The oldest unequivocal evidence of bipedal hominins to date is the 3.7 million-year-old footprints discovered at Laetoli, Tanzania in 1978.

An adjacent site also had footprints, Site A. In fact, they were discovered two years before Site G and S. But they were so different that they were dismissed. Some researchers hypothesized they were made by an upright young bear walking on its hind legs because they looked so different from footprints from other sites. That is until the authors of this new study looked more closely

In June of 2019, the scientists of this current Nature published study, re-excavated five, consecutive footprints. They measured, photographed and 3D scanned these footprints. They teamed up with people who run a bear center and performed a comparative anatomical study of the Laetoli Site A tracks to the footprints of black bears (Ursus americanus), chimpanzees (Pan troglodytes), and humans (Homo sapiens).

Analyzing impressions of the footprints they were able to find some differences. Firstly, the heals of the bear taper. Bear toes and feet are fan-like. Chimpanzees also have relatively narrow heels compared to their forefoot. But the Laetoli footprints, including those at Site A, have wide heels relative to their forefoot.

Secondly, Site A's footprints also contained the impressions of a big toe and a smaller second digit. The size difference between the two digits is something we see in apes like humans and chimpanzees, but not black bears. Lastly, after recording over 50 hours of bears walking, the team observed bears spent less than 1% of that time bipedal. They concluded it is unlikely a bear could have been upright for the length and duration seen at Site A. The summation of these differences demonstrates that the footprints were likely made by a hominin moving on two legs.

But in comparing the Laetoli footprints at Site A and the inferred foot proportions, morphology, and likely gait, the team’s results reveal that the Site A footprints are distinct from those of Australopithecus afarensis at Sites G and S, writing,

“these hominins did not walk with morphologically identical feet.”

We’ve known since the ‘70’s that the Site A prints were different. We just did not have a detailed study like this to say they are a different hominin and not a different mammal. Just which other species of a bipedal hominin from 3.7 million years ago is a question for another day.