The News Minute | February 6, 2015 | 7:15 am ISTA 12-million-year-old fossil found in Pakistan may force anthropologists to rewrite the evolutionary history of how early ancestors of humans first began walking upright on two legs, media reported on Thursday.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 in 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 range, Pakistan, is challenging this belief, the Pakistan Today reported. 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."We always thought if we found this body part, that it would show some of the features we find in the living great apes," said Michele Morgan, curator of osteology and paleoanthropology at the Peabody Museum of Archaeology and Ethnology at Harvard University."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," he said.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, 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.IANSTweetFollow @thenewsminute