In 2009, paleoanthropologists uncovered eight foot bones from ancient human ancestors in 3.4 million-year-old deposits at the Wolanso Mir site in Ethiopia’s Afar Rift Valley. A new study reveals that this fossil, known as Brutele’s foot, belongs to Australopithecus deiremeda. This finding adds to the evidence that two hominin species, Australopithecus deiremeda and Australopithecus afarensis, coexisted in the same region at the same time.
Australopithecus deiremeda and Australopithecus afarensis. Image credit: Gemini AI.
“When we found this foot in 2009 and announced it in 2012, we recognized it was distinct from Lucy’s species, although Australopithecus afarensis has received significant attention since then,” stated Professor Johannes Haile Selassie from Arizona State University.
“Typically, naming a species based on postcranial elements is uncommon in our field, so we anticipated finding something distinctly linked to the feet from the neck up.
“Traditionally, the skull, jaw, and teeth are the primary markers for species identification.”
“When Bartele’s foot was first reported, some teeth had already been found in the same area, but we weren’t certain they were from the same deposit level.”
“Then in 2015, scientists classified a new species, Australopithecus deiremeda, from the same region, but the foot was not included, despite other specimens being unearthed nearby.”
“Over the last decade, our repeated fieldwork has yielded more fossils, allowing us to confidently link Brutele’s foot to the species Australopithecus deiremeda.”
Australopithecus deiremeda exhibits more primitive foot structures compared to Lucy’s species, Australopithecus afarensis.
While retaining an opposable thumb useful for climbing, it is believed that Australopithecus deiremeda likely walked on two legs, with an emphasis on their second toes rather than their big toes, as is the case with modern humans.
“The presence of an opposable big toe in Ardipithecus ramidus was a surprising and unexpected finding, highlighting that 4.4 million years ago, early human ancestors still possessed opposable big toes,” remarked Professor Haile Selassie.
“Then, a million years later, the discovery of Brutele’s foot further amazed us.”
“Currently, we’re in an era where we can observe subsequent species. Members of Australopithecus afarensis had an adducted big toe and displayed complete bipedalism.”
“This indicates that bipedalism, or walking on two legs, manifested in diverse forms among these early human ancestors.”
“The discovery of specimens like Bartele’s foot conveys that there were multiple ways to walk bipedally. It wasn’t until later that a single method emerged.”
To gain insights into their dietary practices, researchers sampled eight of the 25 teeth found in the area related to Australopithecus deiremeda for isotope analysis.
This process involved cleaning the tooth to ensure only the enamel was analyzed.
“I extracted the tooth using a dental drill with a very small bit, similar to what dentists use,” explained Naomi Levin, a professor at the University of Michigan.
“Using this drill, we meticulously remove a small amount of powder, which we store in a vial and return to the lab for isotope analysis.”
“The results were intriguing: Lucy’s species displayed a mixed diet, consuming both C3 (from trees and shrubs) and C4 (tropical grasses and sedges) plants; while Australopithecus deiremeda primarily utilized resources from the C3 category.”
“We were taken aback by how distinctly clear the carbon isotope signal was, mirroring ancient hominin data from Australopithecus ramidus and Australopithecus anamensis.
“I considered the dietary differences between Australopithecus deiremeda and Australopithecus afarensis. Although identifying them was challenging, the isotopic data distinctly indicated that Australopithecus deiremeda was not exploiting the same range of resources as Australopithecus afarensis, known as the earliest hominin to consume C4 grass-based resources.”
Another significant analysis involved accurately dating the fossils and understanding the ancient environments inhabited by these early humans.
“We conducted extensive field research at Wolanso Mir to analyze how different fossil layers interrelate, which is essential for grasping when and in what environments different species thrived,” noted Professor Beverly Thaler from Case Western Reserve University.
In addition to the 25 teeth found at Bartele, researchers also recovered the jaw of a four-and-a-half-year-old child, displaying dental anatomy similar to that of a juvenile Australopithecus deiremeda.
Professor Gary Schwartz from Arizona State University commented: “In juvenile hominins of this age, we observed evident growth discontinuity between front teeth (incisors) and back chewing teeth (molars), akin to patterns in modern apes and early australopiths like Lucy.”
“The most surprising aspect was that, despite gaining a better understanding of the diversity within early australopith (and thus early hominid) species regarding size, diet, locomotion, and anatomy, these early forms appeared surprisingly uniform in growth patterns.”
Findings have been detailed in a paper published in this week’s edition of Nature.
_____
Y. Haile Selassie et al. New discovery illuminates the diet and lifestyle of Australopithecus deiremeda. Nature published online November 26, 2025. doi: 10.1038/s41586-025-09714-4
