There's evidence that plant root fiber toothbrushes are at least as good as nylon bristle toothbrushes (keywords for search "miswak", "chewing stick"), eg:
https://www.ncbi.nlm.nih.gov/pubmed/15643758
(in my personal experience they work just fine, but they are more hassle than nylon toothbrushes because the bristles point in the same direction as the handle instead of to the side)
Of course, medical research is notoriously unreliable, but it's at least plausible that ancient humans had all the technology needed to keep their teeth in good condition.
There's evidence that plant root fiber toothbrushes are at least as good as nylon bristle toothbrushes (keywords for search "miswak", "chewing stick"), eg: https://www.ncbi.nlm.nih.gov/pubmed/15643758 (in my personal experience they work just fine, but they are more hassle than nylon toothbrushes because the bristles point in the same direction as the handle instead of to the side)
There's also evidence that flossing+brushing has no advantage over brushing alone: https://www.ncbi.nlm.nih.gov/pubmed/19138178
Of course, medical research is notoriously unreliable, but it's at least plausible that ancient humans had all the technology needed to keep their teeth in good condition.