The results provide conclusive evidence that several hominid species coexisted in the landscape
The earliest unequivocal evidence of standing walking in the human lineage are footprints discovered in Laetoli, Tanzania in 1978 by paleontologist Mary Leakey and her team. The bipedal pathways date back to 3.7 million years ago. Another series of mysterious footprints was partially excavated at nearby Site A in 1976, but rejected as possibly having been made by a bear. A recent new search of the footprints at Site A in Laetoli and a detailed comparative analysis reveals that the footprints were made by a primitive human – a bipedal hominid, according to a new study published in Nature.
“Given the growing evidence for locomotor and species diversity in the hominid fossil record over the past 30 years, these unusual footprints deserved another look,” said lead author Ellison McNutt, teaching assistant professor at the Heritage College of Osteopathic Medicine at Ohio University. . She began her work as a graduate student in Ecology, Evolution, Environment and Society at Dartmouth College, where she focused on the biomechanics of walking in early humans and used comparative anatomy, including that bears, to understand how the heel bone contacts the ground (a position of the foot called “plantigradie”).
McNutt was fascinated by the bipedal (standing) footprints at Site A in Laetoli. Laetoli is famous for its impressive trail of hominid footprints at sites G and S, which are generally accepted as Australopithecus afarensis – the species of the famous partial skeleton “Lucy”. But because the footprints at Site A were so different, some researchers thought they were made by a young bear walking upright on its hind legs.
To determine the creator of the site A footprints, in June 2019, an international research team led by the co-author Charles Musiba, an associate professor of anthropology at the University of Colorado at Denver, traveled to Laetoli, where they re-searched and fully cleaned all five consecutive footprints. They identified evidence that the fossil prints were made by a hominid – including a large heel and big toe print. The prints were measured, photographed and scanned in 3D.
The researchers compared the traces from Laetoli site A to black bear tracks (American Ursus), chimpanzees (Pan troglodytes) and humans (Homo sapiens).
They teamed up with co-authors Ben and Phoebe Kilham, who direct the Kilham Bear Center, a rescue and rehabilitation center for black bears in Lyme, New Hampshire. They identified four semi-wild juvenile black bears in the center, with paws similar in size to the footprints at Site A. Each bear was lured with maple syrup or applesauce, to stand up and walk on its two hind legs through a mud-filled track to capture their footprints.
Over 50 hours of wild black bear video was also obtained. The bears walked two feet for less than 1% of the total viewing time, making it unlikely that a bear made the footprints in Laetoli, especially since no footprints were found from this individual walking on all fours.
“As the bears walk, they take very wide strides, swaying back and forth,” the lead author said. Jeremy DeSilva, associate professor of anthropology at Dartmouth. “They are unable to walk with a gait similar to that of the Site A footprints, as their hip musculature and the shape of their knees do not allow this kind of movement and balance.” Bears’ heels narrow and their toes and feet resemble fans, while the earliest human feet are square and have a prominent big toe, researchers say. Oddly, however, the site A footprints record a hominin crossing one leg over the other as it walks – a gait known as a “crossed step.”
“Although humans generally don’t cross step, this movement can occur when trying to restore balance,” McNutt said. “Site A footprints may have been the result of a hominid walking over an area that was an uneven surface.”
Based on footprints collected from semi-wild chimpanzees at the Ngamba Island Chimpanzee Sanctuary in Uganda and two captive juveniles at Stony Brook University, the team discovered that chimpanzees have heels relatively narrow compared to their forefoot, a feature common to bears. But Laetoli’s footprints, including those at Site A, have heels wide compared to their forefoot.
The prints from Site A also contained the prints of a large hallux (big toe) and a second smaller digit. The size difference between the two digits was similar to that of humans and chimpanzees, but not black bears. These details further demonstrate that the footprints were likely made by a hominid traveling on two legs. But by comparing the Laetoli footprints at site A and the inferred foot proportions, morphology and probable gait, the results reveal that the footprints at site A are distinct from those at Australopithecus afarensis on sites G and S.
“Thanks to this research, we now have conclusive evidence from the Site A footprints that there were different species of hominids walking bipedally across this landscape but in different ways on different feet,” said DeSilva, who said focuses on the origins and evolution of human walking. “We’ve had this evidence since the 1970s. It only took the rediscovery of these wonderful footprints and more detailed analysis to get us here.
McNutt joined Ohio University as an Assistant Professor of Medical Anatomy Teaching in July 2021. His research uses comparative functional morphology and biomechanics to study the evolution of primates and currently focuses on reconstruction and understanding the evolution of plantigrady in primates and bipedal locomotion in early hominids.