Experts think early humans, who lived in Africa more than three million years ago, mainly ate grass.
Before now, it was thought our ancestors preferred hard nuts and seeds because of their big grinding teeth and powerful jaw muscles.
But after studying fossils of their teeth, scientists have found traces of tropical grasses.
They say humans could have switched from eating nuts and berries to grass when they stopped living in trees.
It means our earliest relations could have had a diet like modern-day savannah baboons, which also feed on African grassland.
Lead scientist Professor Julia Lee-Thorp, from Oxford University, said: "We found evidence suggesting that early hominins, in central Africa at least, ate a diet mainly comprised of tropical grasses and sedges."