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Australopithecus sediba has taken us on a rollercoaster of a ride. First discovered back in 2008, this South African hominin looked like it might be a close relative of ours, perhaps even an ancestor. It combined features seen in earlier, chimp-like Australopithecus and our own genus Homo; prompting countless newspapers to report it as the “missing link”.
However, it’s status as the missing “apeman” was short-lived. In 2011 later it was discovered Australopithecus sediba was only ~1.9 million years old1. Since the first members of Homo lived over 2 million years ago2, it obviously couldn’t have been their ancestor (unless they had a Delorean. That always leads to weird family relations).
But still, that mixture of traits means they were still a close relative. Right?
Stitching together Australopithecus sediba
Since them, follow up research has continued to find Australopithecus sediba had a combination of features from classic Australopithecus and more recent Homo. Like a modern mouth with some ape traits. When wanting to sound fancy, scientists refer to such a mixture as a “mosaic”3.
Perhaps the best examples of this come from their arm; which was long like an ape so they could grab branches4. However, their fingers were short like ours5; which are less great for grabbing but good for tool use. Was this a sign Australopithecus sediba was good with their hands?
Almost every aspect of Au. sediba‘s has this mix of human and Australopith traits; from the jaw through to their backbone. Put them all together and you wind up with image below. Definitely a chimp-like Australopith, but with some key human traits3.
With one key exception: how they walked.
Australopithecus sediba struts!
Australopithecus sediba walked on two legs, like every known member of our family. However, their gait was unique. As their foot hit the ground it twisted, so the outside edge landed first. This is called “hyperpronation” and no other members of our family walk like this6.
Well, some modern people actually do. The resulting damage to their joints was also found in Au. sediba fossils; confirming this was also how they walked. But whilst only some modern people have hyperpronation, every Au. sediba fossil we’ve found did6.
As such, it looks like this was their normal way of walking; not a medical condition. What’s up with that?
Given that most other hominins walk the same, this could mean Australopithecus sediba‘s branch split off early; before our normal gait evolved. This would have happened early in our revolution, potentially pushing their origins back to before even Lucy lived7!
If this is the case then what’s the deal with all of their similarities to us? Were they independent adaptations in a lineage that was evolving along a similar path to us?
Although that’s a fascinating possibility, not all the researchers agree. The sheer number of traits they share with us has led some to conclude they have to be a close relative. Maybe then, they were an “evolutionary experiment” with bipedalism. A weird, short-lived offshoot that tried something different8.
Or maybe something about their lifestyle made them extremely likely to develop foot problems. Without more fossils to study, a mundane explanation like that remains plausible.
- Pickering et al, (2011). Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo. Science 333, 1421
- Aiello, L. C., & Wells, J. C. (2002). Energetics and the evolution of the genus Homo. Annual Review of Anthropology, 323-338.
- Berger, L. R. (2013). The Mosaic Nature of Australopithecus sediba. Science,340(6129), 163-165.
- Churchill, S. E., Holliday, T. W., Carlson, K. J., Jashashvili, T., Macias, M. E., Mathews, S., … & Berger, L. R. (2013). The upper limb of Australopithecus sediba. Science, 340(6129).
- Kivell, T. L., Kibii, J. M., Churchill, S. E., Schmid, P., & Berger, L. R. (2011). Australopithecus sediba hand demonstrates mosaic evolution of locomotor and manipulative abilities. Science, 333(6048), 1411-1417
- DeSilva, J.M., Holt, K.G., Churchill, S.E., Carlson, K.J., Walker, C.S., Zipfel, B. and Berger, L.R., 2013. The lower limb and mechanics of walking in Australopithecus sediba. Science, 340(6129), p.1232999.
- Schmid, P., Churchill, S.E., Nalla, S., Weissen, E., Carlson, K.J., de Ruiter, D.J. and Berger, L.R., 2013. Mosaic morphology in the thorax of Australopithecus sediba. Science, 340(6129), p.1234598.
- Irish, J. D., Guatelli-Steinberg, D., Legge, S. S., de Ruiter, D. J., & Berger, L. R. (2013). Dental Morphology and the Phylogenetic “Place” of Australopithecus sediba. Science 340(6129)