Mountain gorillas eat wild plants and wild Japanese monkeys eat plants, leaves, stems, flowers, nuts and roots found in the mountains. They suffer from neither cavities nor periodontitis.
We humans used to eat lots of vegetables. But, as our culture advanced, so did foods with a lot of sugar as well. This required from us either of two methods for removing food debris from our mouths. One is to eat natural herd foods like the wild gorillas and monkeys. The other is to use a toothbrush to clean our teeth and gums.
The added benefit of using at toothbrush is that it massages our gums, which is similar to the function of natural hard vegetables