Enigmatic footprints, once thought to belong to a bear, linked to unknown human ancestor

A footprint from Laetoli site A, left, that researchers say belongs to a previously unknown type of hominin. On the right is an image of a cast of a footprint belonging to  Australopithecus afarensis.

(CNN)Footprints discovered in 1978 at in Tanzania and dated to 3.66 million years ago were widely thought to be the oldest uncontested evidence of upright walking in the human family tree.

Found at a place known as Laetoli site G, they are generally accepted as belonging to Australopithecus afarensis -- the species of the famous skeleton "Lucy," perhaps the world's best known fossil.
However, the Laetoli site G footprints weren't the only ancient trackways researchers came across at that time. A set of footprints a mile away, at a place called Laetoli site A, were attributed to a young bear walking upright on its hind legs because they were so different from the tracks left by Australopithecus afarensis.
    Researchers now believe the Laetoli site A footprints may belong to a different early human ancestor that also walked on two legs, a revelation that could rewrite this chapter of the human story.
      "These footprints demonstrate that the evolution of upright walking was more complicated and more interesting than we previously thought," said Jeremy DeSilva, an associate professor in the department of anthropology at Dartmouth College and coauthor of the research, which published in the journal Nature on Wednesday.
      "There were at least two hominins, walking in different ways, on differently shaped feet, at this time in our evolutionary history, showing that the acquisition of human-like walking was less linear than many imagine."
      The human version of walking on two legs, known as striding bipedalism, is unique among mammals and conventional thinking was that it had a single evolutionary origin.

        Ancient sediments

        Laetoli is a stark but beautiful grassland environment northwest of the Ngorongoro Crater in northern Tanzania, with acacia trees littering a landscape that is inhabited by giraffes and zebras. Seasonal rains have cut through ancient sediments here and there, exposing a 3.66 million-year-old layer of hardened volcanic ash, which DeSilva said preserves the impressions of thousands of footprints from ancient antelopes, elephants, large cats, birds and insects -- and our ancient hominin ancestors.
        Site A was never fully excavated and was covered up shortly after the footprints were discovered by pioneering paleontologist Mary Leakey in 1977 or 1978, DeSilva said. It's not clear whether the covering was done deliberately to protect the tracks or if the rains washed sediment from the adjacent hillside over them.
        Unlike the now-famous footprints at site G, the tracks had an unusual shape and suggested a upright walking movement that had a peculiar cross-stepping manner, in which each foot moved over the body's midline to touch down in front of the other foot, said Stephanie Melillo, a paleoanthropologist and postdoctoral researcher at the the department of human evolution at the Max Plank Institute for Evolutionary Anthropology, in Leipzig, Germany. She wasn't involved in the research.
        One explanation at the time for the enigmatic footprints was that they had been made by a bear walking on two feet, although Leakey had wondered if they were left by a hominin with an irregular gait.
        "Scientists were not convinced by either explanation. Ultimately, the site A prints were more easily forgotten than explained," said Melillo in a commentary on the research published in Nature.