A trail of 1.43-million-year-old footprints pressed into a Kenyan lakeshore suggests that Paranthropus boisei, the heavy-jawed human relative nicknamed "Nutcracker Man," was built far larger than fossilized bones had indicated — and that it may have moved in same-sex groups, hinting at social behavior not previously inferred from this branch of the human family tree.
The findings, published in the Proceedings of the National Academy of Sciences, come from 21 hominin tracks excavated at a site called GaJi10 in the Koobi Fora Formation, on the eastern shore of the Turkana Basin in northern Kenya. Researchers led by Kevin Hatala of Chatham University and the Max Planck Institute for Evolutionary Anthropology traced the prints to at least eight individuals who crossed a shallow, muddy lakeshore in the same general direction.
Bigger than the bones suggested
Using the depth, length and pressure pattern of the impressions, the team estimated that several of the trackmakers stood as tall as 1.8 meters (5.9 feet) and weighed around 75 kilograms — dimensions on par with modern humans and considerably larger than skeletal remains had previously implied for the species. Foot anatomy visible in the prints, including flattened arches and a mobile big toe, is consistent with P. boisei, a hominin known mainly from a handful of skulls and jaw fragments found across East Africa since the 1950s.
The tracks' directionality and clustering led the researchers to a second, more provisional conclusion: the group was made up mostly of adult males moving together, without evidence of females or juveniles. "The fact that eight, mostly adult male, Paranthropus boisei individuals seemingly traveled together as a group, without females or children, hints at a complex social structure in this species," said Neil Roach, a co-author at Harvard University, per coverage of the findings.
"The sizes of the footprints indicate human-like body sizes — up to 1.8 m (5.9 feet) tall and around 75 kg."
Kevin Hatala, Chatham University / Max Planck Institute for Evolutionary Anthropology
Fossil trackways are prized in this field because they capture a single day's behavior, not an accumulation of bone over centuries. Koobi Fora and the nearby Ileret site on Lake Turkana have already yielded some of the best-known hominin footprint evidence, including tracks attributed to Homo erectus. The new site adds a rare behavioral snapshot for P. boisei, long assumed — based on its robust jaw and molars — to have been a specialized plant-eater living alongside, and possibly competing with, early Homo in the same landscape roughly 1.4 million years ago.
The authors are careful to flag real uncertainty in their interpretation. Species attribution from footprints alone is inherently indirect, and the researchers note they cannot rule out that some tracks belong to Homo erectus rather than P. boisei, nor can they confirm whether the same individual appears in both the east- and west-bound trackways at the site. The larger body-size estimate also sits well above most previous reconstructions drawn from limb bones, a discrepancy the team says warrants further comparison with skeletal collections.
Hatala's group, working with Kenya's National Museums and colleagues including Purity Kiura, says continued excavation at Koobi Fora is planned to search for additional trackways that could clarify group composition and test whether the apparent single-sex travel pattern holds up at other sites from the same era.
