Passage A

Because dental caries (decay) is strongly linked to consumption of the sticky, carbohydrate-rich staples of agricultural diets, prehistoric human teeth can provide clues about when a population made the transition from a hunter-gatherer diet to an agricultural one. Caries formation is influenced by several factors, including tooth structure, bacteria in the mouth, and diet. In particular, caries formation is affected by carbohydrates' texture and composition, since carbohydrates more readily stick to teeth.

Many researchers have demonstrated the link between carbohydrate consumption and caries. In North America, Leigh studied caries in archaeologically derived teeth, noting that caries rates differed between indigenous populations that primarily consumed meat (a Sioux sample showed almost no caries) and those heavily dependent on cultivated maize (a Zuni sample had 75 percent carious teeth). Leigh's findings have been frequently confirmed by other researchers, who have shown that, in general, the greater a population's dependence on agriculture is, the higher its rate of caries formation will be.

Under some circumstances, however, nonagricultural populations may exhibit relatively high caries rates. For example, early nonagricultural populations in western North America who consumed large amounts of highly processed stone-ground flour made from gathered acorns show relatively high caries frequencies. And wild plants collected by the Hopi included several species with high cariogenic potential, notably pinyon nuts and wild tubers.

Passage B

Archaeologists recovered human skeletal remains interred over a 2,000-year period in prehistoric Ban Chiang, Thailand. The site's early inhabitants appear to have had a hunter-gatherer-cultivator economy. Evidence indicates that, over time, the population became increasingly dependent on agriculture.

Research suggests that agricultural intensification results in declining human health, including dental health. Studies show that dental caries is uncommon in pre-agricultural populations. Increased caries frequency may result from increased consumption of starchy-sticky foodstuffs or from alterations in tooth wear. The wearing down of tooth crown surfaces reduces caries formation by removing fissures that can trap food particles. A reduction of fiber or grit in a diet may diminish tooth wear, thus increasing caries frequency. However, severe wear that exposes a tooth's pulp cavity may also result in caries.

The diet of Ban Chiang's inhabitants included some cultivated rice and yams from the beginning of the period represented by the recovered remains. These were part of a varied diet that also included wild plant and animal foods. Since both rice and yams are carbohydrates, increased reliance on either or both should theoretically result in increased caries frequency.

Yet comparisons of caries frequency in the Early and Late Ban Chiang Groups indicate that overall caries frequency is slightly greater in the Early Group. Tooth wear patterns do not indicate tooth wear changes between Early and Late Groups that would explain this unexpected finding. It is more likely that, although dependence on agriculture increased, the diet in the Late period remained varied enough that no single food dominated. Furthermore, there may have been a shift from sweeter carbohydrates (yams) toward rice, a less cariogenic carbohydrate.