Lifestyle is known to affect many aspects of health but now a UK anthropologist says it could even change the shape of our jaws.
Dr. Noreen von Cramon-Taubadel of the University of Kent reports her findings in this week's edition of the Proceedings of the National Academy of Sciences.
She made detailed measurements of the skulls and jaw-bones (mandibles) of nearly 300 individuals from 11 different subsistence cultures — some from hunter-gatherer societies and some from societies with primitive agriculture.
The measurements were of skulls in museum collections, which were from people who lived in the past couple of thousand years.
"These people are likely to be analogous to people living today," Cramon-Taubadel tells the Australian Broadcasting Corporation.
She found that people from hunter-gatherer societies had narrow, long mandibles, while people living an agricultural life tended to have short, broad jaw-bones.
Cramon-Taubadel then looked for correlations between the shape of their jaw-bones with other factors such as climate, geography, genetic make-up and type of lifestyle and found the difference was not genetic.
"The pattern was consistent, regardless of which part of the world the people came from," says Cramon-Taubadel. "The results indicate that there is something biomechanical, rather than something genetic, that is altering the way the mandible grows."
"Presumably the children growing up in these different situations have different chewing behaviour," she says. Rather than happening over an evolutionary time-scale, the change to the mandible that she is talking about happens on an individual level as each child is growing up.
Chew your food
Cramon-Taubadel makes some suggestions for how chewing might differ between the two types of lifestyle. She thinks hunter-gatherers may chew more than people eating farmed food. "Agricultural populations tend to have a starch-based diet," she says, "which may be softer".
She also points out that hunter-gatherers probably chew for more of the day as they tend to browse rather than eating at definite meal times. Their diet may also include nuts and other tough foods.
There is some evidence from another species that jaw-bone shape may change with diet. Some rabbit-like animals called hyraxes have been shown to have about 10% less growth in their mandibles when they are fed soft processed food.
However, Cramon-Taubadel cautions that more research is needed to identify the actual reasons for the difference. For a start, the term 'hunter-gatherer' includes a wide range of diets. "Hunter-gatherers encapsulate Inuits, Australian Aborigines and forest people from the Congo," she says, "and they are all doing very different things".
Weaning behaviour may be another factor, she suggests. She cites research which found that hunter-gatherers wean their children significantly later than agriculturalists. She's not sure how the mechanism would work for the weaning hypothesis, but feels it merits further investigation.
All this may even have relevance to us. Whilst the growing jaw-bone may alter shape quite readily, teeth are much less variable. Cramon-Taubadel wonders whether lack of chewing, because of our soft modern highly processed diet, leads to jaw shortening, resulting in overcrowded teeth.
Professor Peter Brown of the University of New England believes this work provides evidence from cultures around the globe that corroborates similar observations made over the years. He mentions a study done by researchers at Adelaide University back in the 1950s which looked at the Warlpiri Aboriginal people in Yuendumu.
"When Australian Aborigines were first encountered [by Europeans] they were famous for having very large teeth, large chewing muscles, and projecting faces," says Brown. The 1950s Adelaide study looked at changes in the Warlpiri's dental health. By this stage they had adopted a largely Western diet. "They were starting to get crowding, particularly of their front teeth," says Brown.
Brown adds that visits to orthodontists to deal with over-crowded jaws have become the norm in our society, but thinks that getting children to chew more may prevent this. "Because I am an anthropologist, I tell my children to chew their food," he says.