A few million years ago, our ancestors stopped climbing trees and started walking upright, on two feet. To work out how and when this happened, researchers look for fossils -- and recently they found a surprising set of foot bones in Ethiopia. The foot is about 3.4 million years old, making it roughly the same age as 'Lucy' and her species, Australopithecus afarensis. But while Lucy's species had feet much like modern humans, the new foot has an opposable big toe, like a chimp. So do the foot bones represent a new species of hominin? Watch the video and decide.