The Neolithic revolution occurred a number of times over the course of human prehistory. It involved the switch from nomadic hunting and gathering to an agricultural lifestyle, with all the economic, technological, societal, and architectural switches that went along with it. With agriculture came food processing and a softer, more homogenous diet than the one enjoyed by hunter-gatherers.
Hunter-gatherers tend to have longer, narrower lower mandibles, or jawbones, than agriculturalists. It has generally been assumed that the difference in chewing style accounts for the differently shaped mandibles, but this has never been tested. Noreen von Cramon-Taubadel at the University of Kent has now done so; her results are published in this week’s issue of PNAS.
Most of the differences in the shape of human skulls are the results of neutral evolution, of mutations that spread through genetic drift rather than responding to any type of external selective pressure. Not so the mandible. Dr. Cramon-Taubadel demonstrated that the mandible changed shape in agriculturalists because of the decreased mechanical stresses exerted by their chewing.
By analyzing skulls from eleven populations, six agriculturalists and five hunter-gatherers, she showed that mandible shape splits along these lines and does not correlate with geographical location, climate, or the neutral genetic patterns in their respective population histories. This finding raises an interesting question: does the softer food eaten by agriculturalists constitute a selective pressure acting upon the mandible, or does the mandible develop differently in response to dietary stresses? The later case would be an example of what's called "phenotypic plasticity," where development ends up different despite similar genes.
The paper further points out that mothers in contemporary hunting and gathering societies usually wean their children later than those in agricultural populations, so young children don’t start their chewing until later in development. Might this play a role in the different shapes of their mandibles?
Dr. Cramon-Taubadel also notes that the "reduced duration and intensity of mastication" required by our soft, processed foods have been blamed for the increased rate of dental crowding seen in postindustrial urban populations. Stress promotes bone growth, and our pathetic chewing needs are insufficient to stimulate even enough mandible growth to accommodate our teeth. So to avoid hefty orthodonture bills, maybe we should plan some foraging expeditions.