Вадим Дудченко
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Bipedal trackways discovered in 1978 at Laetoli site G, Tanzania, and dated to 3.66 million years ago are widely accepted as the oldest unequivocal evidence of obligate bipedalism in the human lineage. Another trackway discovered in 1976 at nearby Laetoli site A was partially excavated and attributed to a hominin, but curious affinities with bears marginalized its importance to the paleoanthropological community. In new research, paleoanthropologists compared the Laetoli site A footprints with those of American black bears (Ursus americanus), chimpanzees (Pan troglodytes) and humans (Homo sapiens), and found that they resemble those of hominins more than bears.

Laetoli location and site rediscovery: (a) a model of Laetoli site A generated using photogrammetry showing the five hominin footprints; (b) corresponding contour map of the site generated from a 3D surface scan with scale bar; (c) map of Laetoli localities 7 and 8, indicating the positions of bipedal trackways A, G and S; (d, e) topographical maps of the two best preserved A footprints, A2 (d) and A3 (e). Image credit: McNutt et al., doi: 10.1038/s41586-021-04187-7.

“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,” said Dr. Ellison McNutt, a researcher with the Heritage College of Osteopathic Medicine at Ohio University.

To determine the maker of the Laetoli site A footprints, Dr. McNutt and colleagues went to the site, where they re-excavated, fully cleaned, measured, photographed and 3D-scanned the five, consecutive footprints.

They then compared the tracks to the footprints of black bears, chimpanzees, and humans.

“As bears walk, they take very wide steps, wobbling back and forth,” said Dr. Jeremy DeSilva, a researcher in the Department of Anthropology at Dartmouth College and the Evolutionary Studies Institute at the University of the Witwatersrand.

“They are unable to walk with a gait similar to that of the Laetoli site A footprints, as their hip musculature and knee shape does not permit that kind of motion and balance.”

“Bear heels taper and their toes and feet are fan-like, while early human feet are squared off and have a prominent big toe.”

Curiously, though, the Laetoli site A footprints record a hominin crossing one leg over the other as it walked — a gait called cross-stepping.

“Although humans don’t typically cross-step, this motion can occur when one is trying to reestablish their balance,” Dr. McNutt said.

“The Laetoli site A footprints may have been the result of a hominin walking across an area that was an unlevel surface.”

Based on footprints collected from semi-wild chimpanzees at Ngamba Island Chimpanzee Sanctuary in Uganda and two captive juveniles at Stony Brook University, the team found that chimpanzees have relatively narrow heels compared to their forefoot, a trait shared in common with bears. But the Laetoli footprints, including those at Laetoli site A, have wide heels relative to their forefoot.

The Laetoli site A footprints also contained the impressions of a large hallux (big toe) and smaller second digit. The size difference between the two digits was similar to humans and chimpanzees, but not black bears.

These details further demonstrate that the footprints were likely made by a hominin moving on two legs.

But in comparing the Laetoli site A footprints and the inferred foot proportions, morphology and likely gait, the results reveal that they are distinct from those of Australopithecus afarensis at Laetoli sites G and S.

“Through this research, we now have conclusive evidence from the Laetoli site A footprints that there were different hominin species walking bipedally on this landscape but in different ways on different feet,” Dr. DeSilva said.

“We’ve had this evidence since the 1970s. It just took the rediscovery of these wonderful footprints and a more detailed analysis to get us here.”

The study was published in the journal Nature.

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E.J. McNutt et al. Footprint evidence of early hominin locomotor diversity at Laetoli, Tanzania. Nature, published online December 1, 2021; doi: 10.1038/s41586-021-04187-7

 



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