First evidence of distinct human species living cheek by jowl

Imagine sharing your neighbourhood with a different species of human – neanderthals, or the baboon-like homo habilis.

Well, turn the clock back 1.5 million years and that’s what scientists believe occurred in East Africa where two species of human ancestors shared the same land at the same time.

Fossil footprints from the human relatives, Homo erectus and Paranthropus boisei, were discovered side-by-side along the edge of a lake together with bird, bovine and equid (horse) prints by a team of biologists from the US, UK and Kenya.

“Fossil footprints are exciting because they provide vivid snapshots that bring our fossil relatives to life,” says Kevin Hatala, professor of biology at Chatham University, Pittsburgh, US who led the research, which is published in the journal Science.

“That's something that we can't really get from bones or stone tools. The insights they give us, and the questions they raise, can be rather unique. Until now we have struggled to find the necessary data to know whether and how ancient human species may have coexisted and interacted during the early Pleistocene.”

For much of our evolutionary history, multiple fossil human species are believed to have coexisted within the same geographical regions, which has driven hypotheses about the importance of competition between species in human evolution, and niche portioning (where natural selection causes species to use resources differently in order for each to survive).

"One set of tracks is arched and appears to have been made by a hominin moving in a human-like way, but the others are flatter at the bottom, indicating the track maker was not walking like a modern human" - Prof Peter Falkingham

The team examined a recently discovered fossil site in the Turkana Basin in northern Kenya, where human-like footprints, suggested different patterns of anatomy and locomotion.

Using a technique developed last year by Hatala, Stephen Gatesy and Peter Falkingham of Liverpool John Moores University for studying dinosaur footprints, they were able to distinguish two different kinds of footprint.

“The two differ in what we'd call the 'arch', the raised bit between the toes and the heel,” explained Falkingham, a palaeontologist.

“The tracks on this surface are right next to each other, with a trackway of one individual being crossed by tracks from another. One set of tracks is arched and appears to have been made by a hominin moving in a human-like way, but the others are flatter at the bottom, indicating the track maker was not walking like a modern human.”

This is the first direct evidence showing that Homo erectus and Paranthropus boisei, occupied the same lake margin environment and likely interacted with each other. “Despite these two hominins diverging considerably in their anatomy, behaviour and land use, they are both clearly drawn to these important lakeshore environments,” says co-author Neil Roach, of Harvard University. “It raises new questions like did this overlap increase competition between them for the same resources? Were they there for different purposes?”

The research team expanded its analyses to other fossil sites in the surrounding area and found more evidence that these two species occur together, at sites spanning up to 200,000 years, which suggests low to neutral competition between these two species, possibly have enabling their long-term coexistence. Later, environmental shifts could have impacted resource availability, increasing competition and potentially driving the behavioural adaptations that have come to define our genus.

Homo erectus, a possible direct ancestor of ours, persisted for more than one million years after this. The other, Paranthropus boisei, went extinct within the next few hundred thousand years.

“Perhaps changes to climate influenced resource availability and that led to the extinction of Paranthropus and the persistence of Homo,” Hatala adds. “This is a hypothesis that will require further testing, and we're hopeful that by combining fossil footprints with other kinds of palaeontological and archaeological data, we might be able to build a better understanding of how factors like competition and niche partitioning played a role in our evolutionary history.”

The research was supported by the National Geographic Society, U.S. National Science Foundation, the Turkana Basin Institute and UK Research and Innovation.

More information, including a copy of the paper, can be found online at the Science press package.

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