Fossil footprints puzzle scientists: Bear or ancient human?

Prehistoric footprints that have puzzled scientists since the 1970s are getting a second look: Were they left by extinct animals or by human ancestors?

When famed paleontologist Mary Leakey first uncovered the footprints in Tanzania 40 years ago, the evidence was ambiguous.

Leakey focused her attention instead on other fossil footprints that could be more clearly linked to early humans. Those footprints, found at a site called Laetoli G, are the first clear evidence of early humans walking upright.

Decades later, a new team re-excavated the confusing footprints, found at a site called Laetoli A, and made photos and 3-D scans available for other researchers to continue the debate.

The research was published Wednesday in the journal Nature.

"These footprints have been in the mystery category for 40 years," said Rick Potts, who directs the Smithsonian Institution's Human Origins Project.

"It's a really exciting idea to re-exhume them and study them again," added Potts, who was not involved in the research.

What's long perplexed scientists is that those tracks, broad footprints with enlarged fifth toes and estimated to be around 3.7 million years old, don't closely match anything scientists have elsewhere identified.

"They didn't have the right weight and foot movement to be easily identified as human, so other explanations were sought," including that they may belong to an extinct species of bears, said co-author and Dartmouth paleoanthropologist Jeremy DeSilva.

He and other researchers returned to the site in 2019 and used Leakey's original maps to locate the enigmatic prints, preserved in a layer of volcanic ash that had cooled and hardened.

Co-author Ellison McNutt of Ohio University studied the...