Some 3.6 million years ago, an upright walking creature trudged through layers of freshly fallen volcanic ash in what today is northern Tanzania. When anthropologists uncovered five of its fossilized footprints nearly 50 years ago, they couldn’t say whether this ancient biped was a hominin, a bear, or some other ape. Now, a new team claims to have solved this paleontological cold case, identifying the mystery walker as an unknown species of hominin. If true, this creature lived in the same place at the same time as the famed human ancestor, “Lucy.” It would also offer a window into the early day of our distant ancestors’ evolutionary forays into bipedalism.
Back in 1976, paleoanthropologists were combing a site called Laetoli in northern Tanzania’s hill country for fossils. Two members of the team began playfully flinging dried elephant dung at each other. In the fracas, one jumped into a gully and spotted what appeared to be an animal footprint preserved in hardened volcanic ash. The team, led by famed paleoanthropologist Mary Leakey, eventually found tracks left by ancient elephants, hippopotamuses, and more, all later dated to 3.6 million years ago.
Five consecutive footprints stood out. They were semitriangular in shape, with a wide sole that narrowed toward the heel. Whatever left them walked on two legs, “somewhat shambling, with one foot crossing in front of the other,” Leakey wrote.
At the time, nobody knew what to make of the impressions at what came to be called Site A, says Jeremy DeSilva, a paleoanthropologist at Dartmouth College and senior author of the new study. “They looked strange,” DeSilva says. Leakey suggested they might have been made by a hominin, but other experts suggested they were bear prints. Bears do sometimes walk on two legs, and their feet strike the ground heel first, similar to humans.
Then in 1978, Leakey’s team discovered dozens of fossil prints left by multiple individuals, about 1 kilometer away but in the same layer of volcanic ash. They bore little resemblance to the previously discovered tracks. In fact, these Site G tracks were “very similar to the kinds of footprints that you or I would make on a beach,” DeSilva says. Many speculated they were left by close kin of the famous fossil hominin known as Lucy, a member of Australopithecus afarensis, a human ancestor that lived between 3.9 and 3 million years ago.
As for Site A tracks? “The field kind of forgot about them,” DeSilva says.
One of DeSilva’s former graduate students, Ellison McNutt, picked up the thread while working on her doctoral dissertation in the late 2010s. McNutt, now a medical anatomist at Ohio University’s Heritage College of Osteopathic Medicine, had read about the bizarre Laetoli footprints and their proposed bear origin. A few miles up the road from Dartmouth lies the Kilham Bear Center, which rehabilitates orphaned black bear cubs. Why not compare the Laetoli Site A tracks to some actual bear tracks?
Working with the center’s staff, McNutt built a mud-covered trackway and enticed four juvenile black bears—whose paws were approximately the same size as the Laetoli tracks—to walk upright across it to get either applesauce or a maple syrup treat. Then she measured the muddy bear prints, including stride length, gait pattern, and feet dimensions. Next, McNutt and colleagues compared those characteristics with a digital reconstruction of the Laetoli prints—the original casts made in 1976 were lost—and to previously collected data on human and chimpanzee feet and gait.
When the analysis was done, McNutt was confident a bear didn’t leave the Laetoli Site A tracks. Bears have narrow heels and nearly equally sized toes, with exterior toes just slightly bigger than the others. The Laetoli tracks had broad heels and a prominent big toe. Bears also lack the hip or knee flexibility to cross their feet in front of one another, McNutt says. Chimps, too, lack that cross-stepping ability. The closest match, she says, is humans. Whatever left the Laetoli prints had feet “a little bit wider, with a little bit more extended big toe, than what we see in humans now.”
DeSilva was convinced, but he knew others in the field would want more evidence. So, he and colleagues, including a researcher from Tanzania’s Department of Cultural Heritage, returned to Laetoli and consulted Leakey’s old maps to find and re-excavate Site A, which had been covered over the years by sediment washing down a hill. After digging through a few inches of debris, they found the footprints “beautifully preserved,” DeSilva says.
New casts of the footprints reveal a prominent big toe adjacent to a smaller second toe. That’s another strong indication it belonged to a bipedal hominin, DeSilva says. Because the Site A and Site G footprints sit within the same layer of volcanic ash—and because the two sets of prints are so different from each other—the find suggests that 3.6 million years ago, two different species of bipedal hominins at Laetoli were walking within 1 kilometer of each other within the span of a few days, the researchers report today in Nature. “It’s showing there were these different experiments in bipedalism occurring at this time,” DeSilva says.
Although DeSilva agrees with many in the field that the Site G tracks were made by A. afarensis, the identity of Site A’s footprint maker remains a mystery. Candidates living in the region include Kenyanthropus platyops and A. deyiremeda. Researchers haven’t uncovered foot fossils for the former, but they have for the latter, and they share some suggestive similarities with the Site A tracks, DeSilva notes. “That one, to me, is really intriguing as a possible candidate for the hominin that would have made these footprints, but we’re not going to know for certain until we do some more work at that site.”
The idea that bears may have made the Site A tracks “was always a little bit of an odd explanation,” says William Harcourt-Smith, a paleoanthropologist at Lehman College and the American Museum of Natural History, and the researchers here have convincingly debunked it.
But he’s not completely convinced they’re homininmade, either. “I think it’s entirely possible—not likely, but possible—that one of the options for who made these prints could be a nonhominin ape,” Harcourt-Smith says. “Without more prints, it’s quite hard to know.”
That’s exactly what DeSilva and McNutt hope to find as soon as it’s safe to travel again given COVID-19 concerns. “We’d like to go back and continue excavating and try to extend this trackway,” McNutt says.
science.org, 1 December 2021