A new fossil may force anthropologists to rewrite the evolutionary history of how early ancestors of humans first began walking upright on two legs.
For decades evolutionist scientists have believed that our upright posture evolved in a common ancestor shared with the great apes including chimpanzees, gorillas and orangutans around 15 million years ago.
This led to the broad torsos and mobile forelimbs that appear all great apes – known as the orthograde body plan.
But a 12.3 million year old fossilised hipbone belonging a prehistoric ape named Sivapithecus indicus discovered in Siwalik, Pakistan, is challenging this belief.
This species was thought to be an ancient relative of the orangutan that emerged after the great apes split from the gibbons and lived around 12 million to 10 million years ago
Fossilised skull fragments have suggested it had the facial features similar to modern orangutans and scientists assumed it would also have an ape-like body plan.
However, the new hipbone has revealed that these creatures had a narrow torso that more resembles those of monkeys.
This suggests that Sivapithecus may have had both ape-like and monkey-like features, and has left researchers baffled about how it fits into the evolutionary tree.
If it is indeed an ancestor of the orangutan, then it could mean that the upright ape-like body plan evolved at least twice in the past – once in orangutans and once in other great apes.
Dr Michele Morgan, cruator of osteology and paleoanthropology at the Peabody Museum of Archaeology and Ethnology at Harvard University, said: ‘We always thought if we found this body part, that it would show some of the features we find in the living great apes.
‘To find something like this was surprising.
‘Today, all the living great apes—gorillas, orangutans, chimps—have very broad torsos.
‘People had commonly thought that this torso shape was shared among all the great apes, meaning it must have evolved in a common ancestor.
‘We initially believed that Sivapithecus was on the orangutan line, but if that is the case, then the great ape body shape would have had to evolve at least twice.’
The researchers, whose work is published in the Proceedings of the National Academy of Science, used the new hipbone fossil along with other pieces of fossilised skeleton discovered in the past to reconstruct how Sivapithecus may have moved.
The believe that it was probably a slow-moving and deliberate creature that lived in the trees.
They say it may have moved on all fours above supports but perhaps frequently used branches to help support its body in much the same way as modern orangutans do.
There are a number of competing theories for how our ape ancestors first began walking on two legs.
Some biologists believe it was a natural progression as ancient species of ape began using branches to help support their weight.
Others say they used water to help support their weight as they began foraging for food in rivers and pools in the forests, much like gorillas, bonobos and chimpanzees still do today.
However, Dr Lawrence Flynn, assistant director of the American School of Prehistoric Research at the Peabody Museum who was also involved in the research, said that the latest findings suggest that the evolutionary tree of the great apes was probably far more complicated than previously believed.
He said: ‘I think we sometimes take the easy route of trying to understand these fossils based on creatures we find today.
‘What we’re finding out time and again is these 10- or 12- or 15-million-year-old creatures were their own entities. Today is not always a very good model for the past.
‘What this speaks to is a rich tree with a lot of branches. There are not just one or two branches that reach back into the Miocene. It’s a very rich and complex tree.’
