Researchers say a 3.4 million-year-old fossilized foot found in Ethiopia did not belong to a member of Australopithecus afarensis, the hominin species of our early upright-walking ancestor "Lucy", but to a tree-climbing hominin cousin with whom she and her relatives co-existed. They write about how they came to this conclusion in the 29 March online issue of Nature.

The fossil of the partial foot was found in 3.4-million-year-old rocks at Woranso-Mille in the Afar region of Ethiopia, where lead author Dr Yohannes Haile-Selassie, curator of physical anthropology at The Cleveland Museum of Natural History in the US, led the field research.

Bones of Australopithecus afarensis, have also been found in the area, which is known locally as Burtele.

Haile-Selassie and colleagues say the partial foot fossil, which was discovered in February 2009, indicates that more than one species of early human ancestor with different means of locomotion, one walking upright, and the other climbing trees, existed between 3 and 4 million years ago:

"The Burtele 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," said Haile-Selassie in a statement.

"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," he added.

Lucy's big toe is aligned with the other four toes, for walking on two legs, like we do. But the Burtele foot apparently has an opposable big toe, like a thumb, allowing the foot to grasp branches. This is like the earlier Ardi, and similar to modern apes.

The researchers write in their paper that the opposing big toe "not only indicates the presence of more than one hominin species at the beginning of the Late Pliocene of eastern Africa, but also indicates the persistence of a species with Ar. ramidus- like locomotor adaptation into the Late Pliocene".

Haile-Selassie said other features of the Burtele foot show it did not belong to an ape, confirming that it is is truly a hominin, report Nature News, who also report that Daniel Lieberman, an anthropologist at Harvard University in Cambridge, Massachusetts, who was not involved in the study, agrees.

Lieberman said the Burtele foot is "very much like the Ardipithecus foot, which I believe had many hominin features, so it's likely to be a hominin".

Co-author and project co-leader Dr Bruce Latimer of Case Western Reserve University said they were shocked by the discovery. They had never seen bones like this 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," he explained.

The researchers haven't been able to assign the Burtele foot to a species because there is no skull or dental elements to go with it.

The foot was found under a layer of sandstone and the researchers used a radioactive dating method called argon-argon to determine its age. The argon-argon method uses the ratio of radioactive potassium-40 in a sample of rock to the amount of its decay product, argon-40, to work out the age of the rock, which in this case was found to be 3.46 million years.

Co-author Dr Beverly Saylor, also of Case Western Reserve University, said nearby fossils of fish, crocodiles and turtles, as well as the physical and chemical properties of the sediments, indicate the surrounding area was once a "mosaic of river and delta channels adjacent to an open woodland of trees and bushes".

"This fits with the fossil, which strongly indicates a hominin adapted to living in trees, at the same time 'Lucy' was living on land," she added.

The finding adds weight to the idea that human evolution is not a simple linear progression from apes but a more complex affair.

Written by Catharine Paddock PhD