From an article on BBC News web site:

Our human ancestors may have taken a close interest in dental hygiene.
Palaeontologist Dr Leslea Hlusko, of the University of Illinois, claims to have evidence ancient man used rudimentary tooth picks.

She has shown that curved grooves found on fossil teeth dating back 1.8 million years could be the result of erosion caused by repeated rubbing with grass stalks.

Sceptics argue today’s toothpicks leave no such marks, but Dr Hlusko said grass is more abrasive.

More...