Fossil footprints still are puzzling scientists: Bear or ancient human?

In this photo from video, Ellison McNutt collects data from a juvenile female black bear (Ursus americanus), who walks bipedally, unassisted through the mud trackway at Kilham Bear Center in Lyme, N.H. 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. (Jeremy DeSilva/Dartmouth College via AP)