now with modern medical approaches to childbirth, a significant proportion of babies are delivered surgically through C-sections because the baby's head is too large.

Without some modification through evolutionary processes the human species, with it's unique gait, would have gone extinct. Two adaptations worked against this:
  • relaxation of ligaments that hold together bones of the pelvis, hips and back
  • gaps in the baby's skull
Both ultimately helped make birthing big brains possible. The 16-30 hours of labor are just one cost. In contrast, other monkeys (with a few notable exceptions) and apes give birth in about 30 minutes!

Although that may sound idyllic, it's not something I or any other pregnant woman is ever going to experience. But we can be very thankful that human birth and labor is not nearly, not even remotely as awful as what female hyenas endure. More on them next time... It will blow your mind!


Cecile/DreamCreateRepeat said...

I enjoy reading posts like these very much....looking forward to the hyena installment!

The Quiet One said...

I've just rediscovered my love of evolution given the weight put on the subject this year of university. Evolutionary psychology I find internally facinating, so was very happy to come across your blog.

Ronald Brak said...

In some cultures women do give birth unasssited, but it's not very common and in my opinion not that bright. Mind you, my own culture is kind of famous for being a bit dense, so it doens't surprise me when other cultures are a little thick too.