Orang-utan study suggests that upright walking may have started in the trees

A common theory of human evolution says that after our ancestors descended from the trees, they went form walking on four legs to two. But a new study in orang-utans could overturn that theory, by suggesting that our ancestors evolved a bipedal walk while they were still in the trees.

Did and upright posture evolve in a tree-dwelling ancestor?Walking on two legs, or bipedalism, immediately sets us apart form other apes. It frees our arms for using tools and weapons and is a key part of our evolutionary success. Scientists have put forward a few theories to explain how our upright gait evolved, but the ‘savannah theory’ is by far the most prolific.

It’s nicely illustrated by this misleading image that has become a mainstay of popular culture. It suggests that our ancestors went from four legs to two via the four-legged knuckle-walking gait of gorillas and chimps. Dwindling forests eventually pushed them from knuckle-walking to a full upright posture. This stance is more efficient over long distances and allowed our ancestors to travel across open savannahs.

But this theory fails in the light of new fossils which push back the first appearance of bipedalism to a time before the forests thinned, and even before our ancestors split from those of chimpanzees. Very early hominins, including Lucy (Australopithecus afarensis) and Millennium Man (Orrorin) certainly ambled along on two legs, but they did so through woodland not plains.

Our arms provide a further clue. Even though our ancestors’ back legs quickly picked up adaptations for bipedalism, they steadfastly kept long, grasping arms, an adaptation more suited to moving through branches. To Susannah Thorpe at the University of Birmingham, these are signs that bipedalism evolved while our ancestors were still living in trees.

Two legs good, four legs bad?

Orang-utans can go bipedal and our ancestors may well have done the same in the trees.But there is a snag – an adaptation must provide some sort of benefit. And, as many children painfully discover, it is hard to imagine how walking on two legs could benefit sometime in a tree.

But Thorpe has an answer to this too. She spent a year in the Sumatran jungle, studying the orang-utan – the only great ape to spend the majority of its life in the trees.

She carefully documented over 3,000 sightings of wild orang-utans moving through the treetops. On large sturdy branches, they walk on all fours (below right), and on medium-sized ones, they start to use their arms to support their weight.

But on the thinnest and most unstable branches, the apes use a posture that Thorpe calls ‘assisted bipedalism’ (below left). They grip multiple branches with their long, prehensile toes and use their arms to balance and transfer their weight. And unlike chimps which bend their knees while standing up, bipedal orang-utans keep their legs straight, just like humans do.

An orang-utan used both two-legged and four-legged postures.

It’s a win-win posture – the hands provide extra safety, while the two-legged stance frees at least one hand to grab food or extra support. With it, the apes can venture onto the furthest and thinnest branches, which provides them with several advantages.

As Thorpe says, “Bipedalism is used to navigate the smallest branches where the tastiest fruits are, and also to reach further to help cross gaps between trees.” That saves them energy because they don’t have to circle around any gaps, and it saves their lives because they don’t have to descend to the ground. “The Sumatran tiger is down there licking its lips”, she said.

A new view of ape & human evolution

With these strong adaptive benefits, it becomes reasonable to suggest that bipedalism evolved among the branches. Based on this theory, Thorpe, along with Roger Holder and Robin Crompton from the University of Liverpool, have painted an intriguing new picture of ape evolution.

It begins in the same way as many others – with the rainforests of the Miocene epoch (24 to 5 million years ago) becoming increasingly patchy. For tree-dwelling apes, the gaps in the canopy started becoming too big to cross. But in Thorpe’s view, these ancestral apes were already using a bipedal stance, and different groups took it in separate directions.

Our ancestors were bipedal long before they came down from the trees.The ancestors of orang-utans remained in the increasingly fragmented canopy and became specialised and restricted there. The ancestors of chimps and those of gorillas specialised in climbing up and down trees to make use of food both in the canopy and on the ground. The postures used in vertical climbing are actually very similar to those used in four-legged knuckle-walking and this became their walk of choice on the ground.

The ancestors of humans abandoned the trees altogether. They used the bipedal stance that served them well on thin branches to exploit the potential of the stable land environment. Over time, they brought in further adaptations for efficient walking, culminating in the human walking style that we now neglect by sitting at a computer all day.

Thorpe’s reconstruction is delightfully non-human-centric. It suggests that in the evolution of movement, we were conservatives who relied on a walk that had been around for millions of years. Chimps and gorillas with their fancy new knuckle-dragging gait were the true innovators.

Reference: Thorpe, Holder & Crompton. 2007. Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science 316: 1328-1331.

Image: Black and white image from Science magazine.

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Related stuff:
On ape and human evolution:
Chimps show that actions spoke louder than words in language evolution
Hidden ‘junk’ gene separates human brains from chimpanzees
Chimps have more adaptive genetic changes than humans

On the evolution of movement:
Salamander robot walks, swims and sheds light on evolutionary step from sea to land
Microraptor – the dinosaur that flew like a biplane

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7 Responses

  1. Great article! Thanks for posting it!

  2. I was struck by several fact recently that touch upon bipedalism in primates.

    First, I saw a Siamang (brachiating ape – either a gibbon or closely related) at the zoo. I was impressed by the way he walked on two legs even though he held his long arms awkwardly in the air.

    Second, often nature programs show lemurs hopping about on two legs. That’s not bipedalism, but certainly their hind legs are being used beyond the ordinary four-legged gait of most mammals.

    Third, seeing the leaps of “bush babies”, also primates indicates again a long-standing growth in the strength and general usefulness of the hind legs of primates.

    Certainly, all facets of arboreal living have enhanced the flexibility and general adaptability of the hind legs of the entire primate group.

    The orangutan article questioned –why—the chimpanzee and gorilla might have moved away from bipedalism toward an more quadruped gait. I don’t find this surprising. Consider that the new world monkeys are much more arboreal with their prehensile tails than the old world monkeys. I think that the ape group has been moving in various directions regarding an arboreal life style over time including toward a more ground based lifestyle than some primates.

    I would see the human-chimp-gorilla-orang ancestor as being more oriented toward bipedalism than any of the four except humans. The orang was pushed by evolution further toward the arboreal lifestyle abandoning the early flirtation with bipedalism. The chimp and gorilla abandoned bipedalism for the front knuckle walking “re-adaptation” to a quadrupedal gait. Humans took the bipedal gait to its best usage thus far.

    The move of the chimps and gorillas away from a flirtation with bipedalism is a parallel evolution to the movement of new world monkeys from a highly arboreal lifestyle to a more ground exploiting niche.

    This probably has to do with the changes over prehistory in Africa and the more varied environments relative to South America.

  3. Max – great post! I like the comparisons to bushbabies, siamangs and lemurs.

  4. Someone called Tom posted here saying they like orangutans – sorry Tom, I deleted your post by mistake :-)

  5. i do not like evaltion

  6. Beautiful. I was about to moderate against the post above, but the fact that this self-confessed opponent of evolution saw fit to explain their view in less than six words, and can’t even spell the thing they’re against, made me chuckle too much to ignore. I almost suspect this of being a prank.

  7. harlie, what the heck is “evaltion”

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