In early medieval England, a lot of people died from dental infections. Many more had some pretty nasty cavities that must have been unpleasant. You sometimes see human remains where a person chewed most of their food on one side of the mouth (more wear on the teeth in one side of the mouth than the other), suggesting that it hurt too much to chew on the other side.

Diet probably did make a difference. In some cemeteries, the men with the nicer stuff in their graves (swords or other weapons, usually thought to accompany people of higher social status) tended to have the most cavities, suggesting that rich men were the most likely to eat sugary foods (this fits well with the literary stereotype of wealthy warriors drinking mead and eating honey).

It's hard to tell what kinds of treatments were available in early medieval England; most bodies archaeologists find are missing teeth, but it's usually hard to tell if they were lost before or after burial and decomposition. While I wouldn't be surprised if many people had bad teeth pulled, it's clear that some didn't, and that the teeth became infected, grew into swolen abcesses so nasty that they damaged the persons' jaw bone, and almost surely were the persons' cause of death.

Dental caries (cavities) were actually less common prior to the development of agriculture. Diet is hugely influential in the matter, and many people lived with dental pathologies for quite some time before dying. Unfortunately, early treatment methods are largely unknown as they don't preserve archaeologically, but I'll discuss the other aspects.

Hunter-gatherer societies have a much lower rate of caries than those which practice agriculture. In addition, some foods are more cariogenic than others. Starchy foods, such as potatoes, cause much higher rates of caries than foods like rice. In addition, diets with many marine resources have a lower prevalence of dental pathologies. For example, in Jomon period Japan, groups who lived on the coast had much lower rates of pathologies than those who lived further inland and relied on mostly plant based diets. By the following Yayoi period, caries rates increased with the introduction of agriculture. People with diets with low carb intake have fewer cases of severe caries than those with high carb diets. Early people had diets high in protein and low in carbohydrates, so the overall dental health was much better than that of early agriculturalists.

There's quite a bit of bioarchaeological research on the subject, as teeth are the most commonly found skeletal remains when excavating a site. We look for antemortem tooth loss, abscesses in the maxilla and mandible, dental caries, and calculus for signs of overall health. In instances with high rates of these problems, especially abscesses, it likely had an impact on mortality, but unfortunately skeletons rarely can show exact cause of death. I'd be glad to send more sources for further reading from other regions of the world, I have a large collection on the topic.

Sources: Lanfranco, Luis Pezo and Sabine Eggers. 2010 The Usefulness of Caries Frequency, Depth, and Location in Determining Cariogenicity and Past Subsistence: A Test on Early and Later Agriculturalists From the Peruvian Coast. American Journal of Physical Anthropology 143:75-91.

Temple, Daniel H. 2007 Dietary Variation and Stress Among Prehistoric Jomon Foragers from Japan. American Journal of Physical Anthropology 133:1035-1046.

Temple, Daniel H. and Clark Spencer Larsen 2007 Dental Caries Prevalence as Evidence for Agriculture and Subsistence Variation During the Yayoi Period in Prehistoric Japan: Biocultural Interpretations of an Economy in Transition. American Journal of Physical Anthropology 134:501-512.

I'm currently involved in a project that is looking in to Seattle's earliest history, which starts in 1851.

On page 116 of her rambling "diary" of Seattle's early days, E. I. Denny (one of the children that lived here with the first white settlers) made the following statement:

"The Indian children were usually quite amiable in disposition, and it seemed hard to refuse their friendly advances which it became necessary to do. In their primitive state they seemed perfectly healthy and happy little creatures. They never had the toothache; just think of that, ye small consumers of colored candies! Unknown to them was the creeping horror that white children feel when about to enter the terrible dentist’s den."

In reference to your question, this indicates to me that "primitive" cultures did not necessarily suffer from the ailments of "more developed" cultures, so it may not be accurate to assume a lack of dentistry = a lack of dental health.

How "early" are you talking about?

Additional question, I've read in fictional but mostly historically accurate books(as far as I'm aware), that chewing evergreen bark, or minty kinds of plants was a common practice for dental hygiene in early human history. Any truth to that?

Also, how come skeletons always seem to have straight teeth?