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Tracing the Evolution of Human Birth
He’s One of the Go-to Guys
Wenda Trevathan
Darren Phillips

Wenda Trevathan

Tracing the Evolution of Human Birth

Anthropologist renowned for pioneering research
By Bob Nosbisch

If cats, dogs, horses and other animals seem to have little difficulty when giving birth, then why is human birth relatively complicated?

To answer this question, biological anthropologist Wenda Trevathan has spent much of her career examining human birth from an evolutionary perspective. She has gained wide recognition for her work as a researcher and as a teacher, with awards ranging from the American Anthropological Association’s Margaret Mead Award to the New Mexico Professor of the Year Award presented by the Carnegie Foundation for the Advancement of Teaching.

Trevathan’s pioneering research on human birth has focused on two seemingly unrelated things that happened during evolution: Our pelvises became narrower and our brains grew larger. While narrow pelvises enabled us to walk more easily on two legs, “we now are trying to pass big-brained babies through narrow birth canals,” Trevathan said, noting that “complications will be inevitable.”

As she and other scholars traced changes in childbirth back to the times of our ancestors, they discovered that about half of the brains in other animal species are developed at birth, but for humans, only one-quarter of our brains are developed at birth. In other words, humans with bigger, less developed brains in bigger heads tried to squeeze through smaller pelvises.

“So our babies are much more vulnerable and much more helpless,” Trevathan said. “Monkeys can usually hold on to their mothers right away, so if the mother squats and gives birth, the baby sometimes crawls out of the birth canal, holds on to her fur, and the mother can take off running if she needs to escape from a predator. Our babies couldn’t possibly do that, because they don’t have the required motor skills.”

Charting childbirth through time also has shown that humans today, like their ancestors, want someone with them when they deliver their children. Six years ago, Trevathan and Karen Rosenberg, a University of Delaware paleoanthropologist, wrote an article showing similarities between the modern custom of seeking assistance during childbirth to the actions of our ancestors who also sought help.

Their article caught the attention of the publishers of the Williams Obstetrics Textbook, who included the researchers’ work in the first chapter of the book’s 22nd edition so all obstetricians should be aware of it.

In the article, Trevathan and Rosenberg argue that, unlike most animals that want to be alone when giving birth, the complications associated with human childbirth make it advantageous for women to look for help in the delivery process.

“Our argument is that we can physically give birth without any help, but if you have somebody there to help receive the baby, to help clear off the facial region to help it breathe, and to help flip the umbilical cord if it’s wrapped around the neck, this shows one of the successes of the human species,” Trevathan said.

“For women throughout history, birth has always been supported by close friends and relatives so the normal place for birth was with sisters or mothers,” she said. “It was more of a woman-centered thing with a lot of social and emotional support.”

In addition to her work on the evolution of childbirth, Trevathan’s research interests include human sexuality, mother and infant biology, nutritional anthropology and evolutionary medicine.

She is working with psychology professor Laura Thompson on research into mother-infant interaction, cortisol levels and cognitive development, funded by the National Institutes of Health.