The earliest evidence for teeth cleaning dates back to the beginning of humankind. The evidence suggests cleaning with flint tools. From later evidence (can't remember the source right now) there is evidence for cleaning with an early form of toothbrush. Thin twigs gnawed on at one side so it splits into a "brush". Works remarkably well. Have tried it myself. I would suggest this was also used from the earliest moments as this is such a simple cleaning tool. As for "modern" toothbrushes. Late 15th century China there was use of a toothbrush we would recognise as one. Only made with boar bristles and bamboo. The later synthetic one came about in 1938.