Our human ancestors may have taken a close interest in dental hygiene.
A surprising interest in pesonal 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.
Unlike wood, it contains large numbers of hard, abrasive silica particles.
Dr Hlusko said grass stalks were the right size to leave the marks - between 1.5 to 2.6 millimetres wide.
They were also widely available, and required little modification to become an effective toothpick.
It is thought ancient hominids may have started picking at their teeth to try to alleviate the pain of gum disease.
New Scientist magazine reports that Dr Hlusko spent eight hours grinding a piece of grass along a tooth taken from a baboon.
She then replicated the experiment for three hours on a modern human tooth.
In both, the grass left marks almost identical to those seen in scanning electron microscopic images of early hominid teeth.
Dr Hlusko, whose work is published in the journal Current Anthropology, said: "Toothpicking with grass stalks probably represents the most persistent habit documented in human evolution."
Tooth picks were known to be popular in ancient China, Japan, India, Iran and other early Eastern civilisations.
It is thought they often took the form of sharpened, fibrous sticks taken from the lentisk tree. Others were made from gold or bronze.