Scientists Found Fossil Footprints In Kenya And Made A Fascinating Discovery

Not all fossils are mineralized bones. Some fossils are of feathers, such as the ribboned tail feathers of a tiny dinosaur. Others reveal ancient burrows, nests, teeth marks, and footprints. These are called trace fossils, since they're made of the marks that ancient organisms left behind. Trace fossils left by hominins can provide anthropologists with deep insights into the behaviors and lifestyles of our distant ancestors, and one set of trace fossils is showing for the first time just how closely we interacted with other human species.

In a 2024 study published in Science, researchers analyzed trace fossils from the Turkana Basin in Kenya and discovered several distinct pairs of footprints walking along an ancient muddy lake shoreline. Among impressions of birds and ungulates, a few of the 1.5-million-year-old footprints appear to be from two different species of human: Paranthropus boisei and Homo erectus. And, due to the short window of time required for such footprints to become fossilized, the researchers concluded that the footpaths were likely formed within days or even hours of one another. This means that H. erectus, our direct ancestor, most likely interacted with another human species. Whether such interactions were primarily peaceful or violent is unknown.

Anthropologists have long recognized that our ancestors coexisted with other human species. The most well-known are the Neanderthals, which probably even interbred with human beings. But the footprints in the study come from a geological layer 1.5 million years old, well before modern H. sapiens emerged as a distinct species. Instead, at the time, there were at least four human species walking the Earth, H. habilis, P. robustus, P. boisei, and H. erectus (our direct ancestor). It's the latter two — P. boisei and H. erectus — that the 2024 study's footprint fossils show crossing paths.

While the fossil record makes clear that multiple hominin species lived at the same time and places within Africa (some of them more bizarre than others), it's unclear exactly how they shared their spaces. Were different human species hostile to one another? Perhaps 1.5 million years ago, different hominins competed for resources and territory with the same passion that human beings and Neanderthals did 30,000 to 600,000 years ago. Indeed, clashes between H. sapiens and H. neanderthalensis likely often broke out into all-out warfare. Even today, H. sapiens fight amongst themselves when contesting land and resources, so it's not difficult to imagine that our primitive ancestors defended their claims with similar ferocity.

But such assumptions are conjecture. The anthropologists behind the 2024 study have just simple pedal impressions with which to work. Scant though the evidence may be, the researchers were able to conclude the following: One, the footprints were formed by two separate species of bipedal human, most likely H. erectus and P. boisei. Two, based on differences in morphology and gait, the species were adapted to different locomotion. Specifically, the H. erectus footprints display adaptation for long-distance travel, while the P. boisei footprints display adaptation for shorter, unsustained travel. Three, the footprints appear to have formed within a few days or hours of each other, meaning the two species shared the space.

The 2024 discovery inspired a hunt for more evidence. New analyses of footprints from other sites have identified several more examples across multiple sites in the region. In the words of the research team, the abundant evidence highlights the "influence of varying levels of coexistence, competition, and niche partitioning in human evolution." Such forces continue to shape our species today.