By Will Dunham
WASHINGTON (Reuters) – Scientists have solved the mystery of 3.4-million-year-old fossils called “Bertel’s feet” found in Ethiopia in 2009. They belong to a mysterious human ancestor that lived with another closely related species during a incomprehensible period of human evolution.
Based on recent discoveries of 25 teeth and a nearby jawbone of a 4-1/2-year-old child, scientists have determined that the eight-legged bone represents the species Australopithecus deirameda, which combined ape-like and human-like traits and was first identified a decade ago.
Bones found at a site called Burtele in the Afar region of northeastern Ethiopia, so named Burtele foot, show that the species was bipedal but still paradoxically had large feet, useful for climbing trees – evidence that it did so differently than humans today when walking upright.
The fossils show that two closely related hominins—species in the human evolutionary lineage—lived at the same time and place, along with other species such as Australopithecus afarensis. This raises the question of whether these close relatives used the same resources or were sufficiently different to avoid direct competition.
Australopithecus afarensis is the species that includes the famous fossil Lucy, discovered in 1974 in the Afar region.
New discoveries add depth to the understanding of this period of human evolution long before our species, Homo sapiens, arose nearly 300,000 years ago.
“They provide us with the most conclusive evidence that Australopithecus afarensis – Lucy’s species – was not the only human ancestor that lived between 3.5 and 3.3 million years ago,” said Johannes Haile-Selassie, director of Arizona State University.
“As a result, we now know that our earlier stages of evolution were not linear, meaning that there was only one species living at any given time,” Haile-Selassie said.
The fossils show that the two species moved differently and had different plant-based diets.
“Understanding the differences and similarities between these closely related hominins is critical to understanding their environments and perhaps how interactions with one another may, even indirectly, have shaped their evolution and how they are related to our own species,” said University of Michigan geochemist and study co-author Naomi Levin.
The big toe of Lucy’s species was not paradoxical and resembled ours. Australopithecus deyiremeda was an ancestral form, similar to the big-toed tree-climbing ape. When on land, this species walks on two legs and pushes off with its second digit instead of its big toe like Lucy’s species and modern humans.
“Walking on two legs would certainly be less efficient when on the ground. However, it was more efficient for climbing trees — not a bad trade-off, especially in an area where there are large predators,” Haile-Selassie said.
These included large saber-toothed cats and hyenas.
“We know that our lineage comes from an ancestor who had a big thumb that couldn’t be resisted,” Haile-Selassie said. “Human-like bipedalism must have undergone many experiments and modifications at different times with some aspects of the development of the legs, feet and pelvis.”
Chemical analysis of enamel samples from eight Australopithecus deirameda teeth revealed the type of plants this species ate.
Lucy’s species was a generalist, with a diet of grass-based foods and a broad diet of trees and shrubs such as leaves, fruits or nuts. Australopithecus deyiremeda, on the other hand, was limited to a diet based on trees and shrubs, like more primitive hominins. And leg anatomy advantageous for climbing may explain that.
“These species were walking in different ways. There were many ways to be human at this time, and each way probably had an advantage. It’s exciting to me that we can now associate these different ways of bipedalism with different diets. We can associate different morphological adaptations with different behaviors,” Levine said.
Eating a wide variety of foods may have given Australopithecus afarensis a competitive edge.
“But we also need to consider,” Levin said, “if it was Australopithecus deirameda that somehow had the edge, forcing Australopithecus afarensis to broaden its dietary strategy. Now that we know they ate different things and they moved around in different ways, we’re a lot closer to solving this co-op.”
(Reporting by Will Dunham; Editing by Daniel Wallis)
