New fossils link a strange 3.4-million-year-old foot to Australopithecus deyiremeda, a species that combined climbing skills with its own bipedal walking style.
Recently discovered fossils have helped researchers determine that an unusual 3.4-million-year-old hominin foot discovered in 2009 does not belong to Lucy’s species. The evidence now shows that it belonged to another early human ancestor, strengthening the hypothesis that two hominin species lived in the same region at the same time, writes the publication Scitech Daily.
In 2009, a team led by paleoanthropologist Yohannes Haile-Selassie of Arizona State University discovered eight leg bones belonging to a human ancestor in sediments of the same age in the Afar Rift, Ethiopia. The fossil, known as the Burtele Nature Foot, was excavated at the Woranso-Mille site and formally described in a 2012 publication.
“When we found the foot in 2009 and announced it in 2012, we knew it was different from Lucy’s species, Australopithecus afarensis, which was widely known at the time”said Haile-Selassie, director of the Institute on Human Origins (IHO) and professor at the ASU School of Human Evolution and Social Change.
“However, it is not common practice in our field to name a species based only on postcranial elements – that is, elements below the neck – so we were hoping to also find something above the neck, clearly associated with the leg. Skulls, jaws and teeth are usually the elements used to recognize a species.”
When the Beltele leg was first reported, several teeth had already been recovered from the same general area. Scientists were hesitant to link them to the leg because they weren’t sure they came from the same sediment layer. In 2015, the team identified a new species in the region, Australopithecus deyiremeda, but did not assign the foot to this species, despite its proximity, Haile-Selassie explained.
Over the next decade, repeated fieldwork led to more discoveries. According to Haile-Selassie, the team can now confidently link the Belly leg to A. deyiremeda.
Why is this foot important to human evolution?
Determining the species of the foot Belly is only part of its meaning. The Woranso-Mille site remains the only site that provides direct evidence that two closely related hominin species occupied the same territory at the same time.
Beltele’s leg shows features considered more primitive than those of Lucy’s species. It retained an opposable thumb suitable for climbing, but A. deyiremeda still walked bipedally and appears to push itself primarily with the second toe, not the big toe, as modern humans do.
“The presence of an overgrown thumb in Ardipithecus ramidus was a big surprise, because 4.4 million years ago there was an early ancestor that still had an opposable thumb, which was totally unexpected”said Haile-Selassie.
“Then 1 million years later, 3.4 million years ago, we find the Belly foot, which is even more surprising. It’s a time when we see species like A. afarensis, whose members were fully bipedal, with the big toe adducted. This means that bipedality – walking on two legs – in these early human ancestors existed in various forms. The idea of finding specimens like the Belly foot shows that there were many ways to walking on two legs when they were on the ground, not just one, as happened later.”
Isotopic clues reveal distinct dietary habits
To learn more about A. deyiremeda’s diet, Professor Naomi Levin from the University of Michigan analyzed eight of the 25 teeth collected from the Belly area using isotopic tests. The process begins with cleaning the surface of the tooth and carefully harvesting only the enamel layer.
“I harvest the tooth with a small dental drill and a very small tip (<1mm) - the same type of equipment dentists use to work on teeth"Levin explained. “With this drill I carefully remove small amounts of dust. I keep it in a plastic container and transport it to our laboratory at the University of Michigan for isotopic analysis.”
The results showed clear differences in diet.
Lucy’s species consumed a combination of C3 (from trees and shrubs) and C4 (grasses and tropical plants) resources. A. deyiremeda, however, relied more on C3 vegetation.
“I was surprised that the carbon isotope signal was so clear and so similar to the isotopic data of the older hominins A. ramidus and Au. anamensis”Levin said. “I expected that the differences between the diets of A. deyiremeda and A. afarensis would be harder to identify, but the isotopic data clearly show that A. deyiremeda was not using the same resources as A. afarensis, which is the earliest hominin known to use C4 grass-based food resources.”
Determining the age and environment of fossils
An important part of the research involved determining the age of the fossils and reconstructing the environment in which these early hominins lived. This required extensive geological work to understand how the fossil layers at the site are related.
“We have done extensive and careful fieldwork at Woranso-Mille to establish how the different fossil layers are related, which is crucial to understanding when and in what environments different species lived”said Beverly Saylor, professor of earth, environmental and planetary sciences at Case Western Reserve University. Saylor led the geologic analysis that clarified the stratigraphic connection between the Burtele foot and the Au. the remedy
