In 1976, at the Laetoli site in Tanzania, paleontologist Andrew Hill discovered 3.7 million-year-old footprints by literally stumbling upon them. He tripped to avoid elephant dung thrown by a mischievous colleague. The story of the latest discovery, at the Koobi Fora site in Kenya, lacks the flavor of legend. But these paths, dating back 1.5 million years, are just as captivating: They interweave the footsteps of two species of hominins – a term that designates representatives of the human lineage since its separation from that of the chimpanzees.

"These tracks were discovered in 2021 by a colleague of mine, Richard Loki [Stony Brook University, New York], while he was working with a team excavating fossilized skeletons lying in sediment just above," said Kevin Hatala (Chatham University, Pittsburgh; Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany), first author of the study published on November 29 in Science that describes these tracks. He has been commissioned to continue uncovering these footprints and taking photogrammetric images to obtain three-dimensional models for analysis.

The researchers focused on the path dubbed "TS-2," where they distinguished a sequence of 13 steps attributed to the same individual, as well as isolated footprints. The former, according to an analysis of the plantar arch curvature, do not resemble those of modern humans, and are attributed to Paranthropus boisei – a species belonging to an extinct lineage. The latter, by contrast, are more "human" – they resemble 500-year-old traces described in comparable soils in Walvis Bay, Namibia, and are presumed to have been left by Homo erectus (also known by its African name of Homo ergaster), evolutionarily closer to us.

Finding these tracks, contemporaneous within hours or days of each other, confirms that these two species, whose fossils have been found in the region, coexisted. "Given clear differences in the diet, life history, and encephalization of Homo and Paranthropus, this is a fascinating suggestion," commented William Harcourt-Smith (American Museum of Natural History, New York) in Science.

The revelation of such cohabitation led the team to take another look at tracks found 40 kilometers away, at the Ileret site in Kenya. "We found that there too, there was evidence of footprints from several hominin species, which we hadn't identified before," said Hatala.

You have 22.14% of this article left to read. The rest is for subscribers only.