Biology Magazine

Lucy Wasn’t a (tree) Swinger After All

Posted on the 16 March 2017 by Reprieve @EvoAnth

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.

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.

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?

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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