Scientists solve the mystery of the prehistoric ‘Burtele Foot’

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By Will Dunham

WASHINGTON (Reuters) -Scientists have solved the mystery of 3.4 million-year-old fossils called the “Burtele Foot” discovered in Ethiopia in 2009, finding they belonged to an enigmatic human ancestor that lived alongside another closely related species during a poorly understood time in human evolution.

Based on the recent discovery nearby of 25 teeth and the jawbone of a 4-1/2 year-old child, scientists have determined that the eight foot bones represent the species Australopithecus deyiremeda, which combined ape-like and human-like traits and was first identified just a decade ago.

The Burtele Foot, so named because the bones were found at a site called Burtele in northeastern Ethiopia’s Afar region, showed that this species was bipedal but still had an opposable big toe, a feature useful for tree climbing – evidence that while it walked upright it did so in a different manner than people today.

The fossils show that two closely related hominins – species in the human evolutionary lineage – lived at the same time and place, with Australopithecus afarensis as the other species. This raises the question of whether these close cousins leveraged the same resources or were sufficiently different as to avoid direct competition.

Australopithecus afarensis is the species that includes the famous fossil Lucy, discovered in 1974 in the Afar region.

The new findings add depth to the understanding of this period of human evolution, long before our species Homo sapiens arose roughly 300,000 years ago.

“They provide us with the most conclusive evidence showing that Australopithecus afarensis – Lucy’s species – was not the only human ancestor that lived between 3.5 and 3.3 million years ago,” said paleoanthropologist Yohannes Haile-Selassie, director of Arizona State University’s Institute of Human Origins and lead author of the study published this week in the journal Nature.

“As a result, we now know that the earlier phases of our evolution were not linear, meaning only one species living at any given time,” Haile-Selassie said.

The fossils showed that the two species walked differently and had different plant-based diets.

“Understanding the differences and similarities among these nearby hominins is key to understanding their environment and perhaps even how interactions with each other, even indirectly, may have shaped their evolution and how they relate to our own species,” University of Michigan geochemist and study co-author Naomi Levin said.

The big toe of Lucy’s species was not opposable and was more like ours. The Australopithecus deyiremeda big toe was more of an ancestral form, similar to tree-climbing apes. When on the ground, this species walked on two legs and probably pushed off not from its big toe, like Lucy’s species and modern humans, but from its second digit instead.

“It would definitely be less efficient walking on two legs when on the ground. However, it was more effective for tree-climbing – not a bad trade-off, especially in areas where there were large predators,” Haile-Selassie said.

These included large saber-toothed cats and hyenas.

“We know that our lineage descended from an ancestor that had an opposable big toe,” Haile-Selassie said. “Human-like bipedality must have undergone numerous experiments and modifications with some aspects of the foot, the legs and the pelvis evolving at different times.”

Chemical analysis of enamel samples from eight Australopithecus deyiremeda teeth revealed the type of plants eaten by this species.

Lucy’s species was more of a generalist with a broader diet including grass-based foods and foods from trees and shrubs such as leaves, fruits or nuts. Australopithecus deyiremeda, on the other hand, was restricted to a diet based only from trees and shrubs, similar to more primitive hominins. And foot anatomy beneficial for climbing may explain that.

“These species were moving around in different ways. There were multiple ways to be human at this time, and each way likely had an advantage. To me it’s exciting that we can now associate these different ways of moving around on two feet with different diets. We can link different morphological adaptations with different behaviors,” Levin said.

Eating a greater variety of foods may have given Australopithecus afarensis a competitive edge.

“But we also need to consider,” Levin said, “if it was Australopithecus deyiremeda that somehow had the edge, forcing Australopithecus afarensis to broaden its dietary strategy. Now that we know they ate different things and that they moved around in different ways, we’re that much closer to solving this puzzle of co-existence.”

(Reporting by Will Dunham; Editing by Daniel Wallis)