New research by ASU paleoanthropologists gives valuable insight into how two ancient human ancestors coexisted in the same area

With the help of newly identified bones, an enigmatic 3.4-million-year-old hominin foot found in 2009, is assigned to a species different from that of the famous fossil Lucy providing further proof that two ancient species of hominins co-existed at the same time and in the same region.

In 2009, scientists led by Arizona State University paleoanthropologist Yohannes Haile-Selassie, found eight bones from the foot of an ancient human ancestor within layers of 3.4-million-year-old sediments in the Afar Rift in Ethiopia. The fossil, called the Burtele Nature Foot, was found at The Woranso-Mille paleontological site and was announced in a 2012 article. 

“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.” 

When the Burtele foot was announced, some teeth were already found from the same area, but the scientists were not convinced the teeth were from the same level of sediments. Then, in 2015, the team announced a new species, Australopithecus deyiremeda, from the same area but did not include the foot into this species even though some of the specimens were found very close to the foot, explained Haile-Selassie. 

Over the past 10 years of returning to the field and finding more fossils, Haile-Selassie said they now have specimens that they can confidently associate with the Burtele foot and with the species A. deyiremeda. 

What's in a name – and a foot?

The assignment of the Burtele foot to a species is just part of the story. 

The site of Woranso-Mille is significant because it is the only site where scientists have clear evidence showing two related hominin species co-existed at the same time in the same area. 

The Burtele foot, belonging to A. deyiremeda, is more primitive than the feet of Lucy’s species, A. afarensis. The Burtele foot retained an opposable big toe which is important for climbing. But when on the ground, A. deyiremeda walked on two legs and it most likely pushed off on its second digit rather than its big toe like we, modern humans, do today.

“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 which 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.”

Teeth reveal different diets 

To get insight into the diet of A. deyiremeda, Naomi Levin, a professor at the University of Michigan, sampled eight of the 25 teeth found at the Burtele localities for isotope analysis. The process involves cleaning the teeth, making sure to only sample the enamel.

“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 were surprising.

While Lucy’s species was a mixed feeder, eating C3  (resources from trees and shrubs) and C4 plants (tropical grasses and sedges), A. deyiremeda was utilizing resources that are more on the C3 side. 

“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.”

Another key data analysis was carefully establishing the age of the fossils and understanding the surrounding ancient environment in which the ancient hominins lived in.

"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 work that established the stratigraphic association between the foot and Au. deyiermeda. 

A juvenile jaw

Along with the 25 teeth found at Burtele, Haile-Selassie’s team also found the jaw of a juvenile that based on the anatomy of the teeth clearly belonged to A. deyiremeda. This jaw had a full set of baby teeth already in position, but also had a lot of adult teeth developing deep down within the bony mandible, explained Gary Schwartz, IHO research scientist and professor at the School of Human Evolution and Social Change. 

The team used CT scanning technology to look at all the developing teeth because there is a close connection between both the pattern and pace of tooth development with a species’ overall growth biology. This allowed the scientists to estimate that this jaw belonged to a hominin that was around 4.5 years old when it died. 

“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.”

Ancient hominins co-existing

Knowing how these ancient ancestors moved and what they ate provides scientists with new knowledge about how species co-existed at the same time without one pushing the other to extinction. 

“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.”

The paper, “New finds shed light on diet and locomotion in Australopithecus deyiremeda,” is published in the journal Nature (https://doi:10.1038/s41586-025-09714-4). 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 are: 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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