Scientists have solved the mystery of 3,4 million-year-old fossils, dubbed the "Burtele Foot," discovered in Ethiopia in 2009, determining that they belonged to an enigmatic human ancestor that lived alongside another closely related species, during a poorly understood period of human evolution.
Based on the recently discovered 25 teeth and jawbone of a four-year-old child near the site, scientists determined that eight foot bones belong to the Australopithecus deyiremeda species, which combined ape-like and human-like traits and was first described only a decade ago, Reuters reports.
The "Burtele foot," so named because the bones were found at the Burtele site in the Afar region of northeastern Ethiopia, showed that the species was bipedal but still had a big toe that could oppose the other toes—a feature useful for climbing trees. It's evidence that, while it walked upright, it walked differently from modern humans.
Fossils show that two closely related hominin species - species from the human evolutionary lineage - lived at the same time and in the same area, the other species being Australopithecus afarensis. This raises the question of whether these close "cousins" used the same resources or were sufficiently different to avoid direct competition.
Australopithecus afarensis found in the leg of a separate fossil Lusi, discovered in 1974 in the Afar region.
The new findings deepen our understanding of that period of human evolution, long before our species Homo sapiens emerged about 300.000 years ago, Reuters reports.
"They provide us with the most compelling evidence that Australopithecus afarensis - Lucy's lineage - was not the only human ancestor that lived between 3,5 and 3,3 million years ago," said paleoanthropologist Johannes Haile-Selasie, director of the Institute for Human Origins at Arizona State University and lead author of the study published in the journal Nature.
"As a result, we now know that the early stages of our evolution were not linear, in the sense that there was only one species at any given time," Haile-Selasie added.
Fossils show that these two species walked differently and had different plant-based diets.
"Understanding the differences and similarities between these neighboring hominins is key to understanding their environments and perhaps even how their interactions, even indirect ones, shaped their evolution and their relationship to our own species," said geochemist Naomi Levin of the University of Michigan, a co-author of the study.
The big toe of Lucy's genus was not opposable and was more like ours. The big toe of Australopithecus deyiremeda was a more "hereditary", more primitive form, similar to the thumb of apes that climb trees. When on land, this hominin walked on two legs and probably did not rely on the big toe for support, like Lucy's genus and modern humans, but on the second toe. reports Reuters.
"This certainly made walking on two legs less efficient on the ground. However, it was much more efficient for climbing trees - which is by no means a bad adaptation in areas where there were large predators," Haile-Selasie said.
Among these predators were large saber-toothed cats and hyenas.
"We know that our lineage originates from an ancestor that had an opposable big toe," Haile-Selasie said. "The human mode of bipedal walking must have gone through numerous 'experiments' and adaptations, with some aspects of the foot, leg and pelvis changing at different times."
Chemical analysis of the enamel of eight teeth of Australopithecus deyiremeda revealed what types of plants this species ate.
Lucy's genus was more of a "generalist," with a broader diet that included grass-based foods, but also tree and shrub foods—such as leaves, fruits, or nuts. Australopithecus deyiremeda, on the other hand, had a diet limited to tree and shrub foods, similar to more primitive hominins. This is likely related to the anatomy of the foot adapted for climbing.
"These species moved in different ways. There were multiple 'ways to be human' at the time, and each of those ways probably had some advantage. What's exciting to me is that we can now link these different ways of bipedal locomotion to different types of diets. We can link different morphological adaptations to different behaviors," Levin said.
Eating a more diverse diet may have given Australopithecus afarensis a competitive advantage.
"But we also have to consider the possibility," Levin added, "that perhaps Australopithecus deyiremeda had some advantage, forcing Australopithecus afarensis to expand its feeding strategy. Now that we know they ate different foods and moved in different ways, we are one step closer to solving this puzzle of their coexistence."
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