In 2009, researchers discovered eight foot bones belonging to an ancient human ancestor in Ethiopia’s Afar Rift. The fossil, known as the Burtele Foot, was unearthed by a team led by Arizona State University (ASU) paleoanthropologist Yohannes Haile-Selassie at the Woranso-Mille paleontological site. Initially, the team did not assign a species to the 3.4-million-year-old specimen.
“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 and professor at ASU’s School of Human Evolution and Social Change. “However, it is not common practice in our field to name a species based on postcranial elements — meaning 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.”
Some teeth had been found near the original discovery site but were not immediately linked to the same sediment layer as the Burtele Foot. In 2015, Haile-Selassie’s team identified a new species from the area—Australopithecus deyiremeda—but could not definitively associate the foot with this species at that time.
After another decade of fieldwork and additional fossil discoveries, researchers now say they can confidently connect the Burtele Foot to A. deyiremeda.
The Woranso-Mille site is notable for providing evidence that two related hominin species lived side by side during this period. According to Haile-Selassie: “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.”
The Burtele Foot retains an opposable big toe suited for climbing and features longer, more flexible toes compared to those of A. afarensis (“Lucy’s” species). When A. deyiremeda walked upright, it likely pushed off its second digit rather than its big toe.
Haile-Selassie noted earlier surprises about bipedality: “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.” He added: “Then 1 million years later…we find the Burtele Foot…This is a time when we see species like A. afarensis whose members were fully bipedal with an adducted big toe.”
To better understand A. deyiremeda’s diet, Naomi Levin from the University of Michigan analyzed eight teeth using isotope techniques similar to those used by dentists today. She explained: “I sample the tooth with a dental drill and a very tiny (< 1mm) bit…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.”
Levin found clear differences between A. deyiremeda and Lucy’s species: while A. afarensis ate both C3 (trees/shrubs) and C4 (grasses/sedges) resources, A. deyiremeda primarily consumed C3 plants.
“I was surprised that the carbon isotope signal was so clear and so similar to…A. ramidus and Australopithecus anamensis,” she said. “I thought distinctions between…A. deyiremeda and A. afarensis would be harder to identify, but…A. deyiremeda wasn’t accessing the same range of resources as A. afarensis.”
Researchers also recovered a juvenile jaw belonging to A. deyiremeda containing both baby teeth and developing adult teeth deep within its mandible structure—a detail revealed through micro-CT scans conducted by Gary Schwartz from ASU’s Institute of Human Origins.
Schwartz commented: “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 back chewing teeth (molars), much like is seen in living apes and other early australopiths.” He added: “Despite our growing awareness of how diverse these early australopith…species were—in their size, diet…and anatomy—these early australopiths seem to be remarkably similar in how they grew up.”
Understanding how these ancestors moved and what they ate helps scientists study how multiple hominin species coexisted without one driving another extinct.
“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,” Haile-Selassie said. “It is our eagerness to learn about our present and future as well.” He continued: “If we don’t understand our past, we can’t fully understand present or future…What happened in past…happened so many times during times of Lucy and A. deyiremeda…What we learn from that time could actually help us mitigate some worst outcomes of climate change today.”
The study titled “New finds shed light on diet and locomotion in Australopithecus deyiremeda” appears in Nature journal.
Arizona State University has recently been recognized for its innovation achievements over several years according to U.S News & World Report, being named number one for innovation for eight consecutive years.
