Did they have some sort of paste, or is there proof of brushes being used in the past?
More can be written on this, but in the interim, you might like to see the small bits in "What did people use before the invention of the toothbrush, floss and scissors?" by /u/Aerandir , /u/speculativereply , and /u/adso_of_melk .
This is not to discourage discussion. Further questions, data, and debate are welcome.