The human family is a weird bunch, with many strange offshoots. But perhaps one of the weirdest is Australopithecus sediba. It walked with a strange, twisted gait, unlike anything we'd seen before. Now, this 2 million-year-old relative is getting weirder. New research revealed they were swinging from the trees!
This might sound like a typically ape-like thing to do. So what's the big deal? Well, it comes after new evidence suggests that swinging from the trees was a fairly uncommon behaviour in our family. It looks like Au. sediba acted strange both on the ground and in the trees.
The black sheep of the family
Australopithecus sediba was announced to the world back in 2010. Initially, it seemed like a "new Lucy" (although I'd never cheat on my bae like that). A beautifully preserved fossil capturing a critical moment in our evolution: the last moments of our ape-like ancestors Australopithecus.
However, things started getting strange almost straight away. For starters, it was dated to ~2 million years ago. This means it was living at the same time as Homo erectus. So it can't have been that closely related to our direct ancestors. Instead, it was a late-surviving member of Australopithecus. Which is very interesting in its own right, and raises the question of how did it differ from earlier Australopithecus?
More and more evidence began suggesting that it differed in key ways. Notably, it seems to have had a strange gait. Their feet twisted outwards as they moved. This is something that only happens pathologically in modern humans and can put a lot of strain on the joints.
This is quite a big deal when it comes to evolution, as just about every other member of our family has a fairly similar way of walking upright (if it walked upright). Going against such a well-established pattern is quite a bit deal.
Australopithecus sediba: the only swinger?
Now there's more evidence suggesting Australopithecus sediba was weird. An analysis of their arm suggests the hung from tree limbs and swung from branch to branch. This is fairly common behaviour for an ape, and is an orangutan's favourite way of navigating through the tree tops.
In fact, the arm of Au. sediba turns out to be most similar to an orangutan's. Like them, it had a long, strong forearm. They both also had a similar set-up in the elbow, which would keep the joint stable through a wide range of motion. Many other members of Australopithecus also had some of these features. For example, the arms of the genus are famously long in all cases. In fact, it's common in most apes. But Australopithecus sediba really dials it all up to 11.
These climbing forelimbs are particularly notable when compared to Lucy's species, Australopithecus afarensis. This would be significant all on its own, suggesting that some significant arm evolution happened over the couple of million years that separated them.
However, this work comes just after research into Lucy et al.'s shoulder. This suggested that the evidence they were hanging below trees is a lot less substantial than it was thought to be. It's possible they still did it, but the evidence has gone from pretty clear cut to circumstantial at best. This means the evolution between them isn't just a bit of "tinkering" but possibly a fundamental change to how our lineage moved through the trees.
Diversity in our family
Australopithecus sediba is unusual. Which is pretty usual. Our family is a diverse bunch with all sorts of weird specialisations and offshoots. Even we are far from normal, being the only species in our family with a notable forehead and chin. This work simply identifies that there was even more variation than we thought. But crucially, it's variation in fairly fundamental stuff, like locomotion.
References
Berger, L. R. (2013). The Mosaic Nature of Australopithecus sediba. Science, 340(6129), 163-165.
Rein, T.R., Harrison, T., Carlson, K.J. and Harvati, K., 2017. Adaptation to suspensory locomotion in Australopithecus sediba. Journal of Human Evolution, 104, pp.1-12.
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.
