Fossilized footprints that were found in Tanzania in the 1970s, dismissed for decades as having been made by bears, may have been left by an unidentified early human ancestor around 3.6 million years ago, new research suggests.
The footprints were discovered in 1976 near the site at Laetoli in northern Tanzania where, two years later, the paleontologist Mary Leakey and her team found another set of prints — believed to have been made by the same species that left behind the famous “Lucy” skeleton — that offered the first clear evidence of early humans walking on two feet.
The first set of prints was overshadowed. A paleoanthropologist’s suggestion that they could have been bear tracks only diminished interest in the discovery, and the prints had largely been forgotten by archaeologists until now.
But a study based on a new analysis of those prints, published in the journal Nature on Wednesday, indicates that they were made by an unidentified hominin, or early human. The findings suggest that Lucy’s species, Australopithecus afarensis, was not the only hominin walking the earth 3.6 million years ago.
“Upright walking is a defining characteristic of our lineage,” said Jeremy DeSilva, an associate professor of anthropology at Dartmouth and a senior author of the study. “It is a hallmark of being humans. Despite that, our understanding of the origins and evolution of bipedal locomotion is still something we’re trying to figure out.”
Ellison McNutt, an assistant professor at the Heritage College of Osteopathic Medicine at Ohio University and the lead author of the study, was seeking a better understanding of the postures that preceded bipedalism. Through her research into bears and their movements, she came across the set of five footprints that had been partially excavated in 1976 and thought they could help untangle the mystery of what led humans to walk on two legs.
The prints, known as the A trail, were an unusual shape, like a shorter and more stout version of a modern human’s footprint. They showed a cross-step walking movement — not unlike a model on a catwalk — in which each foot crosses the body’s midline to touch down in front of the other.
The researchers said that the prints’ ratio of foot width to length indicated that they had been made by a different species than Lucy’s, one that did not share an evolutionary trajectory with chimpanzees. The foot is wider than that of a typical early human, the researchers said, and the cross-walk pattern that the prints show can happen only if a species walks on two legs, with the aid of the hips.
The researchers recorded almost 60 hours of video of wild American black bears. Unsupported bipedal posture and movement occurred only 0.09 percent of the time, they said. Only once did a bear take four unassisted bipedal steps, according to the study. The archaeologists concluded that this “makes it unlikely” that the fossilized prints belonged to a bear.
Having more than one hominin species living during the same time period, walking a little differently with different foot sizes, “tells us that there wasn’t sort of a one-track way to our evolution,” Dr. McNutt said. “And it just turns out the kind of way that we do is the only one that still survives today.”
The study comes as more research is challenging and changing the understanding of how many early human species occupied the earth three million to 3.7 million years ago, during what is known as the Pliocene Epoch, said Stephanie M. Melillo, a paleoanthropologist with the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Dr. Melillo was not a part of the study, but summarized its findings for Nature.
William Harcourt-Smith, an associate professor of anthropology at Lehman College and a resident research associate at the American Museum of Natural History, said that both sets of footprints could have been made within days of each other.
“What’s news about this particular finding is that these are footprints made in almost exactly the same time,” said Dr. Harcourt-Smith, who was not involved with the study.
“This is the real deal,” he added. “It is the smoking gun of two different fossil hominins at the same time in the same landscape, if they are indeed both hominins.”
But Tim D. White, a paleoanthropologist and a professor of integrative biology at the University of California, Berkeley, was skeptical, saying that it was “a step too far” to conclude that a new hominin species had been identified.
Dr. White, who was present at the excavations of both sets of footprints in Laetoli, said that the differences between them were minimal, and not enough to definitively indicate the existence of another bipedal species. When footprints are made in volcanic ash, as these were, the prints at their deeper layers can become deflected, flatter or broader, changing their size and shape, he said.
The experts did agree that the new research disproves the original hypothesis that the A trail prints had been made by bears. There are no bears in the fossil record at Laetoli, Dr. White said.
The researchers said they planned to continue to excavate the site in search of more footprints.