Several Appear to Have Been Considerably Larger Than Earlier Estimates of Paranthropus Boisei Body Size Based on Limited Skeletal Remains.

Fossil footprints discovered in northern Kenya reveal that eight Paranthropus boisei individuals may have traveled together about 1.4 million years ago, with some reaching surprisingly large, human-like body sizes. The tracks, preserved near an ancient lakeshore, offer a rare glimpse into the social behavior of this extinct human relative and suggest the species may have lived in more complex groups than previously thought. Researchers describe the discovery as a snapshot of "fossil behavior" because, unlike bones that can accumulate over long periods, footprints capture a brief moment in the lives of individuals.

The international team, made up of researchers from the US and Kenya, concluded that all eight trackmakers were likely adults. Several appear to have been considerably larger than earlier estimates of Paranthropus boisei body size based on limited skeletal remains. The researchers used analytical techniques first developed by their team in 2024 to identify the footprints as belonging to Paranthropus boisei, based on clues in foot anatomy and the way the individuals moved.

"The sizes of the footprints indicate human-like body sizes - up to 1.8 meters tall and around 75 kilograms, says lead author Kevin Hatala of Chatham University and an associated researcher of the Department of Human Origins at the Max Planck Institute for Evolutionary Anthropology in Leipzig. For decades, most known fossils of Paranthropus boisei consisted of skulls, including specimens with exceptionally large jaws and teeth. Much less has been known about the species below the neck because fossils from the rest of the skeleton are far rarer.

The new tracks challenge earlier research suggesting Paranthropus boisei was substantially smaller than Homo erectus, another hominin that lived during the same period and is considered a possible direct ancestor of modern humans. Footprints can reveal more than anatomy. Because a walking surface may be formed and preserved over a very short span of time, fossil tracks can provide direct evidence about movement, group size, and possible social interactions.

In this case, the eight individuals appear to have been traveling together, most likely adult males, with no obvious evidence of females or children among them. "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, says co-author Neil Roach of 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 discovery is part of a much larger collection of hominin footprints found in ancient lakeshore environments around Lake Turkana. That pattern suggests these areas were important habitats that offered resources valuable enough to draw extinct human relatives back repeatedly. "To me, it is amazing that we have the same kind of lake margin deposits in two areas of East Turkana that are 40 kilometers apart, at about the same age, says co-author Kay Behrensmeyer of the Smithsonian Institution.

Researchers have now documented hundreds of hominin footprints at more than a half-dozen sites across the region. Behrensmeyer predicts that if we can understand what geological conditions allowed these tracks to be preserved, we should know more about why hominins continued to return to the lake shore environment over more than 100,000 years. The research team plans to return to Kenya this year to continue investigating the fossil footprint sites.

The species shared its world with early members of the genus Homo but followed a very different evolutionary path. Why ancient hominins returned to the lakeshore remains a key question, with the new tracks adding a new dimension to what scientists know about Paranthropus boisei.