A collection of 21 footprints discovered in northern Kenya, dating back over 1.4 million years, suggests that an ancient relative of modern humans, Paranthropus boisei, may have traveled in all-male groups. Researchers analyzing the tracks believe they capture a single moment when eight individuals walked together along the shore of an ancient lake.
The size and pattern of the footprints indicate they were made by multiple adult male P. boisei, pointing to a complex social structure for the species. Scientists hypothesize that these males may have traveled together for protection in a dangerous environment, despite potential competition for mates.
"The fact that eight, mostly adult male P. boisei individuals seemingly travelled together as a group, without females or children, hints at a complex social structure in this species," said co-author Neil Roach from Harvard University. "They may have lived in large groups, where males competed for mates, but also tolerated each other at times for safety in a dangerous environment."
The footprints were found in the Koobi Fora Formation on the eastern edge of Lake Turkana, a region known for its rich fossil record of early human evolution. The sediment layers have preserved evidence of hominin activity spanning millions of years.
After detailed 2D and 3D imaging analysis, researchers compared the tracks to those of modern humans and other ancient hominin species. They identified the prints as belonging to P. boisei, an extinct hominin species often nicknamed "Nutcracker Man" due to its large jaws and molars. Although P. boisei lived alongside early humans like Homo erectus, it represented a separate, extinct branch of the human family tree.
IMAGE4 The analysis also revealed that P. boisei individuals may have been larger than previously estimated. "The sizes of the footprints indicate human-like body sizes – up to 6ft (1.8 metres) tall and around 75 kilograms (12 stone)," stated lead author Kevin Hatala from Chatham University. These findings challenge earlier beliefs that P. boisei was significantly smaller than its contemporary Homo erectus.
"The body size estimates for six of the individuals fall well above published species average. We hypothesise these tracks likely represent adult males," the research team wrote in the journal Proceedings of the National Academy of Sciences. "At a minimum, the trackways indicate mutual tolerance of multiple P. boisei adult males on the same landscape at approximately the same time, and perhaps within the same social group."
IMAGE2 The lakeside environment, likely abundant with resources such as papyrus and shellfish, may have drawn these ancient hominins. Researchers suggest P. boisei consumed relatively large quantities of plants and mollusk shells available at the lake's edge.
This discovery adds to a growing body of evidence from the Lake Turkana region, which has yielded hundreds of ancient hominin footprints across multiple sites. These sites collectively span over 100,000 years of repeated visits to the lakeshore. The researchers plan to continue excavations in the area, hoping to uncover more information about the behavior and lives of early human relatives.
In 2024, footprints from both P. boisei and Homo erectus were found at Lake Turkana, providing the first direct evidence that these distinct human relatives inhabited the same locations simultaneously and possibly interacted.