Lucy wasn’t a (tree) swinger after all

For a long time the evidence suggested that lucy was a tree swinger. However, a re-evaluation of shoulders suggest this isn’t the case


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Modern humans are very peculiar. In particular, we have this weird habit of walking upright on the ground. Although this form of locomotion now defines us, for most of our evolution we didn’t use it. Instead, our ancestors spent a lot of time climbing through the trees. Understanding how early hominins climbed is crucial. After all, how else will we figure out why they stopped? A lot of current evidence suggests she was a swinger, dangling below branches on long arms held above her head. But new evidence is challenging this view.

Lucy the swinger

The famous Lucy fossil plays a key role in this story. Her shoulder blade is one of the best preserved from our climbing period. Notably, her shoulder socket is orientated upwards. Since its discovery, this has been viewed as an adaptation to help the arms move above the head. It was thought that she was a swinger, dangling below branches with the aid of her unique shoulder joint.

Lucy belongs to Australopithecus afarensis, a crucial species for understanding our shift out of the trees. This is because they have many adaptations for an arboreal lifestyle (like the aforementioned shoulder). But they also have some features well suited for a life on the ground. In fact, the species spent enough time on the ground to leave behind the oldest footprints of any hominin; creating the famous fossil tracks at Laetoli.

In other words, it seems that Lucy and her kin were on the cusp of transitioning between the two. Understanding how she moved in the trees could help reveal how we began our transition to life on the ground. And it seems that she was a swinger.

Other fossils from this species appear to confirm this pattern. The Dikika child is a three-year-old Au. afarensis that is beautifully preserved. In fact, the kid has a one up on Lucy, as both of their shoulders are preserved compared to Lucy’s single shoulder (and an incomplete one at that). There’s also Kadanuumuu, a large male of the species. He only has the one scapula left, but it is arguably even better preserved than the Dikika child. Despite this variation in age and completeness, this extra evidence tells the same story. The shoulder joint is orientated upwards, for swinging.

Dikika’s shulder compared to other species, showing a slightly more upward orientated socket than modern humans

Changing the story

Of course, there’s no denying that Lucy and her species was a climber. The species had a long upper limb with curved fingers. Great for grasping onto branches. Their toes also showed greater curvature, which would also have helped with climbing. But whilst we might have accurately figured out that they climb, how they climbed is once again a topic for debate.

This challenge comes from a review of other primate shoulders. These confirm that Lucy and friends did indeed have a shoulder socket orientated a bit more towards the head than modern humans. However, it turns out that this difference might not be as significant as once thought.

Notably, the research found that many other primates have a similar orientation, yet don’t engage in swinging behaviour. As such, we can’t reliably say that this species did either. This key “indiactor” of swinging turns out to be nothing of the sort. In particular, Colbus monkeys (who spend most of their time on top of branches rather than swinging underneath) have a similar angle of shoulder to Australopithecus afarensis.

The orientation of the shoulder in many species. Humans and our fossil ancestors are on the far left. Although Lucy (AL 288-1) is different to us, it isn’t different to other non-swingers

Implications for upright walking

As I said, understanding how early hominins moved is important for understanding why they changed. Why did they make this switch from climbing – however they did actually climb – to bipedalism?

One idea growing in popularity is that bipedalism actually started out in the trees. Perhaps our ancestors “walked” along branches. As they began to find themselves exploiting more open environments, this form of climbing was easily adapted to life on the ground. It continued to adapt and evolve to better perform out of the trees until you wind up with the fully modern bipedalism we all know and love.

This model takes its inspiration from some modern primates, like orangutans, who also find “walking” in the trees to be useful. Earlier fossils support it, showing that even the most arboreal of our family still seemed to have some adaptations for bipedalism. What if it was because they were bipedal in an arboreal context?

An orang-utan walking along a branch

If Lucy wasn’t a swinger then perhaps she also fit into this pattern, using her bipedal capabilities in the trees and on the ground. In the process, she was laying the groundwork (pun intended) for the terrestrial lifestyle we now know and love.

References

Green, D.J. and Alemseged, Z., 2012. Australopithecus afarensis scapular ontogeny, function, and the role of climbing in human evolution. Science, 338(6106), pp.514-517.

Haile-Selassie, Y., Latimer, B.M., Alene, M., Deino, A.L., Gibert, L., Melillo, S.M., Saylor, B.Z., Scott, G.R. and Lovejoy, C.O., 2010. An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proceedings of the National Academy of Sciences, 107(27), pp.12121-12126.

Lovejoy CO, & McCollum MA (2010). Spinopelvic pathways to bipedality: why no hominids ever relied on a bent-hip-bent-knee gait. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 365 (1556), 3289-99

Selby, M.S. and Lovejoy, C.O., 2017. Evolution of the hominoid scapula and its implications for earliest hominid locomotion. American Journal of Physical Anthropology.

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12 thoughts on “Lucy wasn’t a (tree) swinger after all”

  1. Belac says:

    Always thought she was more of a climber than a swinger. Good to know I was on the right track.

  2. clayton says:

    The arboreal bipedal ism makes sense to anyone who’s read any of the original Tarzan books by Edgar Rice Burroughs. I think he even made fun of the vine-swinging in some of the books, as he was still writing them when the early movies came out. Sure would be awesome to see how an Australopithecus would move—I bet they would look similar to chimps in the way they hunt, but move like men…extremely agile and strong, little men. Wow. Someone needs to make a movie.

    1. Adam Benton says:

      There’s been a lot of work trying to recreate how she walked on the ground. That’s what a lot of people are interested in. Plus we have the Laetoli footprints so we can catch her “in action” so to speak. Sadly a bit less effort has been put on the climbing, hence why seemingly basic discoveries are only coming out more than 30 years after she was found. You’d have thought a comparison with other primates would have been something that was done earlier!

  3. Carol Kellogg says:

    Engaging birth walker hypothesis
    Need for longer pregnancy prompted bony landmarks of fetal head to wedge in maternal pelvis. Upon walking prompted by labor contractions, angles changed, fetal head engaged, a cardinal stage of labor, head descended onward to birth!

    1. Adam Benton says:

      That’s an interesting idea. Are there any citations expanding on it you can give?

      My one qualm would be that there doesn’t seem to be a massively compelling link between encephalization and bipedalism. Early competend bipeds, like the Dmanisi fossils, only have marginally bigger brains than earlier species.

  4. Brett Martin says:

    The fossil tracks at Laetoli were made by something that appeared to have anatomically modern feet, and a human walking gate, Lucy had neither of these, although you would need to find another specimen to examine the feet. Both knees and hips would indicate Lucy to be just like a monkey also.

    1. Adam Benton says:

      Lucy’s hip is markedly wider and deeper than the ape norm, an adaptation for supporting the weight of an upright body. Her knee curves inwards, like ours, bringing that weight over the midline for better balance. Her foot features an adducted toe, to push off with, and an early arch for propulsion. All of this adds up to a competent biped who could walk with a gait not miles apart from our own; the same story the Laetoli footprints tell.

  5. Belac says:

    Just found this and thought it might be worth your time. It’s a new book on early hominin evolution which argues that our LCA with chimps was chimp-like rather than anything else. I personally think our LCA was more of an orthograde climber than a knuckle-dragger, but it’s still interesting.
    https://www.amazon.com/gp/aw/d/067496795X/ref=mp_s_a_1_11?ie=UTF8&qid=1489893835&sr=8-11&pi=CB192198896_AA75_QL70&keywords=david+pilbeam

    1. Adam Benton says:

      I mean yes, chimps probably are one of the best models out there. Similar environment, anatomy, and cranial capacity all indicate they’re useful models. However, just because they might be the “best” model doesn’t make them perfect. There are still key differences, so you still need to be careful when extrapolating from them. Having not read the book I can’t say if it’s appropriately cautious. However, the blurb on amazon doesn’t seem to be.

    1. Adam Benton says:

      What don’t you like about what he’s saying?

      1. Belac says:

        Nevermind, the title misled me and by the time I realized it wasn’t saying what I thought it was saying, I had already sent the comment. Sorry about that.

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