Orangutan (Pongo pygmaeus) with a ‘toothpick groove’ on the lower left second molar (specimen FMNH 19026; Field Museum, Chicago). An orange arrow indicates the position of the groove. Credit: Ian Towle For decades, small grooves on ancient human teeth were thought to be evidence of deliberate tool use – people cleaning their teeth with sticks or fibers, or easing gum pain with makeshift “toothpicks.” Some researchers even called it the oldest human habit . But our new findings, published in the