Were different species of early humans neighbours?

But now, researchers say they have found the 1.5-million-year-old criss-crossing footprints of two different species of early human ancestors preserved in mud at the same spot in Kenya. Nearly half a century ago in the same geography the famous British palaeoanthropologist Mary Leakey made a similar discovery of footprints dating back over three and a half million years. Those footprints were some of the strongest evidence at the time that those ancestors walked on two feet. The international team who’ve made the new discovery, which actually also includes one of Mary Leakey’s descendents, say their findings prove that at least two different ancient hominins were near neighbours and possibly even friends. Here’s Craig Feibel at Rutgers University…

Craig - This is a trackway from one and a half million years ago that represents a beach on an ancient lake. On that beach surface, we have the trackways of not only a lot of birds and other animals, but two of our ancient cousins; one walking along the beach and one at some point crossing that trackway. We can see two different trails on this surface.

Chris - And are these two different species, effectively? They're two different branches of life's evolutionary past.

Craig - For the first time, I think we can actually distinguish individual footprints and begin to tell that at least they're different, that there are two different forms here. We can tell them apart. That's really the novelty of this particular discovery, I think, that we can recognise that they're in the same place, maybe not at exactly the same time, but that they passed across the same beach probably within a few hours or a couple days of each other.

Chris - You're calling them cousins, but what were the two groups that you think made these footprints?

Craig - One of them is the lineage that leads directly to us, so our direct ancestor. That would've looked very much I think as we do today, a very long bodied, tall form. But the other cousin was very, very different. It was a shorter, stouter, more robustly built cousin. Perhaps the main distinction between the two is that we think our ancestors were already into scavenging for meat and marrow and having an omnivore's diet, whereas this other stockier cousin was perhaps still more of a vegetarian.

Chris - We often think of the timeline of evolution as, one thing turns into the next, turns into the next. But what this is saying is actually something quite different, which is something we've suspected. That these things were all going on and around in an overlapping way. They were around simultaneously. It's like nature was doing an experiment with lots of different forms alongside each other.

Craig - In this particular time span, one and a half million years ago, there was quite a bit more diversity than we have in the world today. Not only were there different kinds of giraffe or different kinds of antelope, even in our own group we had cousins that were very similar to ourselves, but were different species. Today we're the only surviving species, but we had a number of these cousins who have all gone extinct over time.

Chris - How did you find these footprints? Where are they? And how do you know, just on the basis of a footprint, that one is homo, our lineage, and the other is this other group which are a bit stockier, a bit shorter and into eating vegetables and so on?

Craig - The footprints are found in northern Kenya, up towards the Ethiopian border on the Eastern side of Lake Turkana. They're in deposits that represent an ancient lake where a river was coming in near the shoreline of the lake. That would have been the setting for this particular several days of fossil record. A number of different birds and animals were wandering along the beach, and all of their tracks are preserved on this surface. Footprints preserve very nicely in damp sediment, and that's part of the story here. They're very nicely formed in the soft sediment and then a nearby river gently buried them and preserved them to the present day.

Chris - And that enables you, with the detail you've got there, to say, well, these are definitely the footprints of one group and these are definitely the footprints of the other.

Craig - That was the surprise in this discovery. In part the technology and our understanding of feet and how feet operate, the locomotion of human ancestors, has advanced a lot. Looking at the details of these feet, it was very apparent that there were two different ways of walking here, two different kinds of getting across the landscape. That made these two sets of footprints quite distinctive.

Chris - It's interesting, isn't it, because that part of the world has form for this kind of thing. Mary Leakey made her name and put it on the map, this area, this part of the world, for this very reason.

Craig - This is one of the richest places in the world for fossils from the last few million years time. Over the last 50 years there has been extensive exploration and discovery there. The way in which these footprints were found, Mary Leakey's granddaughter Louise Leakey now runs a team of fossil hunters who prospect the landscape and search for fossils there. They had actually found some very interesting bones on the surface at this locality. When you find bones on the surface, you're often curious if there's anything still in the ground still buried. They were starting an excavation to see if they could find more of those bones, and that's when they stumbled across the footprints.

Chris - And just for clarity, how do you know how old they are?

Craig - We managed to date these sites because there are volcanoes that are very active in the Rift Valley at that time. In some of the materials that are thrown out of the volcano, volcanic ash, and particularly pumice, they have some crystals in them that originate with that eruption. Those crystals can be dated by radiometric decay to give us an age for that particular eruption. Then, we can relate those to where the fossils are found.

Chris - What do you think the main learning point here is to emerge from this?

Craig - One of the most interesting things we've learned from this is that these two forms were actually on the same part of the landscape. The problem has been that, when we find the fossil bones, they're not always in the place that the animal lived. Bones can be transported by water, or often carried by a carnivore or a scavenger. We knew there were two different species present, but we really had no idea if they would've ever encountered each other on a given day. This tells us that they're very much in the same space.

Chris - Were they friends or were they enemies, do you think?

Craig - It's very hard to say. They probably recognised each other as we recognised any other animal on the landscape. They may have seen the similarities, but they were probably very different in their lives, not competing with each other and so probably less likely to have interacted very actively.