Homo naledi changes history

Here at the Naked Scientists we pride ourselves on bringing you the latest in science news, direct from the horse's mouth and in September we covered a story which gave a whole new angle on the origins of humanity. The discovery of Homo naledi, in South Africa, introduced a new species of human ancestor to the world with clues to their culture. Chris Smith spoke to the people who made the discovery Lee Berger, from the University of Witwatersrand, Charles Musiba from the University of Colorado, and, up first, Wisconsin University's John Hawks...

John - If you look at Homo naledi from some distance, they would stand about 1.4 metres high, the size of a small human. They stand upright. They're very thin-looking in build. As you look closer, you'll notice that there are some things wrong about them. Their heads are very small: their heads are around a third the size of ours in terms of their brain size. Their hips are cast much more like a more primitive hominin, something like "Lucy", the famous skeleton. Their shoulders are sort of canted upwards in a way that we associate with some of the most primitive hominins - we think that that's probably related to climbing - but it's very clear that when you look at the details, the things that strike us as being so human-like. The feet, they're clearly adapted for walking long distances in the way that humans have; their hands, very human-like through the wrist and the palms. Their thumb is appropriate for humans in its length. But the fingers are very curved, and that thumb is immensely powerful, something that we've never seen before in the fossil record. Their teeth, also really quite human-like in their size, in what they look like: they would have been suited for in terms of eating, but they have features in them that we've never seen before in humans or any other kind of hominin.

Chris - Lee, how did you find them in the first place?

Lee - Well, they were found as part of an organised search. I'd actually enlisted a former student of mine, employed him to actually been going underground in the caves just outside of Johannesburg. He in turn enlisted two amateur cavers - Rick Hunter and Steve Tucker. What that led to in mid-September of 2013 was Steve and Rick, entering a very tiny narrow passage, about 20 metres underground. It's about 17.5 centimetres wide, dropping down 12 metres into a chamber where they chanced upon this remarkable discovery. I saw the first photos on October 1st that led to a 60-person expedition that we launched on November 7th which led to the recovery of this remarkable sample of fossil hominins.

Chris - When you saw that photograph, just describe the scene for us in that situation. What did you see?

Lee - I was at home at night, 9:00 o'clock in the evening, working on some emails and the doorbell rang and there was Pedro on the other side. He said, "You're going to want to let me in." I almost didn't, given that tone. He came in with Steve in tow and they opened up this laptop and there sitting in the middle of this picture - was a man with a jawbone with these beautiful teeth that I could immediately tell were from a very primitive or ancient hominin. I could tell by the shape of them. The next slide was that of a skull - or at least half a skull - embedded in the dirt of the ground. Just to say, we don't see fossils like that in Southern Africa. Most of our fossils are embedded in concrete-like rock. These are sitting in dirt, loose. The next slide was more bones and I thought I was looking at a skeleton. I was stunned. I've never seen anything like that.

Chris - Charles Musiba, who's also a part of the team from the University of Colorado, when you look at those teeth, where did they fit in?

Charles - When you look at those teeth, you realise that they're not modern humans. They have some features transitional between modern humans and some of our earliest ancestor. It's very interesting in that it may be signalling some completely different type of adaptation to maybe a different type of dietary behaviour which may not necessarily be exactly like ours.

Chris - John, given where these specimens were found, how do you account for them being there?

John - You know, this is the thing that occupied us as we were excavating. This is the largest assemblage of bone that we found for early hominins anywhere. We found them together with no animal bones other than a few little fragments. So, there's something very curious about the way that these hominins entered this chamber. There's no evidence that these bones were ever altered or chewed on by carnivores. It's clearly not some sort of predator that's dragged them into this cave. There's no signs of it at all. We've looked at the sediments within the chamber where we find them and we can show that those sediments originated within the chamber. They don't have grains that have come from the external environment and, in fact, the nearby chambers don't have grains that have come the external environment. We look at that and we think it's very likely that the entrance to the chamber in the past was always pitch black and isolated from the outside environment. That explains why other creatures besides the hominins were not able to reach the chamber and it creates a problem in that, Homo naledi has to have been able to access the entrance of this and reach it inside with bodies.

Chris - So, you're saying that these pretty primitive, small-brained individuals must have been intentionally depositing either themselves or their dead or dying in this chamber and then they remained in-situ for you to find potentially up to 2.5 million years later?

John - What we were working on in here is a bed that's full of hominin bone and that includes articulated elements like complete hands and feet, things that would've been disarticulated rapidly if the bodies had not entered this chamber hole. We have got them in a situation where they could not have been washed in, where there's no evidence that there's a catastrophe that's happened to them, where they clearly entered the chamber over some period of time - we don't know how long, but not instantaneously. We can in other words, exclude the things that seem simple like some sort of catastrophic event, some sort of flood, some sort of predator that's a death trap that had them fall in. we're left with the explanation that Homo naledi itself must have been intentionally depositing bodies at the entrance of this chamber or into the chamber itself.