There's a nice section in popularly asked questions regarding dental care.

But I'll do what I can do sum up here.

Primarily, as a result in the shifting of diets in recent history dental issues have become more of an issue than they ever were before. Before sugar and other widely available modern foods were available en masse to general populations they likely didn't face the same issues that we do today. That said, people did (and in some places still do) use certain twigs such as the miswak which functioned similarly to a toothbrush, or they would simply chew on the stick to receive the benefits.