Five fossil footprints left in volcanic ash in Tanzania 3.66 million years ago are offering scientists a new look at a milestone in human evolution —the upright gait— while also showing that its origins are more complex than was previously known. now.
Researchers said on Wednesday that a new in-depth analysis of the footprints, done nearly half a century after they were discovered, revealed that they were made not by a bear, as originally thought, but by a hominin — in other words, a species of the human lineage—possibly as yet unknown.
The footprints reveal a curious way of walking, which adds to the mystery around them.
Bipedalism — that is, walking on both feet — is a typical characteristic of humanity. But scientists are still piecing together the puzzle of how and when it started.
The footprints were found in 1976 at a site called Laetoli, an arid landscape northwest of Ngorongoro Crater in northern Tanzania, about a kilometer away from two sets of fossil footprints found two years later.
The footprints found in 1978 were attributed to a Australopithecus afarensis, a hominin exemplified by the famous skeleton discovered in Ethiopia and nicknamed Lucy.
The study determined that the various tracks found at Laetoli — made days, hours, or possibly minutes apart, in the same layer of ash — were created by two different species of hominin.
Paleoanthropologist Ellie McNutt of the Heritage College of Osteopathic Medicine at Ohio University and lead author of the study published in the journal Nature, pointed out that the tracks at Laetoli represent the oldest unambiguous evidence of bipedal locomotion in the human fossil record.
“There were at least two hominins walking in different ways, with different shaped feet, at this time in our evolutionary history, showing that the acquisition of walking in a human way was less linear than many imagine”, commented the co-author of the study, the paleoanthropologist Jeremy DeSilva of Dartmouth College.
“In other words, throughout our history there have been distinct evolutionary experiments on how to be bipedal.”
The footprints found in 1976 and re-excavated in 2019 exhibit different characteristics from those discovered in 1978, in particular a way of walking called “cross-stepping”, or taking crossed steps.
“The trail consists of five consecutive bipedal footprints. But the left foot crosses over the right, and vice versa. We still don’t know for sure what this means,” said DeSilva.
“Crosswalk sometimes occurs in humans when we’re walking on uneven terrain. Maybe that explains this bizarre gait. Or maybe it’s just this individual hominin that walked in a peculiar way. way,” added DeSilva.
Based on the footprints, the researchers estimate that the individual who left them was just over three feet tall, pressed his heel heavily when walking, and had a big toe that protruded slightly to the side, although not as much as it is. the case in a chimpanzee.
DeSilva said scientists can only speculate about other aspects of this hominin’s appearance and behavior or whether it is one that has already been identified, such as the Kenyanthropus platyops or the Australopithecus deyiremeda, or a hitherto unknown.
The human lineage departed from the chimpanzee lineage about 6 million to 7 million years ago. A key moment came when our ancestors adopted upright walking on two feet, possibly to adapt to life in the African savannah.
Bipedalism required anatomical transformations, particularly in the feet, legs, hips and spine, which evolved long before the emergence of our species, Homo sapiens, more than 300 thousand years ago.
The Laetoli site is a savanna area, with acacia trees dotted around the landscape and giraffes and zebras in abundance. At the time the footprints were left, it was a place that held many dangers for the little hominin, traversed by the ancestors of modern lions, leopards and hyenas, as well as the now extinct saber-tooth cats.
“The ancestors of many of the same animals that live in Laetoli today lived there millions of years ago, including, of course, humans,” said DeSilva.
Translation by Clara Allain
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