Footprints reveal the coexistence of two human species 1.5 million years ago

In 2021, a team of paleoanthropologists unearthed something unusual in Kenya. They were searching for human fossils in sediments dating back 1.5 million years, but instead they found the footprint of a prehistoric stork, which was enormous, judging by the size of its fossilized footprint. Next to it was something much more interesting: a footprint that looked human.

The following year, scientists continued their excavations with the help of two experts in human locomotion. Two parallel footprints were found, one meter apart. After two more years of work, the team behind the investigation announced on Thursday that this was the first conclusive proof that two different human species, Homo erectus and Paranthropus boisei, lived together in the same place at the same time.

“They passed by within a few hours, or a few days at most,” says Kevin Hatala, a researcher at the Max Planck Institute for Evolutionary Anthropology in Germany and co-author of the discovery, which was published on Thursday in the prestigious journal Science. The discovery of the fossilized footprints allows researchers to explore “a lot of key questions about human evolution and competition between species,” he tells EL PAÍS.

Modern humans are accustomed to being the sole human species on the planet, but this has been the case for only a few tens of thousands of years. Throughout most of history, multiple human species coexisted, continually evolving in parallel. Louise Leakey, a paleoanthropologist at Stony Brook University, who led the research, highlights this diversity with a quick calculation: 1.5 million years ago, at least six distinct human species inhabited Africa. Among them was Homo erectus, a species that walked and ran in a manner remarkably similar to modern humans and had reached Asia.

“Fossilized remains of Homo erectus, Homo habilis, Paranthropus boisei, and possibly other species have been found from this period,” Leakey explains. “However, there may be hundreds or thousands of years of difference between the sedimentary layers in which they are found.” Instead, “this is the first time we have a snapshot of a coastal environment where two different hominins walked on the same surface,” and at almost the same time. “It is possible that they saw the large stork, horses and other animals that were also on the coast,” she says.

Leakey is part of the third generation of a renowned family of paleoanthropologists who discovered some of the most important fossils in history in Africa. In 1978, her grandmother found a trail of fossilized footprints in Tanzania, dating back 3.6 million years. These prints, left by early hominins, revealed bipedal locomotion and behaviors that seem distinctly human, such as stepping into the footprints of those ahead.

Using advanced 3D scanning technology, the team analyzed the newly discovered footprints in Kenya and compared them with hundreds of footprints made by modern humans. The findings revealed two distinct species. One set of prints closely resembles those of modern humans and is likely attributed to Homo erectus, a species known for its omnivorous diet and behavior similar to Homo sapiens. The other footprints belong to Paranthropus boisei, a smaller-brained hominin with unique physical traits, such as massive jaws and molars adapted for chewing fibrous plants — similar to gorillas today. Unlike the more human-like Homo erectus, P. boisei walked upright but with less human-like foot mechanics and greater big toe mobility. “Given their different dietary adaptations, it’s possible the two species did not directly compete for resources,” explains Hatala, a lead researcher.

P. boisei became extinct around a million years ago due to climatic and environmental changes that eliminated its food sources. In contrast, Homo erectus thrived for 1.5 million years — making it the longest-surviving human species. African populations of Homo erectus were the ancestors of the first Homo sapiens, which emerged on the continent approximately 200,000 years ago. These early humans left Africa and reached Europe, where they encountered the local human species, the Neanderthals, with whom they inter-bred. Until now the most widely accepted theory was that the Neanderthals became extinct about 40,000 years ago, but a study published this year suggest Neanderthals may have been absorbed into the human lineage, as Neanderthal DNA is found in all modern humans outside of Africa.

The Kenyan footprints were found in the Lake Turkana basin in the north of the country. The team reviewed other fossil footprints previously discovered in the same area and found more examples of the two species moving in close proximity to each other. The sediments span 100,000 years, a long period of coexistence.

“There are a thousand questions about what happened, how they interacted and how their coexistence could have influenced human evolution,” says the researcher. “Perhaps one of the biggest questions to be answered is why both hominins were attracted to the margins of the lakes. They were dangerous, and were plagued by hippos, crocodiles and other threatening animals. Whatever attracted them to these areas must have been worthwhile. Hopefully, by combining the footprints with other lines of palaeontological and archaeological data, we can begin to understand exactly what they were doing.”

William Harcourt-Smith, a paleoanthropologist at New York University, believes that “the findings help to elucidate the complex evolutionary history of hominin locomotion.” The work adds new data to the knowledge of a “unique” and “highly specialized” form of locomotion in humans, which is “as complex and interesting as other more studied forms, such as diet and development,” he adds in an article accompanying the study.

Adrián Pablos, a biologist and researcher at the National Center for Human Evolution, believes that “the main novelty of this study, apart from the fact that they are footprints that were previously unknown, is that among these tracks two different ways of walking can be observed.” “Both are bipedal, with the big toe abducted [extended] typical of bipedalism and different from the footsteps of chimpanzees,” he points out.

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