When archaeologists recently examined the upper teeth of skeletons of humans who lived 15,000 years ago in North Africa, they found widespread incidence of rotten teeth. It seems that the hunter-gatherers of this era were quite fond of eating pine nuts, acorns, and other carbohydrate-rich foods. Then, as now, if left to accumulate on tooth surfaces, these foods break down into sugars, which oral bacteria feed on, producing acid by-products that dissolve tooth enamel. High levels of tooth decay seem to have been a problem for other hunter-gatherer groups located in northern Mexico, who ate a corn-based diet. Fortunately, we now have the education and means to stave off tooth decay. Our ancestors were not so fortunate.
P.S. The research mentioned above helps underscore the importance of flossing and brushing at least twice daily. If brushing and flossing are not possible at lunchtime, try to use a tooth pick, and rinse your mouth with water.