Missing Sediba vertebrae fill in gaps in hominid posture

In the late "noughties", Chris Smith was fortunate enough to be in South Africa at a cave site called Malapa, which turned out to harbour some of the most important fossils of our early ancestors. Almost complete skeletons, much of them in full articulation - and even the hands - are preserved from several australopithecines. They almost certainly fell - 2 million years ago - into what would have been a mud-filled hole. That mud turned to stone and has preserved a near pristine snapshot of these early hominids. But there were some bits missing: because miners had got there first, with dynamite, a century ago. Luckily, the missing material wasn’t blown far away and analysis of the surrounding terrain has turned up some crucial pieces, including the parts that Scott Williams was extremely keen to get his hands on…

Scott - My research has largely focused on the evolution of bipedalism, how we walk on two legs. And so we're looking for fossils that have a mix of traits indicating they climb trees and a mix of traits indicating that they walked on the ground, like we do. And species like Australopithecus, that we're working on, are really demonstrating a mosaic mixture of these features.

Chris - How old are they?

Scott - They're quite variable in their age. The oldest ones push to 4 million years, and some of the younger ones, like Australopithecus sediba, date to just under 2 million years.

Chris - And that's the one we're talking about here. Isn't it? Because that was discovered about 15... 14... Years ago now?

Scott - That's correct, yeah, 2008.

Chris - So tell us a bit about what you're actually doing, then. Tell us about the project.

Scott - So my role in the project, has been really to work on the spinal remains of Australopithecus sediba. I was writing my dissertation when these fossils were initially published, and Lee was kind enough to invite me on the team to work on the vertebrae, the backbones.

Chris - This is Lee Berger, whose son Matthew - actually we should give proper acknowledgement, shouldn't we - I think he was about 12 at the time when he actually found what was a very important piece of our evolutionary history?

Scott - So I've been working on the vertebrae for over a decade now and you know, we were missing really important parts of the lower back, and those fossils are what we published on recently in eLife. They were recovered a few years ago and we were able to visualize them, using some 3d methods, and then describe and publish them.

Chris - Now, if the original fossils came out of this hole in the ground in 2008, why has it taken such a long time since to get your hands on these key bits that you wanted?

Scott - So the initial fossils that were recovered, some of them were outside of the pit at Malapa, which is an old cave that has since weathered away quite a bit. And it is really just an open pit. The initial fossils that Matthew discovered there were outside that pit and that's because miners were dynamiting the site over a hundred years ago. Lee then excavated the site itself, once they found out where those fossils came from, and that produced a juvenile male and an adult female that were published. Now, it's taken a long time because there are lots of blocks, other blocks of this thick material called breccia, which contains the fossils plus other minerals that have sort of encased them. And those blocks are all around Malapa, they've been pulled out of the pit. They've been taken from areas that were blasted. In this case, the fossils that I worked on, they had actually been blasted by the miners out of the pit and then used to make sort of a little mining trackway where they could roll carts and things like that. That was taken apart in 2016. And when that was done, blocks were removed and those blocks went to the lab and in the lab, they were slowly, very slowly excavated apart until fossils were discovered. And those fossils are what this new paper is based on. And they happened to interestingly fit perfectly with the fossils we had previously from the adult female individual.

Chris - They're basically bits that were blown out of the hole where she had been laying and they ended up part of that path. I've walked on that path! I didn't realise, when I first went there 14, 15 years ago, that I was walking probably on your work; it sounds terrible, doesn't it? That, basically, people were dynamiting some of the most precious things that we have in terms of documenting our evolutionary past!

Scott - Exactly. Yep. But we wouldn't probably wouldn't have discovered the sites initially if they hadn't been dynamited. A bit of a mixed bag!

Chris - Well, that's true. That's true. So you've now got this amazing, I mean, it's the gift that keeps giving, Malapa, isn't it? Not only have you got these fossils, but you've now got the bits that were missing. So why is the backbone, though, so important from your perspective?

Scott - If we're talking about the evolution of bipedalism, we're basically envisioning an animal that is something that is evolving from more of a tree-living animal, that probably had different posture than us and perhaps carried itself more like a living chimpanzee, that is going to give rise to a bi-ped, something like us, something that walks on two legs. And when you're doing that, obviously one of the main targets is the lower limb; the feet, thighs, the pelvis; but another really important area is the lower back. And that's because we humans have what's called lumbar lordosis. Our lower backs have a curvature to them that allows us to sort of balance our upper bodies over our pelvis and lower limb. The lumbar vertebrae have clear adaptations to upright posture and bipedalism, so that's what we're looking for in these fossils. And we had basically a couple of vertebrae of the very low back that sit right above the pelvis. These new fossils basically fill in the rest of the lower back all the way up to the top of the lower back. And they demonstrate that not only are there these adaptations to bipedalism, but there are some other features we didn't quite expect that indicate that this animal was also spending a lot of time climbing in trees.

Chris - So you really have got something that's a hybrid. It would have walked upright, as we had speculated, but it was also well adapted to doing other things like tree dwelling as well?

Scott - Exactly, yeah. I mean, this is something that we kind of, we knew about, right? We knew that members of this genus Australopithecus, going back to Lucy and even before that, we knew that they had this sort of mosaic morphologies. They were pretty clearly bipeds like us, but their upper limb bones, largely their arms and hands, had features suggesting they could climb trees. And what's really interesting about these new fossils is that it sort of bridges the gap. If we think anatomically it's sort of a missing link in the sense that the lower back connects the upper body to the lower body. And it demonstrates this mosaic set of features, clear evidence for bipedalism, but also strong evidence for climbing around in trees and spending lots of time in trees.

Chris - Have you got more specimens where this came from? Because you know, talking to Lee Berger and, other researchers, there's an enormous amount of material still there or still to be processed. So have you got more individuals? Are we going to get more clarity from this or, have we now eventually got to the end of that road that had been dynamited?

Scott - No, I think there'll be a lot more to come. You know, I mean one major piece that's been missing is the head of the adult female; the skull, we don't have it. We have the head of the juvenile male, which is really amazing and interesting, but it would be really nice to have an adult skull to compare with other adult Australopiths, and other species. So that's yet undiscovered, but could be there somewhere. We definitely have other materials still. I mean, I'm aware of some vertebrae that have been recovered recently that I haven't had a chance to study yet. There's more material from Malapa for sure. You know, there's still many, many blocks that are in the lab waiting to be excavated.