The human family contains around 20 species. Some look recognisably human. Others, less so. But Australopithecus sediba has to take the cake for balancing the two in the weirdest way possible. This 1.9 million-year-old species shared so many similarities with us some thought it might be a direct ancestor. But a growing body of evidence suggests it may be a much more distant cousin, and these similarities just coincidental. It was " human by accident ".
A new analysis of their jaw has added yet another mystery to the pile. Parts of it indicate adaptations to a diet of tough foods. Yet other aspects show that it had lost bite power. It's an evolutionary contradiction.
There can be only one
Fortunately, their jaw isn't all bad news. It does help clear up some of the other weirdness of the species. Currently, two relatively complete Australopithecus sediba fossils had been found. MH1 (a young male) and MH2 (an older female). Both come from the same cave in South Africa, as the MH prefix should tell you. As such, the classic interpretation was that they belonged to the same species.
However, there were some notable differences between the two individuals. Given they were different sexes and ages, most chalked this up to intraspecies variation. But some remained skeptical, suggesting that there were sufficient differences to justify calling them two different species. For example, the spines of early and later human species varies in key ways. It has been claimed that MH1 had a spine closer to modern species, whilst MH2 had a much "older" spine.
The jaw allegedly follows a similar pattern. The young MH1 has a more modern looking mouth, whilst MH2 has an old-school jaw. Although this conclusion remained a fringe view, it did have some support from modern primate data. There was more difference in the jaws of Australopithecus sediba than you might see in chimps or gorillas. However, this data could only place jaws in the correct species 82% of the time. That's a fairly high margin of error.
Digitising Australopithecus sediba
So a group of researchers repeated this approach. This time, they used advanced statistical techniques to help recreate some parts of the missing jaw. The real trick here was recreating (and re-analysing it) several different times. What if the missing parts were like a chimp? Gorilla? The other Australopithecus sediba mandible? A statistical prediction?
Each of these recreations produced a slightly different result for the difference between MH1 and MH2. These were then paired with jaws from living great apes to see if the differences were more than would be expected for a single species. This time around, the jaws were also digitally scanned to further increase the accuracy.
The result was that MH1 and MH2 are very different, regardless of how they were reconstructed. However, they are no more different than the variation seen in most extant apes. With the exception of orangutans, all other apes have a similar degree of variation within their species.
The orangutan data acts as a bit of a caveat. It's still possible that there are two species. But given Australopithecus sediba is more closely related to humans, gorillas, and chimps it's a slim possibility. There can be only one species in Malapa cave.
Mandibular mystery
Whilst the jaw of Australopithecus sediba clears up one mystery, it exposes another. Most members of the genus Australopithecus ate tough food. The degree to which they actually did is debated, but it seems to have been quite common. Although it's worth noting Australopithecus sediba did consume other things as well.
Accordingly, most species of Australopithecus were adapted to process this tough diet. One branch took it to the extremes, but everyone had some capacity.
However, there is a growing body of evidence that Australopithecus sediba wound up bucking this trend. Their muscles were ill-positioned to produce the forces needed to eat tough food. Although they could still crunch down, they were at serious risk of dislocation. This new work on the jaw of Australopithecus sediba supports this conclusion. It finds that their stiffness and strength were well below expected for a hominin this size.
Now, the authors note that this might just be a side-effect of the fact the species was already very large to begin with. Its size meant that it could process tough foods without the most efficient jaw. Thus, it saved energy growing the unnecessary bones and muscles needed. So whilst it might seem relatively week, it's actually just as strong as anyone else.
However, there is one definite conclusion from this work. It makes Australopithecus sediba even more distinct from more modern species. The overall similarity with modern humans is seeming more and more like a coincidence. The implications of which are fascinating. Is the modern human form something "inevitable" (given the relevant environment conditions) many different lineages were pushing towards.
References
Been and Rak. 2014. The Lumbar Spine of Australopithecus sediba Indicates Two Hominid Taxa. PaleoAnthropol
Daegling, D.J., Carlson, K.J., Tafforeau, P., de Ruiter, D.J. and Berger, L.R., 2016. Comparative biomechanics of Australopithecus sediba mandibles. Journal of Human Evolution, 100, pp.73-86.
Rak and Been. 2014. Two hominid taxa at Malapa: the mandibular evidence. PaleoAnthropol
Ritzman, T.B., Terhune, C.E., Gunz, P. and Robinson, C.A., 2016. Mandibular ramus shape of Australopithecus sediba suggests a single variable species. Journal of Human Evolution, 100, pp.54-64.
