Ancient Footprints Reveal Another Hominin Species Lived Beside Lucy

A comparison of two footprints made probably on the same day 3.66 million years ago by two species of hominins and preserved by ash from a volcano. Laetoli A3 (left) and a cast of Laetoli G1 (right). The lengths are similar, but the former is wider indicating a different shaped foot, and therefore a different species. Image Credit: Jeremy DeSilva (left) Eli Burakian/Dartmouth

The footprints discovered by Mary Leakey and her team at Laetoli, Tanzania are some of the world's most famous. Their discovery in 1978 revealed that hominins, possibly our direct ancestors, had walked upright 3.66 million years ago. Nearby, the Leakey survey also discovered another set of footprints in 1976, but the identification of these was less certain, leaving them to fall into neglect. Now, anthropologists have returned to the ambiguous prints, found at Laetoli Site A, and found they prove two bipedal hominin species once lived in close proximity.

The team have re-examined the prints using more advanced techniques available today. Following detailed investigations of bear and chimpanzee prints, they report in Nature that the Laetoli Site A prints were almost certainly the product of a hominin, but shockingly not the same species as the one that made the more famous set nearby.

An astonishing 18,000 animal prints have been found in the volcanic ash at Laetoli Site A, but interest centers on five clearly made by an animal walking on its hind legs. Mary Leakey proposed they were made by a hominin. Other members of the team, however, thought they could be from a bear walking upright, and were reluctant to make a fuss about something so uncertain. When the unambiguous, and now famous, prints were found at Laetoli Site G, with a subsequent find at site S, Laetoli A prints were largely forgotten.

The Laetoli A prints in natural light (a), contour mapped (b), their location (c), and with topographical maps of A2 (d) and A3 (e). Image Credit: (a) and (b) Austin C. Hill/Catherine Miller (c): Illustration using GoogleMaps by Ellison McNutt. (d) and (e) by Stephen Gaughan and James Adams.

“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,” first author Dr Ellison McNutt of Ohio University said in a statement. McNutt is part of a large team that used Leakey's records to re-discover the prints and 3D scan them.

The authors then lured four semi-wild American black bears young enough their prints matched in size those at Site A, with maple syrup to walk bipedally across mud. The prints were so different that, even allowing for the fact an ancient Tanzanian bear would have been from another species the authors are confident no member of genus Ursus made the prints at Laetoli Site A.

“[Bears ] 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,” Dr Jeremy DeSilva of Dartmouth said. Moreover, although bears can walk on two legs, they do so only occasionally, so bipedal prints are unusual. Similar analysis ruled out near relatives of chimpanzees.

Ellison McNutt collects data on a bipedal female black bare for science. Right: Left footprint from one of the juvenile male black bears. Image credit: Jeremy DeSilva (left)/Ellison McNutt (right)

The prints show a cross-step in a way humans don't when moving freely, but resemble those of a person struggling to keep their footing on uneven ground.

Additional prints from the makers of the Laetoli G tracks would have some value, but the two don't match. Instead, Site A's tracks maker was smaller and had feet shaped differently enough to be another species.

“The footprints at site A are preserved in the same ash layer as the tracks made at sites G and S, meaning that they were made within (at most) days of one another, but more likely hours or minutes,” DeSilva told IFLScience. “These two hominins were contemporaries on the landscape.”

The Laetoli Site G prints have long been associated with Australopithecus afarensis, including the famous Lucy fossil, and the authors don't challenge that.

“The foot bones from A. afarensis match the footprints at sites G and S much better than they match the prints at site A,” DeSilva told IFLScience. “In this paleoanthropological version of Cinderella, the challenge now will be to discover which hominin foot fits the slipper [of Laetoli A].” DeSilva suggested Kenyanthropus platyops and Australopithecus deyiremeda as candidates.



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