WASHINGTON, DC.- In a paper published in the journal Science, an international team of researchers from the U.S., Kenya and the UK provide the first direct evidence of two different ancient human relatives, Homo erectus and Paranthropus boisei, simultaneously occupying the same immediate landscape. The research team analyzed a newly discovered fossil footprint site in northern Kenya that records two different kinds of ancient footprints left along the margins of a lake around 1.5 million years ago. These fossil footprints reflect different patterns of anatomy and locomotion, providing the first direct documentation that H. erectus and P. boisei occupied the same environment and could have interacted with each other.
The study was led by Kevin Hatala, associate professor of biology at Chatham University in Pittsburgh, and includes contributions from Kay Behrensmeyer, senior research geologist and curator of vertebrate paleontology at the Smithsonian’s National Museum of Natural History, who studied the paleoenvironmental context of the trackways.
“Fossil footprints are exciting because they provide vivid snapshots that bring our fossil relatives to life—with these kinds of data, we can see how living individuals, millions of years ago, were moving around their environments and potentially interacting with each other, or even with other animals,” Hatala said. “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. For example, 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 humans’ evolutionary history, scientists have thought that multiple human relatives may have coexisted within the same geographical regions and time periods. This has driven numerous hypotheses about the importance of niche partitioning and competition between species in human evolution. However, the paleontological record has been unable to definitively establish whether these ancient human relatives actually lived together on the same landscapes simultaneously, until now.
In addition to studying the newly discovered site, the research team expanded its analyses to other fossil footprint sites known from the surrounding area. These sites yielded further evidence that H. erectus and P. boisei lived alongside each other at sites spanning up to 200,000 years. This prolonged overlap suggests low to neutral competition between these two species, which may have enabled their long-term coexistence during the early Pleistocene. Later, environmental shifts could have impacted resource availability, increasing competition and potentially driving the behavioral adaptations that have come to define the genus Homo. Resolving these kinds of questions may be possible in the future.
“Documenting the strata revealed that there are many more trackway surfaces that could be excavated nearby,” Behrensmeyer said. “These might hold more clues that could address questions about how different hominin species interacted, what they were doing wading in the shallow water and why this behavioral pattern recurred over 200,000 years.”
“Despite these two hominins diverging considerably in their anatomy, behavior and land use, they are both clearly drawn to these important lakeshore environments,” said 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 fossil footprints were found in 2021, when Cyprian Nyete and other co-authors from the Stony Brook University’s Turkana Basin Institute were excavating skeletal fossils from overlying sediments. Richard Loki of the Turkana Basin Institute was a member of that excavation team who recognized the first hominin footprint. Louise Leakey of Stony Brook University then coordinated a team, led by Hatala, Roach and Nyete, that excavated the footprint surface in July 2022. Kenyan excavators Apolo Alkoro Longaye, Hilary Sale, Ben Sila, Losogo Nyakitala and David Kipkebut provided assistance with Behrensmeyer’s geological work on the site in 2023.
During this time and place in human evolution—about 1.5 million years ago in the Turkana Basin of Kenya—scientists have long hypothesized that these ancient human relatives coexisted together. H. erectus, a possible direct ancestor of humans today, persisted for more than 1 million years after this. The other, P. 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 said. “This is a hypothesis that will require further testing, and we’re hopeful that by combining fossil footprints with other kinds of paleontological and archeological 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.”
In addition to Hatala, Behrensmeyer, Roach and members of the research team at Stony Brook University and the Turkana Basin Institute, the study also includes authors affiliated with the Max Planck Institute for Evolutionary Anthropology, Liverpool John Moores University, Brown University, George Washington University and Rutgers University.
