What's the News: Parents going broke to pay for their offspring's braces and orthodontistry can finally blame somebody besides their mildly malformed children: our farmer ancestors. A study published this week in the Proceedings of the National Academy of Sciences found that people living in subsistence farming communities around the world have shorter, wider jaws than those in hunting and gathering societies. This leaves less room for teeth, which have changed little in size or abundance over human history—and may help explain why crooked choppers and a need for orthodontia are so common, study author Noreen von Cramon-Taubadel tells the BBC. "I have had four of my pre-molars pulled and that is the only reason that my teeth fit in my mouth," she says. How the Heck:
von Cramon-Taubadel, a University of Kent anthropologist, made 3-D images of 322 craniums and 295 mandibles from 11 groups of subsistence tribes around the world. Six of the groups were farmers; five were hunter-gatherers.
She found no link between cranium size and the type of subsistence economy the person came from.
The jawbone was a different story. After controlling for genetic, geographic, and climatic factors, she found a strong correlation between the type of subsistence economy and jawbone length and width.
What's the Context:
The study found a worldwide correlation toward shorter, broader jaws in groups that had domesticated plants or animals, which had never been shown before.
von Cramon-Taubadel's hypothesis is that longer, narrower jawbones are more efficient for the hunter-gatherer diet, which is more varied and requires more frequent and intense chewing bouts.
Previous studies have shown that differences in the length of jawbones seem to arise relatively quickly after a change in subsistence economy, suggesting the shift to a shorter jawbone in agricultural groups could reflect a selective pressure for downsizing.
Changes in jawbone size can also arise within a single generation due to phenotypic plasticity, wherein environmental differences lead to anatomical changes. For example, a 2004 study found that the mandibles of hyraxes (a small rodent-like mammal) given soft, processed food grew to be 10 percent shorter than those fed a heartier unprocessed diet. This hints that longer jaws are advantageous for more intense chewing.
The Future Holds:
Shorter jaws may be more efficient for consuming agricultural fare like wheat, corn, or rice, which is relatively soft and consistent compared to hunter-gatherer grub. But this means less room for teeth, whose size and abundance haven't changed much over the centuries, the anthropologist tells NPR. "It's interesting that there's this mismatch between teeth and bone."
Perhaps one day this "mismatch" will correct itself. Until then, were stuck with braces. This Thanksgiving, orthodontists should consider raising a glass to our farming ancestors.
Reference: Noreen von Cramon-Taubadel. Global human mandibular variation reflects differences in agricultural and hunter-gatherer subsistence strategies. Proceedings of the National Academy of Sciences. Published online 21 November 2011. DOI: 10.1073/pnas.1113050108
Image credit: instafoto / Flickr
