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Wednesday, June 17, 2020

Rethinking evolutionary clichés

URI anthropology professor challenges evolutionary narratives of big, competitive men and broad, birthing women
Buster Keaton and Margaret Leahy in Three Ages (1923)
Buster Keaton and Margaret Leahy in 1923 silent movie Three Ages 
Men are taller than women because millennia ago big, strong men beat out their shorter rivals for access to mates. The female pelvis is broader than the male pelvis because women have evolved to give birth. So the thinking goes.

They’re compelling evolutionary narratives that have lasted in textbooks, classrooms and pop culture as explanations for the skeletal differences between men and women. But as explanations, these simple stories no longer stand up to current science, says Holly Dunsworth, associate professor of anthropology at the University of Rhode Island.

Poring over decades of existing research, Dunsworth has reevaluated and rewritten the narrow, reigning theories for sex difference in height and pelvic width in a new paper, “Expanding the evolutionary explanations for sex differences in the human skeleton.” 

The paper, published online by the journal Evolutionary Anthropology, maps out the critical role of estrogen production on bone growth in men and women.

“A lot of these conventions and how they support these old stories, such as sexual selection made men taller, are out of a tradition where we really only had skeletons to study,” says Dunsworth. 

“People hadn’t done behavioral observations, or studied the physiology or the genetics. There have been so many advances in 150 years of human biology, and when you put all these things together, the old origin stories don’t add up.”

In rewriting the explanations, Dunsworth waded through hundreds of existing studies. Her paper cites 94 references, but she reviewed five times that. “I tried not to go too far back. The further I went the more misconceptions I found,” she says. “I think there is an old assumption out there that testosterone makes men taller, but that’s just not the science.”

In her paper, Dunsworth focuses on how different levels of estrogen production dictate bone growth in both sexes, with ovaries producing more estrogen than testes. Boys and girls grow at roughly the same pace, reaching about 62 inches by age 13. 

At that age, greater estrogen production in girls causes long bone growth plates to fuse. Boys continue to grow taller for about five more years, until they reach levels of estrogen that fuse their bones. 

In that time, boys grow another 8 inches on average; girls just 2. As with height, sex differences in the pelvis skeleton are also rooted in the differing levels of estrogen and its effects over time on differing systems of gonads, genitals, ligaments and bones.

“There are ways that men and women are so obviously different in their evolved reproductive physiology,” Dunsworth says. “It’s really as if the reigning theories just look at the skeleton to claim that men are taller because they evolved to be dominant and competitive – as if women didn’t – and to claim that women are broader because they evolved for reproduction – as if men didn’t. 

Conspicuous sex differences in our bodies lead to assumptions about gender differences. They feed our narratives about what a man is and what a woman is, and what our different roles in society should be. These myths about human nature haven’t exactly worked wonders for women and they fuel toxic masculinity.”

Dunsworth, a biological anthropologist, sees it as her job as a professor and researcher to overturn outdated and false evolutionary traditions and to retell origin stories that are inclusive and unbiased.
“We make meaning out of human evolutionary origin stories,” she says. 

“Whether they really dig human evolution or not, people are using it to make sense of the world and they’re thinking that some of these very narrow, very outdated ideas are the science, are the facts,” she says. 

“There are facts and then there are stories we tell about them. But we can improve our stories. There are more inclusive stories to tell, more complicated, more dynamic, more interesting, more scientific ways of describing the facts and telling stories about those facts.”

Despite their flaws, theories of sexual selection for height and natural selection for pelvis size continue to be taught in classrooms, Dunsworth says, even in hers.

“We’ve taught it for years because there’s an obsession with comparing the degree of difference between men and women to the much larger difference between male and female gorillas. Somehow, it’s supposed to show that we are more peaceful and more cooperative, while still acknowledging that, because human men are bigger than women, the big men in our ancestry have been the big winners,” she says. 

“I was teaching sexual selection. It’s canon. I thought this is how we explain this until I sat back and thought it through.”

Dunsworth had doubted the use of sexual selection to explain male and female body size differences. But the tipping point came in 2016 after she took exception on social media to comments by a well-known evolutionary biologist who was defending the theory in a politically charged rant.

“I’m a feminist and I’m trying to be part of this inclusive, diverse future of the world,” Dunsworth says. “I knew that this one simple, narrow story wasn’t even scientific. So, I spoke out. That’s when I realized this is a huge problem.”

She started her research immediately and submitted her paper in 2018 for peer-review in Evolutionary Anthropology. Already available online, it appears in the May/June issue of the journal.

“To have this new way of thinking in a major journal in my field and reviewed by my peers is the gold standard of knowledge,” she says. “It’s not just me on my blog, raising my feminist fist in the air. This is how you advance knowledge.”