We might have misunderstood one of the greatest mysteries of human evolution. For decades, its been unclear why we began walking on two legs instead of moving on all fours as it is assumed our great ape ancestors did. But new evidence suggests the first great apes may have moved on two legs a little like we do.
This means the real mystery is not why we walk on two legs, but why we stuck with this walking style while the other living great apes dropped down to all fours.
Humans are one of four major forms of great ape today. Animals in the other three groups – the chimps and bonobos, the gorillas, and the orangutansĚý– typically use all four limbs when they move around on the ground.
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Researchers have always thought that walking on all fours is the primitive condition, and hominins were the only great apes that evolved to walk on two legs. But Carol Ward at the University of Missouri says Earth has been home to dozens of great ape species over the past 20 million years, and the more we learn about them, the more it seems that today’s four-legged great apes are the unusual ones. “The modern great apes – like chimpanzees – are real oddballs,” she says.
The latest evidence comes from an early great ape called Rudapithecus that lived in what is now Hungary about 10 million years ago. Ward and her colleagues, including David Begun at the University of Toronto, have spent several years carefully examining a Rudapithecus pelvis that Begun unearthed in 2006. Their analysis shows that it was surprisingly like ours in one important respect: the distance between the hip sockets and the point the pelvis contacts the base of the spine was relatively short.
Ward says this implies that Rudapithecus had a long flexible lower back like ours. Because of the way our spine flexes below the rib cage, we can curve our lower back and bring the weight of our torso over our hips for easy upright walking. Ward suspects Rudapithecus could do this too to some degree, implying it could walk on two legs.
In our living great ape relatives the . “The pelvis almost reaches the rib cage,” says Ward. “It’s a huge difference.” This means chimpanzees and the other living great apes have a rigid lower back that can’t flex and curve, making walking on two legs awkward.
We’ve already seen a short pelvis, indicating a flexible lower spine, in early hominins including the 4.4-million-year-old Ardipithecus. “The reason Rudapithecus is so important is that it’s much older than Ardipithecus and the other hominins,” says Ward. This suggests that the short pelvis we see in Ardipithecus wasn’t a unique feature that first appeared in the hominins. It was a trait with deep roots stretching back to the earliest great apes that lived at least 10 million years ago.
Read more: Who are you? How the story of human origins is being rewritten
Ward says these ideas have been “lurking around in the shadows” for some time. For instance, a 2017 study by Sergio Almécija and Ashley Hammond, both now at the American Museum of Natural History in New York, across living apes to predict pelvis shape at various points in the evolutionary tree. They concluded that the earliest great apes lacked a chimp-like long pelvis. But Ward says that we can have much more confidence in such conclusions when they are based on real fossils.
All this means we may have been asking the wrong questions about the evolution of our walking style. The real puzzle is not why we walk on two legs, but why our living great ape cousins don’t.
Size matters
Ward thinks the answer comes down to size. Rudapithecus was smaller than a chimp, and it probably spent most of its time in trees, holding its back vertically as it walked on and hung below branches. But heavier animals that do the same would risk damaging their backs, Ward says. She reckons this is why chimps, gorillas and orangutans independently evolved a taller pelvis to stiffen the back and protect the spine.
Hominins are large-bodied apes too though, so why did we stick to two legs? Ward suspects it’s just that our ancestors had evolved to spend less time in the trees before their bodies evolved to be larger. This means that when they did grow bigger there was no need to stiffen the back to protect the spine from the stress of hanging from branches.
Almécija thinks that’s a plausible idea. “I think that the earliest hominins just found an alternative way to move on the ground while having an erect body posture,” he says. “Give me a time machine and I’ll test it.”
Journal of Human Evolution
