This 3.4 Million-Year-Old Foot Changes the Story of Human Origins

New fossils link a strange 3.4-million-year-old foot to Australopithecus deyiremeda, a species that mixed climbing skills with its own style of bipedal walking.

The evidence shows that multiple early human ancestors inhabited the same region while relying on different diets and behaviors.

Ancient Foot Fossil Reassigned to a Different Early Human Species

Newly uncovered 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. Instead, the evidence now indicates that it belonged to another early human relative, strengthening the case that two hominin species lived alongside each other in the same region at the same time.

In 2009, a team led by Arizona State University paleoanthropologist Yohannes Haile-Selassie uncovered eight foot bones from an ancient human ancestor in sediments of the same age in Ethiopia’s Afar Rift. The fossil, known as the Burtele Nature Foot, was excavated at the Woranso-Mille site and was formally described in a 2012 publication.

“When we found the foot in 2009 and announced it in 2012, we knew that it was different from Lucy’s species, Australopithecus afarensis, which is widely known from that time,” said Haile-Selassie, director of the Institute of Human Origins (IHO) and a professor in the ASU School of Human Evolution and Social Change.

“However, it is not common practice in our field to name a species based on postcranial elements –elements below the neck – so we were hoping that we would find something above the neck in clear association with the foot. Crania, jaws, and teeth are usually the elements used in species recognition.”

Linking the Burtele Foot to A. deyiremeda

When the Burtele foot was first reported, some teeth had already been recovered from the same general area. Scientists were hesitant to connect them to the foot because they were not certain the teeth came from the same sediment layer. In 2015, the team identified a new species from the region, Australopithecus deyiremeda, but they did not assign the foot to this species despite its close proximity, explained Haile-Selassie.

Over the next decade, repeated fieldwork led to more discoveries. According to Haile-Selassie, the team can now confidently link the Burtele foot with A. deyiremeda.

Why the Foot Matters for Human Evolution

Determining the species of the Burtele foot is only part of its significance. Woranso-Mille remains the only site that provides direct evidence of two closely related hominin species occupying the same landscape during the same period.

The Burtele foot shows traits considered more primitive than those of Lucy’s species. It retained an opposable big toe suited for climbing, yet A. deyiremeda still walked upright and appears to have pushed off mainly from the second toe rather than the big toe, which is how modern humans walk.

“The presence of an abducted big toe in Ardipithecus ramidus was a big surprise because at 4.4 million years ago, there was still an early hominin ancestor that retained an opposable big toe, which was totally unexpected,” said Haile-Selassie.

“Then 1-million-years later, at 3.4-million-years ago, we find the Burtele foot, which is even more surprising. This is a time when we see species like A. afarensis whose members were fully bipedal with an adducted big toe. What that means is that bipedality – walking on two legs – in these early human ancestors came in various forms. The whole idea of finding specimens like the Burtele foot tells you that there were many ways of walking on two legs when on the ground, there was not just one way until later.”

Isotope Clues Reveal Distinct Dietary Habits

To learn more about what A. deyiremeda ate, University of Michigan professor Naomi Levin analyzed eight of the 25 teeth collected from the Burtele area using isotope testing. The process begins with cleaning the tooth surface and carefully sampling only the enamel layer.

“I sample the tooth with a dental drill and a very tiny (< 1mm) bit — this equipment is the same kind that dentists use to work on your teeth,” said Levin. “With this drill I carefully remove small amounts of powder. I store that powder in a plastic vial and transport it back to our lab at the University of Michigan for isotopic analysis.”

The results revealed clear differences in diet.

Lucy’s species consumed a mixture of C3 resources (from trees and shrubs) and C4 resources (tropical grasses and sedges). A. deyiremeda, however, relied more heavily on C3 vegetation.

“I was surprised that the carbon isotope signal was so clear and so similar to the carbon isotope data from the older hominins A. ramidus and Au. anamensis,” said Levin. “I thought the distinctions between the diet of A. deyiremeda and A. afarensis would be harder to identify but the isotope data show clearly that A. deyiremeda wasn’t accessing the same range of resources as A. afarensis, which is the earliest hominin shown to make use of C4 grass-based food resources.”

Establishing the Age and Environment of the Fossils

A major part of the research involved determining the age of the fossils and reconstructing the environmental setting in which these early hominins lived. This required extensive geological work to understand how the fossil layers relate to each other across the site.

“We have done a tremendous amount of careful field work at Woranso-Mille to establish how different fossil layers relate, which is crucial to understanding when and in what settings the different species lived,” said Beverly Saylor, professor of earth, environmental and planetary sciences at Case Western Reserve University. Saylor led the geological analysis that clarified the stratigraphic connection between the Burtele foot and Au. deyiremeda.

Juvenile Jaw Sheds Light on Early Growth

Along with the 25 teeth recovered at Burtele, Haile-Selassie’s team also identified the jaw of a young individual that clearly belonged to A. deyiremeda based on its dental anatomy. According to Gary Schwartz, IHO research scientist and professor at the School of Human Evolution and Social Change, this jaw contained a complete set of baby teeth as well as developing adult teeth deep within the lower jawbone.

The researchers used CT scanning to examine the developing teeth. Since tooth formation is closely tied to overall growth patterns, the scans helped them estimate that the juvenile died at about 4.5 years old.

“For a juvenile hominin of this age, we were able to see clear traces of a disconnect in growth between the front teeth (incisors) and the back chewing teeth (molars), much like is seen in living apes and in other early australopiths, like Lucy’s species,” said Schwartz.

“I think the biggest surprise was, despite our growing awareness of how diverse these early australopith (i.e., early hominin) species were – in their size, in their diet, in their locomotor repertoires and in their anatomy – these early australopiths seem to be remarkably similar in the manner in which they grew up.”

How Multiple Ancient Hominins Shared the Same Region

Understanding how these early human relatives moved, ate, and grew provides insight into how several hominin species could coexist without one eliminating the other.

“All of our research to understand past ecosystems from millions of years ago is not just about curiosity or figuring out where we came from, said Haile-Selassie. “It is our eagerness to learn about our present and the future as well.”

“If we don’t understand our past, we can’t fully understand the present or our future. What happened in the past, we see it happening today,” he said. “In a lot of ways, the climate change that we see today has happened so many times during the times of Lucy and A. deyiremeda. What we learn from that time could actually help us mitigate some of the worst outcomes of climate change today.”

Reference: “New finds shed light on diet and locomotion in Australopithecus deyiremeda” by Yohannes Haile-Selassie, Gary T. Schwartz, Thomas C. Prang, Beverly Z. Saylor, Alan Deino, Luis Gibert, Anna Ragni and Naomi E. Levin, 26 November 2025, Nature.
DOI: 10.1038/s41586-025-09714-4

The paper, “New finds shed light on diet and locomotion in Australopithecus deyiremeda,” is published in the journal Nature. The international research team included scientists from Arizona State University, Washington University, St. Louis, Case Western Reserve University, Berkeley Geochronology Center, Universitat de Barcelona, University of Tampa and University of Michigan. The full list of authors is: Yohannes Haile-Selassie, Gary T. Schwartz, Thomas C. Prang, Beverly Z. Saylor, Alan Deino, Luis Gibert, Anna Ragni, and Naomi E. Levin.

Funding for this project was provided by the National Science Foundation and the W.M. Keck Foundation. Field and Laboratory research in Ethiopia was facilitated by the Ethiopian Heritage Authority.

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