A set of footprints dating back more than 1.4 million years suggests our ancient relatives may have travelled in all-male groups, according to researchers who analyzed 21 footprints preserved in sediment along the edge of an ancient lake in northern Kenya.
Close analysis revealed the footprints belonged to Paranthropus boisei, an extinct ancient cousin of modern humans. Scientists believe the footprints capture a single moment in time as eight individuals walked together along the shore.
The size of the footprints suggests the tracks were made by a group with multiple adult males, hinting at an unexpectedly complex social structure. The researchers say the males may have travelled together for protection despite competing with one another 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."
Discovery at Koobi Fora
The tracks were discovered in the Koobi Fora Formation on the eastern edge of Lake Turkana, one of the world's richest archaeological and fossil sites. Thick layers of sediment there have preserved some of the most important evidence of early human evolution over the past two million years.
Researchers analyzed the footprints using detailed 2D and 3D imaging before comparing them with tracks made by modern humans and other ancient hominin species. They determined the tracks belonged to P. boisei, a muscular, gorilla-like species nicknamed "Nutcracker Man" because of its enormous jaws and huge molar teeth.
Although P. boisei lived alongside early humans such as Homo erectus, it belonged to a separate branch of the human family tree that eventually died out.
Larger than expected
The size of the footprints was especially surprising, suggesting the species was far larger than previously thought. Lead author Kevin Hatala, from Chatham University, said the sizes of the footprints indicate human-like body sizes, up to 6ft (1.8 metres) tall and around 75 kilograms (12 stone).
The findings challenge the long-held belief that P. boisei was much smaller than the contemporary Homo erectus.
Writing in the journal Proceedings of the National Academy of Sciences, the team said that if the tracks were made by P. boisei, the large body mass estimates would alter many reconstructed aspects of the species' biology and behavior. The body size estimates for six of the individuals fell well above the published species average, leading the researchers to hypothesize that the tracks likely represent adult males.
The team wrote that, 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.
The researchers suggest the species consumed relatively large quantities of plants such as papyrus, which would likely have been abundant in the area at the time. This, along with nutritionally rich shellfish and mollusc shells, could have drawn the ancient relatives to the lakeside.
Hundreds of tracks, more digs planned
The team has now documented hundreds of ancient hominin footprints from more than half a dozen sites across Lake Turkana. Together, the sites span more than 100,000 years of repeated visits to the lakeshore, with each footprint adding to what researchers describe as a photo album of human evolution.
The researchers hope that studying how the footprints were preserved will explain why ancient human relatives kept returning there. They plan to return to the area later this year to continue excavating the footprint sites, hoping future discoveries will reveal more about how the ancient relatives lived nearly one and a half million years ago.
In 2024, a team of scientists announced the discovery of footprints from both P. boisei and Homo erectus at Lake Turkana. Until that point, the two species were thought to be unknown to each other. The researchers said the discovery was the first direct evidence that distinctly different kinds of human relatives inhabited the same place at the same time, and the tracks opened the possibility that the two groups may have interacted with and influenced one another.
