(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.
Enigmatic footprints, once thought to belong to a bear, linked to unknown human ancestor
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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.
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.
DeSilva said they decided to re-excavate the site after he and his colleagues collected footprint data from humans, chimpanzees and bears that cast doubt on the bear hypothesis. However, it was a challenge to re-examine the five consecutive footprints.
"Mary Leakey made exquisitely detailed maps of the footprint localities. From her map, we were able to approximate where the tracks should be. We began to dig, hoping for the best, but fearing instead that forty years of seasonal rains had washed them away," DeSilva said via email.
"The soil was hard as cement and it took a hammer and chisel to reach the footprint layer, which we then needed to excavate delicately with a hard-bristled brush and tongue depressor. Fortunately, the footprints were beautifully preserved."
Once they had cataloged the original prints, they compared them with prints belonging to black bears (Ursus americanus), chimpanzees (Pan troglodytes) and modern humans (Homo sapiens).
They also obtained over 50 hours of video of wild black bears. The bears walked on two feet less than 1% of the time. This made it unlikely that a bear made the footprints at Laetoli, especially given that no footprints were found of this individual walking on four legs, the researchers said.
DeSilva said that when non-human animals walk on two legs, they cannot balance on a single leg. This means they wobble back and forth as they move forward, producing widely spaced footprints.
However, early in human evolution, changes to the position of our ancestors' hip muscles and knees allowed upright hominins to balance on a single leg at a time and walk in a straight line, without the side-to-side motion.
Melillo agreed that the new excavation had revealed "a combination of features diagnostic of hominins."
"The big toe and second toe are similar in length; the impression made on the ground by the big toe is much larger than that made by the second toe; the impressions made by the toes and the rest of the foot are continuous; and the heel is wide," she said.
"Still, the site A footp