Separate species of ancient human ancestors co-existed 3.4m years ago, a dramatic fossil find has shown.
While one species walked upright on land, another similar but distinct group lived mostly in trees, with feet adapted for climbing.
A team of scientists from the United States said in the journal Nature that a 3.4m-year-old foot found in the Woranso-Mille area of Ethiopia could rock the foundations of early human research.
The foot, found in 2009, did not belong to Australopithecus afarensis, the species of the famous 'Lucy' fossil.
Project leader Dr. Yohannes Haile-Selassie, curator of physical anthropology at The Cleveland Museum of Natural History, said the foot belonged to a distinct species.
The fossil had an opposable big toe often used in climbing, instead of the aligned big toe seen in modern humans - and Lucy's species.
"The partial foot clearly shows that at 3.4 million years ago, Lucy's species, which walked upright on two legs, was not the only hominin species living in this region of Ethiopia," he said.
"Her species co-existed with close relatives who were more adept at climbing trees, like 'Ardi's' species, Ardipithecus ramidus, which lived 4.4 million years ago."
The find was made near to an ancient river. Fossils of crocodiles, fish and turtles were found nearby.
"This discovery was quite shocking," said co-author and project co-leader Dr. Bruce Latimer of Case Western Reserve University.
"These fossil elements represent bones we've never seen before. While the grasping big toe could move from side to side, there was no expansion on top of the joint that would allow for expanded range of movement required for pushing off the ground for upright walking. This individual would have likely had a somewhat awkward gait when on the ground."