Evolution favours shorter and heavier women—like it or not

Natural selection is still at work

Natural selection is still at work

Photograph by Hans Neleman/ Getty

What might our granddaughter’s granddaughter’s granddaughter’s granddaughter’s granddaughter look like? Shorter and stouter, says a report in the Proceedings of the National Academy of Sciences. If current trends continue, its authors predict, then by 2409 descendants of the women in the study will have evolved to be one kilogram heavier and two centimetres shorter than their 2010 foremothers.

For years, some scientists heralded the end of human evolution. The post-industrial homo sapiens, they argued, was free of the kinds of “survival-of-the-fittest” pressures that could drive large-scale genetic change. In 2008, Steve Jones, professor of genetics at University College London, gave a much-hyped lecture entitled “Human Evolution is Over.” “Not so,” says Stephen Stearns, co-author of this latest study, professor of evolutionary biology at Yale University, and founding editor of the Journal of Evolutionary Biology. “The basic take-home is that humans continue to evolve,” Stearns told Maclean’s.

“One [could express] the result as: women are going to get shorter and fatter,” he explains. But he prefers a different bent: “There is natural selection against women being slender.” Stearns’s work shows that plumper, shorter women tend to bear more children—who carry on those same traits. His analysis drew on data from the Framingham Heart Study: a survey, begun in 1948, that collected medical information from 5,209 subjects, and monitored them and their offspring for 60 years.

The weight part of the equation, says Stearns, is straightforward: “A woman has to have about 20 per cent body fat to ovulate and conceive.” But he admits that he “can’t give a good explanation of why they are getting shorter.” A separate study by Open University’s Daniel Nettle found that shorter women are more likely to be in long-term, offspring-producing relationships—perhaps, he hypo­thesized, because men evolved to disfavour tall women, who tend to reach puberty later.

Whatever the cause of the change, it won’t be speedy. Humans won’t evolve as fast as the Galapagos finches that helped Charles Darwin cement his evolutionary theory. Instead, the homo sapiens gal is keeping pace with the New Zealand chinook salmon and the Hawaiian mosquitofish.

Still, it’s that slow pace that is the heart of Stearns’s mission, which goes beyond simply musing about the female physique. He and his colleagues are out “to correct the still widespread misconception that natural selection is not operating on contemporary humans.” He explains that it’s true “hygiene, nutrition and medical care” have helped erase survival-of-the-fittest pressures. But as evolution’s “mortality component” becomes less significant, it makes “the variation between individuals and how many children they have more important.” In other words, evolution continues because of differences in reproductive success. So why all the disbelief? “Charles Darwin himself emphasized survival rather than reproduction,” says Stearns. “I think, though, that if you had the conversation with Darwin, he would get the point.”

The hitch, Stearns warns, is that predicted changes might not materialize. For instance: while evolution is literally pushing women down and out, environmental factors, like better nutrition, allow them to grow taller and stronger. The result of these battling influences is impossible to predict.

Still, more scientists are turning their attention to how the female body evolves to maximize motherhood. For Steven Gaulin, anthropology professor at the University of California, it’s not a woman’s weight that is important, as much as her proportions. “There is a strong correlation between waist-hip ratio and the cognitive ability of a child,” he explains: the bigger a mother’s hips (relative to her waist), the smarter her offspring. Gaulin estimates that with every decrease of 0.01 in a mother’s waist-to-hip ratio, her child’s average cognitive score is raised 0.061 points. The reason? “The brain is fabulously fatty.” And the fats it craves, Omega-3s, are stored disproportionately in hips and thighs. That means pear-shaped moms can better fuel their babies’ brain development.

But for others, like Dr. Andrew Clark, researcher at the University of Bristol, “just pointing out the size [of a hip, thigh or bottom is] too simplistic.” Instead, he says, you must focus on “pertness.” A fit bosom or butt signals “fertility, fecundity, offspring quality,” he explains. “They signify youth; the slings and arrows of time have not had time to work their magic and make things saggy, which is a pretty good indicator that this person still has a long reproductive lifespan.” Clark thinks sexual preference for fertile mates has made pert bums, like J.Lo’s, objects of attraction.

Stearns’s work ultimately considers more than just shape. He predicts that women will also evolve to have “better cardiovascular health” and a larger “reproductive window,” with earlier periods and later menopause. Still, he says his ideas were tough for some to buy. “When news of this result first broke—that women are getting shorter and fatter—there were a lot of inquiries,” he laughs, because the forecast seems to counter “standards of beauty in our culture.” Those standards may well have to change, at least by 2409.