Australopithecus sediba is a South African member of the human family that lived about 1.9 million years ago. And that's about all we can say for sure about it. It turns out it's a really weird species. On the one hand, it has a mixture of ape-like and modern-like characteristics. This is what you might expect for an early member of our genus, Homo. But it combines this with a distinct way of walking unrelated to us. It's also way too young to be an early member of our genus, living nearly a million years after Homo first evolved. And this weirdness is just scratching the surface.
This has led to a lot of debate over where it fits into the human family. The general consensus is that it may be a late surviving branch of a species from around the time Homo first evolved. Hence why it has those transitional modern/ape features. But it survived for a while after Homo split off, becoming isolated in South Africa. There, it developed its own unique set of features. Like it's weird legs.
So have we finally got to the bottom of the mystery. Not quite. Some researchers are becoming increasingly convinced that the Australopithecus sediba fossils are lying to us.
Deceptive children
Of course, the fossils in question aren't trying to lie to us. That's just a side effect of their age. See, the type specimen which defines Australopithecus sediba is MH1. And MH1 is a juvenile.
Now, in most circumstances, this isn't a huge problem. The other fossils we have - like the adult MH2 - can be used to see what adults of the species also look like. And for the most part, they tell the same story, combining a mixture of modern, ancient, and unique features. With one notable exception: the skull.
Currently, the most complete skull is that of MH1. So there's no real point of comparison to ensure it isn't particularly child-like. This issue is further exacerbated by the fact that we do know that ape skulls can change quite dramatically during the course of their development. Notably, young apes can look a lot more human. Could this confound our comparisons between other species?
Rapid ageing of fossils
Now, this isn't necessarily a new objection. Since the species was discovered people have been pointing out that a suspicious amount of evidence comes from juvenile fossils. And we don't know juvenile fossils are great representatives of their species.
At the same time, we don't know that they're that different from adults either. Take a look at Selam, for example. Also nicknamed Lucy's child, this 3 year old ape-like Australopith is still recognisable as a member of Lucy's species. Hence the nickname. Yet it still does considerably from adults of the species. Just not enough to make it a mystery.
So is the juvenile nature of MH1 really a problem? Researchers wanted to find out the answer for sure. So they gathered data on a bunch of ape skulls of various ages to see how it changes during development. From this, they extrapolated out how the MH1 skull should change if it followed a similar trajectory. And then investigated whether the resulting adult would still seem to have that mix of ape and modern features that make the species so unusual.
Australopithecus sediba in the human family tree
Plot twist: no. Examining how Australopithecus sediba should change as it matures produces some notable similarities with other earlier, ape-like species. Not the later members of Homo it's thought to be closely related to.
In particular, it shows many similarities with Australopithecus africanus. This species has the honour of being the first Australopith ever found and lived until 2.1 million years ago in the same region as Australopithecus sediba. Thus the two species seem very close geographically and temporally. And if this extrapolated skull is to be believed, they're very similar anatomically well. In fact, scientists have already found some juvenile members of the species, and they line up with the MH1 skull surprisingly well.
Under this model, Australopithecus sediba would be a descendant of the ape-like Australopithecus africanus (or maybe even fall within the range of that species). It's similarity to modern humans are just juvenile features that occur early in development.
Is this the final answer? Well, this new work is thorough and does a great job of examining how age could impact these fossils. So I find myself being convinced by it. However, at the same time it does have the same flaw as previous work. It's trying to extrapolate from fossils into the unknown. I think it does a very good job of it, but this means there'll be wiggle room for people who don't agree. Ultimatley, no matter how good the work is I think the debate will continue until more fossils are found.
But don't be surprised when the fossils force us to "rewrite the textbooks". People have started already.
References
Alemseged, Z., Spoor, F., Kimbel, W.H., Bobe, R., Geraads, D., Reed, D. and Wynn, J.G., 2006. A juvenile early hominin skeleton from Dikika, Ethiopia. Nature, 443(7109), pp.296-301.
Cherry, M., 2010. Claim over 'human ancestor'sparks furore. Nature, pp.827-847.
Kimbel, W.H. and Rak, Y., 2017. Australopithecus sediba and the emergence of Homo: Questionable evidence from the cranium of the juvenile holotype MH 1. Journal of Human Evolution, 107, pp.94-106.
