Ancient ‘Bear’ Footprints in Laetoli Tanzania Belonged to Human Ancestor
While exploring the Laetoli fossil site in the foothills of northern Tanzania in 1978, a team of anthropologists led by the renowned Mary Leakey discovered the oldest footprints of a human ancestor found anywhere in the fossil record. These tracks were estimated to be approximately 3.66 million years old and are believed to have been left by a member of an archaic hominin species known as Australopithecus afarensis, which shared characteristics with both ancient apes and modern humans.
This was viewed as a landmark discovery, and rightly so. What no one realized at the time was that Leakey’s team had actually made their initial discovery of archaic human footprints two years earlier, at a site less than a mile (about one kilometer) away from the later discovery site.
Incredibly, these earlier footprints were incorrectly identified as having belonged to a bear. Only now, 45 years after the discovery of these walking tracks, has the truth finally been revealed, thanks to an authoritative study led by a former Dartmouth College (New Hampshire) graduate student and her professor. These researchers and their colleagues have just published the results of their work in the journal Nature, explaining how fresh evidence clearly contradicts the bear theory.
Like the footprints found in 1978, the new study reveals that these tracks were also left by a long-extinct human ancestor. Whoever this individual was, however, he or she cannot be identified as an Australopithecus afarensis, which is what caused the misidentification in the first place. These newly recategorized footprints belonged to a different human ancestor species entirely, one that for now remains unidentified.
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This model of five hominin footprints at site A in Laetoli, Tanzania, was made using photogrammetry and a 3D scan. (Austin C. Hill and Catherine Miller / Dartmouth College)
Anatomical Analysis Reveals Laetoli Footprints Are Human
The single track of footprints that were incorrectly credited to a bear were found preserved in a layer of ancient hardened volcanic ash at the Laetoli site in 1976. Their discovery launched one of the most fruitful and productive paleoanthropological ventures of all time, which produced fossilized tracks left by archaic humans and by many species of animals that lived in Africa during the Middle Pliocene Epoch.
Following the eruption of a volcano somewhere in the region, a thick layer of ash had settled over a broad area of eastern Africa, nearly 3.7 million years ago. This created a perfect medium for the long-term preservation of archaic human and animal footprints, as volcanic ash left undisturbed will eventually harden into rock.
The footprints at Laetoli Site A, as the location of the initial discovery was called, were clearly left by a creature walking on two feet. They were nevertheless identified has likely having belonged to a bear, based on the knowledge that this large and heavy animal was known to occasionally walk on its hind legs. The idea that they might have belonged to an ancient human-like species was rejected, since the footprints didn’t seem to bear much of a resemblance to those of modern humans (they also didn’t resemble the Australopithecus afarensis tracks that were discovered at Laetoli Sites G and S two years later).
Images of the Laetoli A3 footprint (top) length matched to Laetoli G1 (bottom). (b) Plot comparing foot length to forefoot width in adult and juvenile humans and bipedal chimpanzees. (c) Histogram of Mahalanobis distances between the mean modern human footprint and the averages of two randomly drawn human footprints. (Nature)
While working on her doctoral dissertation at Dartmouth College in New Hampshire in the late 2010s, anthropology graduate student Ellison McNutt read about the alleged bear footprints that Mary Leakey and her compatriots had discovered at the famed Laetoli site. Knowing their identification was still controversial, she came up with an idea. She wanted to perform a more in-depth study of the fossilized footprints, before comparing them to footprints made by real bears to see if there really was a good match.
“Given the increasing evidence for locomotor and species diversity in the hominin fossil record over the past 30 years, these unusual prints deserved another look,” McNutt, who now teaches anatomy at the Heritage College of Osteopathic Medicine at Ohio University, explained in a Dartmouth College press release.
The Kilham Bear Center, a wildlife rescue and rehabilitation facility, was located just a few miles away from Dartmouth. This gave McNutt the chance to obtain as many bear footprints as she needed to make a comprehensive comparison.
McNutt’s idea attracted the attention of other academics. In 2019, a team of scientists led by University of Colorado-Denver anthropologist Charles Musiba traveled to Laetoli to take new measurements, photographs, and 3-D images of several of the Site A footprints. Later, after the freshly obtained evidence had been examined more closely, members of the research team compared the Site A prints to those made by bears, chimpanzees, and modern-day humans, seeking the closest match.
Research team members, Anjali Prabhat and Catherine Miller survey Laetoli Site A to identify the location of the bipedal footprints. (Jeremy DeSilva / Dartmouth College)
Back in New Hampshire, McNutt and her closest collaborator, Dartmouth College anthropology professor Jeremy DeSilva, visited the bear center on several occasions, and with the cooperation of its staff they were able to collect many samples of footprints made by bears walking on two feet to in order to reach specially prepared snacks. The scientists also watched over 50 hours of video footage of bears in the wild, to see how often they actually walked on two feet in their natural living environment.
Through careful and diligent work, McNutt and her team members were able to debunk the bear hypothesis for the Site A footprints once and for all.
“As bears walk, they take very wide steps, wobbling back and forth,” DeSilva explained. “They are unable to walk with a gait similar to that of the Site A footprints, as their hip musculature and knee shape does not permit that kind of motion and balance.”
The shapes of the feet were also different. Bears have tapered heels and feet and toes that fan outward, while the Site A walker had squared-off feet that featured prominent big toes. These were characteristics the Site A walker shared with modern humans, along with a few other qualities that are unique to hominin (human or near-human) species.
Adding yet another nail to the coffin of the bear theory, McNutt and Silva’s observations of wild bears showed they seldom walked on two feet, doing so less than one percent of the time. When they did rise up on two feet they would go back down on all fours again as quickly as possible, but the footprints found at Site A were made by a creature that walking exclusively on two feet.
To check the possibility that the Tanzanian footprints might have belonged to a non-human ancient primate, the researchers also compared the new Laetoli Site A footprints to those of chimpanzees living at the Ngamba Island Chimpanzee Sanctuary in Uganda. This analysis showed important differences between chimpanzees and the Site A walker, differences that placed the Site A individual much closer to modern humans on the ever-branching evolutionary tree.
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Reconstruction of a male Australopithecus afarensis at the Natural History Museum, Vienna. (Wolfgang Sauber / CC BY-SA 4.0)
But Which Species Was It? Scientists Want to Know
Scientists falsely identified the original footprints at Laetoli based on incomplete knowledge and only a casual study of their features. When footprints that looked much more human were found at Sites G and S, it only reinforced the idea that those other tracks must have been left by a non-human species.
But Australopithecus afarensis wasn’t the only hominin roaming around eastern Africa during the Middle Pliocene. They shared the region with another species of archaic human, which possessed fewer characteristics that linked them to modern humans than Australopithecus afarensis, but still enough that a more thorough analysis was able to uncover the truth.
“Through this research, we now have conclusive evidence from the Site A footprints that there were different hominin species walking bipedally on this landscape but in different ways on different feet,” DeSilva declared.
An obvious next step would be to try to identify this new species. Two possible candidates would be a pair of ancient hominins known to have inhabited the region millions of years ago, Kenyanthropus platyops and Australopithecus deyiremeda. No fossilized foot imprints have been found for the first, but they have been found for Australopithecus deyiremeda, and according to DeSilva there seem to be similarities between those tracks and those found at Site A.
“That one, to me, is really intriguing as a possible candidate for the hominin that would have made these footprints,” he told Science Magazine, “but we're not going to know for certain until we do some more work at that site.”
More than four decades after the original discoveries of Mary Leakey and associates, the Laetoli fossil beds are still revealing some astonishing secrets about the history of human evolution.
Top image: Blaine Maley, from the Idaho College of Osteopathic Medicine, works alongside Prabhat, Fannin, and Montgomery Fellow Charles Musiba at site A in Laetoli where the archaic footprints were found. Source: Shirley Rubin / Trustees of Dartmouth College
By Nathan Falde