Introducing Homo naledi

Where we came from is, arguably, one of the most important questions facing mankind; and this week the story has become even more intriguing. The well-preserved remains of 15 individuals from a new species of human ancestor, called Homo naledi, have been unveiled by scientists in South Africa. The name means "star in situ" in a local language, and it's a nod to the rising star cave system where the remains were uncovered. These primitive people show features very similar to our own; their hands and feet are almost identical to those of a modern human, but their heads and brains were much smaller; their shoulders were tilted, ape-like, to favour climbing, and the pelvis was proportionally wider than our own. They therefore share a number of key features with us but are also clearly distinct, and traits suggest that they might sit very early in our evolutionary time-line. The finds also harbour another secret: it's possible that these individuals might have been put where the scientists found them after they died. Chris Smith spoke with the discoverers, 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 meters 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 meters underground. It's about 17.5 centimetres wide, dropping down 12 meters 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 lesser jawbone with this beautiful teeth that I could immediately tell were from a very primitive or ancient hominin. I cound 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.

Chris - Do you think this is cannibalism possibly? Could these have been a future lunch?

John - We have examined every bone in microscopic detail and there's no sign on them that they've been cut upon, that they've been chewed on. The only marks we find on them are the marks that come from natural decomposers like beetles and snails. So, we can totally exclude that this was some sort of a violent, aggressive event or that they were cannibalising each other. The evidence is just totally against it.

Chris - Charles, can you dig into some of those sediments and some of the context in which these remains are found and begin to refine the date from which they probably do hail?

Charles - The geology of that cave is very complex and we have to also think about the nature of the deposit itself, how those bones were actually deposited. So, we have to reconstruct their deposition history and then we have to look into the timing. So, it's painstaking work which has to be done. Unfortunately, we're still working on that and we are not in a rush to try to actually come up with quick data. This is an assemblage which is very unique. It's so different. We can start to see things which we have never seen before, rather than being blinded by dates, we're not going for that quick shortcut.

Chris - Lee, given the incredible state of preservation of these fossils and their sheer number, where do we stand on trying to get DNA out of them? You've got these pristine teeth and so on.

Lee - The next phase of this project is going for questions like that. Right now, ancient DNA discoveries have been restricted to the last several hundreds of thousands of years, and we pushed it maybe back to 500,000 or 600,000 years at this stage but under remarkable circumstances. But this is a remarkable assemblage and maybe we'll be talking to you in the near future and saying, "Guess what..."

Chris - John, how are you now seeking to share this with other scientists in your field, but also the world?

John - You know, Lee of course has long been a leader in this and from a science point of view, we can say in a very straightforward way, here's the bones that we're finding, but the development of interpretation of, are these something that we've seen before or something new? took us many, many months.

To conduct that process, we wanted to open it to a broader scientific community to allow more participation in the analysis of the bone. We specifically engaged early career scientists from South Africa and around the world. We brought them to Johannesburg to cooperate with Lee's team and really created a dream team of people who had enormous expertise in describing new hominin remains. Now of course, we're doing everything that we can to share them even more broadly.

We have three dimensional images of these that you can examine and you could print them out on computers. We're making them available on a site called Morpho Source so that people can access them. We're doing everything that we can to share every aspect of this discovery with the public and with other scientists and with school teachers. Also, these fossils are right now, on display in Maropeng and I think that is unprecedented. From the moment of the press conference, we've put the largest assemblage of fossil hominins on display at our Visitor's Centre in the Cradle of Human Kind where people could actually go and stand next to this extraordinary discovery.

In addition, every person on this planet can actually download these papers and see the science themselves. We published them in a journal called eLife which is an open source, open access journal. We did that deliberately because it was a good fit with our team's ethos that is of sharing this with not only the scientific community but the public at large.