Washington, Mar 21 : A six million year old fossil has anthropologists suggesting that the species may have been the earliest known hominin to walk upright.
The fossil, belonging to the species called Orrorin tugenensis, was discovered iin the hills of northwest Kenya in 2000.
It showed that the species existed during a critical period in the human evolutionary timeline.
According to scientists, the genetic differences that led to the separation of human and chimpanzee lineages took place between five and eight million years ago.
Since the fossil's discovery, scientists have been debating whether O. tugenensis was upright-walking human ancestor or an ape. Walking on two legs or bipedalism, is often seen as a first fundamental step in human evolution.
In the current study, Richmond and co-author William Jungers of Stony Brook University set out to determine if O. tugenensis was bipedal.
To determine this, the two researchers measured telltale indicators of bipedalism, such as joint size and thighbone shaft strength. These were then compared to other early hominin fossils, living apes, and bones from about 130 modern humans from around the world.
The anthropologists found that the specie's femur, or thighbone, differed from that of modern humans and living apes. However, it was surprisingly similar to early hominids like Australopithecus that lived three to four million years later.
"This provides really solid evidence that these fossils actually belong to an upright-walking early human ancestor," National Geographic quoted lead author Brian Richmond, a biological anthropologist at George Washington University in Washington, D.C, as saying.
"The overall theme of upright walking seems to have stayed fairly consistent as a successful strategy for about four million years, which is most of our evolutionary history," Richmond said.
Arizona State University's Johanson added: "This suggests that you don't immediately become a modern, efficient biped all at once. "As is so often the case in the evolution of any mammal, it happens in stages, and it's interesting to see the sequence of those events."
The current research also contradicts the theory suggested by the fossil's discoverers, Martin Pickford and Brigitte Senut of the College de France, that O. tugenensis was a direct ancestor of the genus Homo, even though that genus doesn't appear in the fossil family tree until about two million years ago.
Pickford and Senut's theory suggested that hominins that lived from six million to two million years ago were not ancestors of modern humans but merely a now-extinct branch of our family tree.
Richmond says that the analysis conducted by him and Jungers shows that O. tugenensis was most likely the ancestor of early hominins.
"Our analysis shows that these fossils resemble early hominin fossils more than they resemble Homo at two million years ago," he said.
"It is likely to be ancestral to these early forms, not requiring a ghost lineage that goes undiscovered for four million years [until Homo's appearance]."
He also agrees that O. tugenensis, like other early human ancestors, was capable of climbing trees.
"The upper limb looks very much like a chimp's does today," he said.
"That points to the idea that O. tugenensis still had a powerful upper limb used in climbing trees. It probably did that regularly to access food, to sleep at night, or to escape predators."
Richmond's results appear in the journal Science.