In addition to all the other arguments here (more chewing for bigger jaw development), I have another theory about tooth decay. I think that pre-civilization humans did not eat much sweets past adolescence. This is because, as soon as a minor cavity starts developing, it hurts like hell to put anything sweet in your mouth. Sweet stuff was for children and adolescents, then we just stopped eating it.
Pre-civilisation humans wouldn't have had "sweets" as we know them. They would have occasionally had unrefined sugars through honey or palm sugar, and at certain times of the year had fruits (which most likely wouldn't even have been as sweet as they are now)