Well, a few things to consider. First, these days we eat a lot more foods with nutrients that bacteria REALLY LOVE, particularly simple sugars. So there is some truth to that whole “candy rots your teeth” thing. But the bigger answer is that animals get terrible tooth decay (including primates) and keepers regularly brush the teeth of animals in zoos to prevent these problems. Finally, the average human these days lives a lot longer than we’re designed to. The life expectancy of our nearest relative, the chimpanzee, is just 40 years in the wild. In captivity, that number goes up to around 60. We just live longer than we’re supposed to…so we have to keep our parts working for longer than they’re designed to work…which means brushing your teeth a couple times a day.