The question of whether our early ancestors walked fully upright or in a crouched position, like apes, has long been a hot debate among scientists.
Now, new research presented at the annual meeting of the American Association of Anatomists during the 2018 Experimental Biology Conference in San Diego suggests that the characteristic bipedalism of modern humans may have evolved much earlier than previously thought—long before our species had even emerged.
For the research, evolutionary anthropologist David Raichlen and his colleagues from the University of Arizona, examined 3.6-million-year-old hominin footprints recently discovered in Laetoli, Tanzania, which represent the earliest direct evidence of hominin bipedalism.
They found that while there may have been some subtle differences, the hominins that made these footprints walked in much the same way as we do.
Our species, known as Homo sapiens sapiens appeared between 200,000 and 300,000 years ago. We are the only surviving member of the genus—or group of species—called Homo which also includes several extinct species of human—such as Neanderthals—the first of which emerged around 2.5 million years ago.
Meanwhile, the term 'hominin' describes the broader group of species that existed before that (although there is some debate as to which species should be included in this distinction). Scientists generally think hominins started to walk on two legs around 7 million years ago, although it is likely that these early ancestors shuffled along with bent legs in a crouched position—a conclusion that has been reached by looking at how other primates evolved.
For their research, the Arizona team reconstructed the walking mechanics of early human ancestors using data from fossilized footprints and skeleton, finding that the Laetoli footprints are consistent with a fully-upright, straight-legged walking posture.
One of the team’s experiments involved comparing the depth and shape of the ancient footprints to those of modern humans, provided in this case by eight volunteers who either walked in an upright or crouched posture, with bent knees and hips.
When analyzing footprints, looking at the difference between the impressions made by the toe and the heel reveals how the center of pressure moves along the foot during a step and therefore provides clues to walking posture. After comparing the footprints, the results showed that the Laetoli prints were much more similar to those made by humans walking upright.
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Walking in this manner with fully extended legs is more efficient—and therefore uses less energy—than shuffling in a crouched position. This means longer journeys can be undertaken, suggesting that the evolutionary switch from one mode of walking to another may have been linked to how human ancestors looked for food, according to the researchers.
"The data suggest that by this time in our evolutionary history, selection for reduced energy expenditures during walking was strong," Raichlen said in a statement. "This work suggests that, by 3.6 million years ago, climate and habitat changes likely led to the need for ancestral hominins to walk longer distances during their daily foraging bouts. Selection may have acted at this time to improve energy economy during locomotion, generating the human-like mechanics we employ today."
While the new research suggests hominins were walking upright at least 3.6 million years ago, when exactly this distinct locomotion style first diverged from other apes remains unclear. Analysis of older footprints will be needed to answer this question.
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