Why do we walk upright?

Walking upright is one of the defining features of humans, separating us from the rest of the living apes. So it’s hardly surprising that the origins of bipedalism are a subject of great interest to palaeoanthropologists. Even I’ve probably spent more time dwelling on the subject than is healthy (particularly when arguing with Answers in Genesis. That’s never good for you). Yet despite all of these posts, I’ve never really addressed the ultimate question: just why do we walk upright?

Dart’s discovery, Taung Child, was the first hint something was wrong with the infamous Piltdown Man

This is a question that has been perplexing evolutionary anthropologists since Darwin himself1, but – like so many tv shows – didn’t get interesting for quite a while. In the 1920s Raymond Dart was working in South Africa, where he discovered the first fossils of Australopithecus, a 4 – 2 million year old, ape-like human ancestor2. This place the origin of our species in Africa and with it scientists could really get cracking on figuring out why we’re bipedal.

Dart’s own hypothesis was that it was linked to the African Savannah. Our non-bipedal ancestors moved out of the forest where all the other apes lived into this exciting new environment. There, they encountered some evolutionary pressure that drove them to become bipedal3.

Since Dart scientists have debated just what that pressure was. Some think it was the heat of the sun. By walking upright our ancestors would’ve exposed less surface area to that fiery ball of death than if they were walking on all fours4 (an idea closely linked to the thermoregulatory explanations for our lack of hair). Others thought the added height of walking upright would’ve allowed us to see further and spot (and thus avoid) predators5. The list goes on and I’ve got better things to do than type it all out.

However, some researchers question whether or not a non-bipedal hominin could survive on the open Savannah (a hominin, fyi, is a human member of the human family that lived after we split from chimps). If there truly was such a strong push to become bipedal, would the intermediate stages have been able to survive? An animal that is dying of heat stroke or a case of lion around the throat isn’t going to do much evolving after all6.

Living in the cliffs is how modern Savannah dwelling primates, like these baboons, survive

Living in the cliffs is how modern Savannah primates, like these chacma baboons, survive

Enter the recent “complex topography hypothesis” (or if you want to sound less pretentious, the “hiding in the rocks hypothesis”). The CTH argues that our ancestors lived on the sides of cliffs or rocky outcrops where they would be protected from the worst the Savannah had to offer. However, they would’ve still spent enough time on the ground gathering food (maybe even occasionally venturing out onto the deadly deadly Savannah) for evolution to push them towards upright walking6.

The CTH has some interesting implications. For example, early bipeds (like Australopihtecus) retained many ape-like features; such as long arms with curved fingers. These were thought to be adaptations for tree climbing retained by a hominin that hadn’t fully abandoned the trees, since chimps use their long curved fingers for grabbing onto branches and all that fun stuff. But under the CTH these traits were actually used by our ancestors to scamper over rocks and up cliffs! It may also help explain some of our other key attributes, like large brains, which the CTH researchers claim may have evolved to help us navigate through the complex landscape6.

The CTH is so new that it hasn’t been properly dissected by the rest of the scientific community yet, so I can offer no academic disembowlings a la Richmond & Strait. In fact, at the time of writing only 2 other articles have mentioned it, according to Google Scholar (the most prestigious of all academic websites), one of which is a news article about the hypothesis!

However, I do have some problems of my own with the CTH. The main one is that even our earliest ancestors were at least partly bipedal, predating our migration into the Savannah (or any rocky environment within the Savannah). This is something the CTH can’t explain, and that the paper on it conveniently ignores. They make no reference of any fossils older than ~4 million years ago6, whilst the earliest evidence of bipedalism stretches back 7 million years5!

Sahelanthropus tchadensis, the first member of the human family since we split from chimps, lived in a mixture of woodland, desert and swamp5,7 yet has a foramen magnum that suggests it was already upright8. In fact all of the early hominins had some bipedal capacity whilst still living in the forest. It would seem the story of human evolution is one of us getting good at walking upright, rather than inventing it from scratch somewhere on the Savannah.

An orang-utan walking along a branch

An orang-utan walking along a branch

This has led many scientists to conclude that the forests these early bipeds were living in is the key to this whole debate. After watching orang-utans move through the trees, a team of scientists discovered that they tend to “walk” over thin branches in a manner very similar to how we walk along the ground9. It appears that life in the trees prepared our ancestors for upright walking, making the transition to life on the ground relatively easy.

So when they eventually wound up on the Savannah they wouldn’t have been immediately murdered by lions, but could survive long enough for other evolutionary pressures (perhaps those identified by Dart, all those years ago) to take effect. Our ancestors became better and better bipeds, refining the design they developed in the trees 7 million years ago until they abandoned the forests entirely. By about 2 million years ago the modern human body plan had been perfected in the form of Homo erectus (Latin for “upright man”).

And the rest, as they say, is (pre)history.

References & Notes

  1. Hawks, 2005. Why be bipedal? Accesed 15/10/13 from http://johnhawks.net/weblog/topics/bipedalism/why_be_bipedal.html
  2. Klein, R. G. (1989). The human career. Chicago: University of Chicago Press.
  3. DART, R.A. 1925. Australopithecus africanus: the man-ape of South Africa. Nature 115: 195–99

  4. Ruxton, G. D., & Wilkinson, D. M. (2011). Avoidance of overheating and selection for both hair loss and bipedality in hominins. Proceedings of the National Academy of Sciences108(52), 20965-20969.
  5. Boyd, R., Silk, J. B., Walker, P. L., & Hagen, E. H. (2000). How humans evolved New York: WW Norton.
  6. Winder, I. C., King, G. C., Deves, M., & Bailey, G. N. (2013). Complex topography and human evolution: the missing link. Antiquity87, 333-349.
  7. Note: The environmental record can be formed over thousands (if not millions) of years, so just because those 3 environments have been found with Sahelanthropus doesn’t mean they all existed at the same time
  8. Russo, G. A., & Kirk, E. C. (2013). Foramen magnum position in bipedal mammals. Journal of human evolution.
  9. Thorpe, S. K., Holder, R. L., & Crompton, R. H. (2007). Origin of human bipedalism as an adaptation for locomotion on flexible branches. Science,316(5829), 1328-1331.

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16 thoughts on “Why do we walk upright?”

  1. Jim Birch says:

    Bipedalism is such a massive remodelling of the body that there’s zero chance it coming as a big bang, even a slow big bang. So the adaptive pressures that initiated it would very likely be different to the ones that refined it as we progressed through niches. Branch walking looks a good first step, doesn’t it?

    1. Adam Benton says:

      Certainly that seems to the case, and most hypotheses do tent to suggest that as hominins became increasingly bipedal they spread into new environments where they encountered new pressures that drove them to further refine bipedalism. Like with the furlessness last week, there probably is no silver bullet explanation and (unlike many of the “all or nothing” ideas in other areas of evoanth) most people in this field seem to understand that. They’re looking for the initial or dominant force that really got the thing going, rather than sole mover of moving upright.

  2. Artem Kaznatcheev says:

    The CTH does certainly have some compelling evidence to support it, such as the fact that a good proportion of hominin fossils are found in these “topographically complex” environments. More than can be explained by chance.

    Could it be that “topographically complex” environments are better at retaining fossils than say a desert? Is that accounted for the the “more than chance” part?

    1. Adam Benton says:

      The CTH paper claims that taphonomy cannot explain the bias towards complex topography, hence my “more than chance” comment. However, the paper at they site notes “the sample of sites that we have so far examined is still quite small and that it is not yet possible to … to eliminate sources of potential bias such as differential preservation and visibility of sites or differential intensity of investigation”.

      That’s not compelling evidence, that’s misrepresenting research. I’m cutting that phrase from the post.

  3. Jim Thomerson says:

    Perhaps increasing bipedialism is linked with two things. First would be the development of hard accurate throwing, which would work better for a biped. Second would be the development of tools, which a biped could better carry around for use..

    1. Adam Benton says:

      However, as I mention, bipedality appears to have been selected for very early in the human lineage. Likely before our use of tools was significantly different from how chimps use it.

  4. andre salzmann says:

    Its you who said it is interesting to speculate about some archeological questions, Adam.
    This one surely is.

    Upright walking must relate to a multitude of factors. What is known, based on data collected and analyzed
    concerning the disappearance of the Neanderthal, climate change, should most probably be correlated
    to the changes we know of in the hominids. So as to apply those facts to speculations like this, as we have relatively little data. Of course the two situations differ seriously as well. Over the million years it took the
    hominids to descend from the trees, climate change must have played a significant roll. Varying this and that way. Forming and destroying savannas,forrests and swamps perhaps.

    Reading about the Vadoma and Twaka peoples, who still live today, should also help to find fact based
    insight into this question. To much to relate here. But it actually seems, reading on them, that the tree
    climbing “gene” has not totally left today’s humans. And they are not quiet hominids either.

    You wrote on the disappearance of our body hair. If the hominids were also almost black haired like today’s
    African great apes originally, chimps and gorillas, and their hair disappeared at some point in time, and they were not so well camouflaged in the forests, what would have happened ? Is the black coat of the apes a forest friendly survival adaption ? Could merit some consideration?

    As the climate changes, diets would have changed. There is quite significant difference between the colons of the chimp and homo sapiens. Modern science says that the matter the colon is constructed of, is similar to the matter the brain consists of. I am under correction, but read that somewhere. If so and diets changed, what did that do to brain development. Lets guess that early hominids diet changed to eating more nuts at some stage. We do have evidence of larger molars at some stage. What did brain development do to adaptive abilities? Being able to remember more food types perhaps. Being more dextrous with ones hands. Being more capable socially. Forming more reliable groups to withstand threats? Could then venture further into the flats.

    In nature all elements survive in symbiosis with others. Consider the possibility that, for any one of a number of reasons, bird life changed drastically in the area the hominids lived. Say their intake of protein (eggs) altered drastically then. What then. Possible changes in muscle development ? Possibly more egg eating snakes in their environment ? Swamps might have formed or disappeared. Rivers changed their course. Could fish have become a diet somewhere ? In other words, not only was there climate change but there could also have been geological/topographical changes that affected them ? Volcano’s. We know it happened.

    Only constant we have in this respect is that the hominids kept on getting better at survival. Leaving the forest must have been one issue. Being capable of bipedal motion another. Adapting to the flats possibly another. But
    most likely all of it was interrelated.

    Lucky for us. all of this. To think that all of that has led to me being able to type on a computer now.

  5. Algis Kuliukas says:

    Er…. wading? The one place 100% guaranteed to induce an otherwise quadrupedal great ape cousin to move (not just pose momentarily) bipedally is in waist deep water. Don’t tell me – space apes are more likely, right?

    1. Adam Benton says:

      Actually, the foot is the one thing space apes can’t explain. Since you can’t guarantee you’ll always be orientated “up” in microgravity, surely it would be beneficial to retain an opposable big toe to grasp onto things when you’re accidentally flipped upside down.

      Personally, that’s why I don’t buy into it.

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  16. leorivers says:

    The arboreal 3-D web of interlace boughs all in motion in the wind is a complex and environment as you might want. Monkey bars are a lattice with depth perception and a recognition of porous surfaces to exercise your brain. I don’t thin rough terrain or even ocean tops that.

    1. Adam Benton says:

      As the somewhat skeptical tone of this article indicates, I am in complete agreement.

Leave your filthy monkey comments here.

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