(CNN) — Scientists have revised the longstanding picture of “Nutcracker Man” — an extinct hominin, or human relative, nicknamed for its large teeth and powerful jaw muscles — thanks to fresh analysis of fossil footprints from a site in Kenya.
Nutcracker Man, whose species is scientifically known as Paranthropus boisei, lived in eastern Africa from about 2.3 million to 1.2 million years ago, and this new evidence suggests that P. boisei was taller and heavier and had more complex social relationships than once thought.
Prior estimates based on P. boisei fossil evidence hinted that males stood about 4.2 feet (131 centimeters) tall and weighed approximately 108 pounds (49 kilograms) on average. Updated calculations based on the length and width of 21 newly identified P. boisei footprints suggest that Nutcracker Man’s size was more humanlike, standing up to 6 feet (1.8 meters) tall and weighing as much as 165 pounds (75 kilograms).
Patterns in the footprints further indicate that the tracks belonged to eight P. boisei individuals who crossed through the site — East Turkana’s Koobi Fora Formation — at the same time, traveling as a group. These preserved prints provide clues about the complexity of their social structure in their lakeside habitat, researchers reported Monday in the journal PNAS.
“The site captures an amazing snapshot of hominin anatomy, locomotion, behavior, and environments,” said lead study author Kevin Hatala, an associate professor of biology at Chatham University in Pittsburgh and an associate researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
“Each set of footprints like this one provides a window to a moment in time when we can directly observe how hominins were living, and engaging with their environments,” Hatala told CNN in an email. “We think that the footprints may capture a group of adult males moving together along the lakeshore,” he said, adding that the threat of dangerous animals such as hippos could explain why the group included only adult males.
“Footprints are important fossils because they provide us with both anatomical and behavioral information,” said Patricia Kramer, a professor in the department of anthropology at the University of Washington in Seattle and an affiliate curator of archaeology at the Burke Museum. Kramer has previously studied these trackways but was not involved in the new research.
“I am excited that the authors have provided additional information and analysis about the trackways at Koobi Fora,” she told CNN in an email.
Coexisting cousins
The tracks, which are about 1.4 million years old, are well-defined with depths reaching 0.4 inches (12 centimeters), and were probably pressed into mud in shallow lake water, the authors reported. Seven tracks were uncovered in 1978 through fieldwork led by senior study author Anna Katherine “Kay” Behrensmeyer, a senior research geologist and curator of vertebrate paleontology at the Smithsonian’s National Museum of Natural History in Washington, DC. Excavations in 2016 and 2023 later revealed 14 more hominin footprints, along with tracks left by animals such as antelope and hippos.
P. boisei wasn’t the only archaic human relative living near Lake Turkana. For hundreds of thousands of years, P. boisei coexisted alongside Homo erectus. Another study from 2024 led by Hatala reported the discovery of fossil footprints in Kenya’s Turkana Basin made no more than a few days apart by two different hominin species, and the researchers hypothesized that the prints belonged to H. erectus and P. boisei.
“We can’t say how the two different hominins interacted on the mudflats, but we know they were both here at this time,” Behrensmeyer said in a statement.
Homo erectus was the earliest relative of Homo sapiens to share similar body proportions with modern humans, demonstrating that the species had adapted to a life of upright walking. By comparison, P. boisei, a more distant cousin of modern humans, had physical features that more closely aligned with those of our tree-dwelling primate ancestors.
Impressions made by P. boisei feet resemble footprints made by H. erectus and modern humans. However, some key differences set P. boisei tracks apart, Hatala said.
In prior research into hominin footprints dating to the early part of the Pleistocene (2.6 million to 11,700 years ago) — when P. boisei was around but H. erectus was not — researchers found that the prints had flatter arches than those made by H. erectus and modern humans. While that doesn’t necessarily mean those older hominins had flatter feet, “it at least tells us they moved their feet against the ground in a different way,” Hatala explained.
“We’ve also found that those same footprints tend to have a big toe that is more angled away from the other toes,” he added. “Both of these features differ from what we usually see in modern humans, and that led us to hypothesize that footprints with that shape were more likely made by P. boisei.”
Footprint impressions can vary greatly depending on the type of ground and how wet or sticky it is, so the researchers controlled for those variations, Hatala said.
“In our experiments we have sampled an extensive range of substrate variation, and we have conducted multiple experiments using a rehydrated version of the same kind of mud in which these Early Pleistocene footprints are preserved.”
Further analysis of P. boisei’s footprints could reveal additional clues about its habits, though it will be challenging to interpret how the species’ toe angle and arch height affected its lifestyle, Kramer said.
“I am cautious about attributing importance to particular morphology because there are many ways to move effectively in the environment as a biped,” she said. “Nonetheless, we desperately need the kind of in-depth, thorough analyses that this new work exemplifies.”
Fossil footprints: A snapshot in time
Scientists discovered the first P. boisei fossils in the 1950s. However, estimating the size of this human relative has been challenging because there have been few fossils other than skulls that could unequivocally be attributed to the genus, according to Hatala. Larger bones from this time and place were typically thought to belong to H. erectus, while smaller bones were associated with Paranthropus.
“But maybe these species were much closer in body size than previously thought,” Hatala said.
Unlike fossilized anatomical remains such as bones and teeth, fossil footprints are a snapshot in time that capture an impression of ancient hominins when they were still alive. Those glimpses do more than preserve the past; they also highlight the connection between modern humans and our long-extinct relatives, Behrensmeyer said.
“They allow people to relate to these fossils much more easily than an isolated piece of bone or jaw,” she said.
This perspective brings the prehistoric world of our extinct relatives into clearer focus, Kramer added. While P. boisei and H. erectus may not have looked much like modern humans, Homo sapiens followed in their footsteps and walked the same African landscape about half-a-million years later.
“It is a story of a world populated by creatures so much like us but so different at the same time,” Kramer said.
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