A long time ago humans ate foods which didn't damage our teeth much. We ate meats we hunted and plants we gathered. It was only when farming came along that we stopped hunting and we then had large groups of people living together and working to produce their food, so instead of hunting what you end up with is alot of people who have enough food, and more time to think of things to do with it because they're not hunting. We eat alot more foods now that damage our teeth like sugar etc. and brushing our teeth is to help keep them healthy. That's why the Tudors who ate alot of sugary things had bad teeth, and people a long time ago didn't. So now we brush our teeth to help keep them clean