The world's oldest known habitual drug use can be traced to an island in Indonesia, where ice age hunter-gatherers sucked on betel nuts — palm seeds that contain a neuroactive substance, a new study finds. Scientists made the discovery after studying strange grooves in the teeth of two prehistoric hunter-gatherers. The deep grooves were caused by regularly sucking on betel nuts (Areca catechu), likely for relief from the pain of toothache, according to the study, which was published Wednesday (Sept. 9) in the journal Science Advances.The partial skeletons of the two people were found on the island of Sulawesi, but their lives were separated in time by thousands of years. One of them, a man w...