About 3.4 million years ago, in what is now the Afar region of Ethiopia, at least two different kinds of early human relatives walked the same landscape.
A new study published in Nature focuses on fossils from Woranso‑Mille, a rich mid‑Pliocene site already famous for yielding both Australopithecus afarensis, the species of the famous fossil known as “Lucy,” and a more recently named species, Australopithecus deyiremeda. When A. deyiremeda was first described, researchers also had a peculiar partial foot from a nearby locality called Burtele, but they could not confidently assign it to any species.
The new paper reports additional jaws and teeth from the same rock layers and argues that these “missing pieces” now firmly link the Burtele foot to A. deyiremeda.
A Foot, a Jaw, and A Few Million Years
In 2009, Paleoanthropologist Yohannes Haile-Selassie and his team found eight bones from the foot of a human ancestor within layers of 3.4-million-year-old sediments in Ethiopia’s Afar Rift. The curious foot, dubbed the “Burtele Foot,” was announced back in 2012.
Around the time the Burtele foot was announced, teeth had already been found from the same area, but the scientists were not convinced the teeth were from the same sedimentary level. In 2015, using the mystery teeth, the team announced a new species, Australopithecus deyiremeda. Though the teeth and the foot were from the same area and were found fairly close together, the research team was not fully convinced they belonged to the same hominin.
In short, they had a mystery foot and some mystery teeth, but paleontologists follow a rule that says new species should be named only from bones of the neck up; therefore, the foot discovery, as it were, temporarily took a hike.
However, as the team has returned to the site over the last ten years and with other fossil findings, they have now officially called it: the foot and the various dental findings belong to A. deyiremeda.
What do we know about Australopithecus deyiremeda?
The fossils themselves are modest in appearance. A juvenile lower jaw packed with baby and emerging adult teeth, several isolated premolars and molars, fragments of adult mandibles, and a juvenile hip bone. What makes them interesting, however, is their geological context.
Radiometric dating of volcanic ash and measurements of Earth’s ancient magnetic field in the rocks place the material between about 3.47 and 3.33 million years ago, overlapping with nearby sites that preserve A. afarensis, the same species as ‘Lucy.’
That means two distinct hominin species were living side by side in the same region, rather than one species evolving neatly into the next.
To pin down the identity of this hominin, the study authors focused on the teeth and jaws, which tend to carry strong species‑level signatures. A. deyiremeda is known for relatively small and narrow first molars, and for canines that are reduced and lack the strong ridges on their inner surfaces seen in A. afarensis. The teeth also show premolars whose overall shape and cusp pattern look more like earlier species, and not like Lucy’s species.
The recently discovered juvenile jaw also lacks features typical of A. afarensis, so, taken together, these traits tighten the taxonomic net and indicate that the owner of these bones was not a close relative of ‘Lucy,’ but someone a bit more distant.
With no evidence for a third hominin species at the locality, a new game was now afoot. It stood to reason that the foot and the dental fossils belonged to the same ancient human, but could it be proven?
As it turns out, it’s all in the toes.
“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,” explained lead author Yohannes Haile-Selassie in a press statement. “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.”
The Burtele foot looks like a compromise between life in the trees and life on the ground. Its toes and some of the long bones of the forefoot are relatively long and curved, a pattern associated with grasping branches rather than pounding pavement. Certain joints in the midfoot appear less rigid than in modern humans or in an A. afarensis foot, suggesting that A. deyiremeda retained some midfoot flexibility useful for climbing.
At the same time, the head of the big toe metatarsal shows subtle “doming” and a more continuous articular surface, shifts that move it away from the very divergent, thumb‑like big toe of Ardipithecus and toward the more forward‑aligned toe seen in later australopiths and humans.
In practical terms, this anatomy points to an animal that could walk upright on the ground with a plantigrade (sole‑on‑the‑ground) gait, but that lacked a fully developed, stiff longitudinal arch and still relied heavily on the trees.
According to the study, a statistical comparison of foot bones from apes, humans, and early hominins places the Burtele foot in an intermediate position: more human‑like than Ardipithecus, but not as specialized for long‑distance ground walking as ‘Lucy’s’ A. afarensis.
So this means that instead of a single “ladder” of bipedal evolution, this region of Ethiopia played host to multiple kicks of the bipedal can.
So how do the teeth connect to the foot?
Tooth enamel locks in a chemical record of the plants an animal ate, distinguishable by the ratio of carbon isotopes (forms of carbon with different masses). Plants like trees, shrubs, and many woodland grasses leave a different isotopic signature than tropical grasses and sedges more common in open savannas. The team measured these signatures in eight A. deyiremeda teeth and found that the numbers strongly point to a diet dominated by woodland resources like tree leaves.
“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 co-author Naomi 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.”
A. afarensis ate a more grass‑heavy diet than you’d see in large open African plains. In other words, A. deyiremeda seems to have stuck with a more wooded forest‑edge menu even as other hominins were exploiting more open, grassy environments.
Combined with the partly grasping foot, that diet hints at a niche that involves frequent use of tree cover for food and safety.
The new paper adds weight to the idea that early human evolution was not a simple, smooth progression from primitive ape‑like ancestors to ever more human‑like australopiths, but instead a diverse mix of different evolutionary runs. A. deyiremeda shows a mosaic of traits that place it closer to later species such as Australopithecus africanus, and even the branch that leads to Paranthropus and Homo than to A. afarensis, underscoring that Lucy’s lineage was not the only human game in town.
“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,” explained Haile-Selassie. “It is our eagerness to learn about our present and the future as well.”
MJ Banias covers space, security, and technology with The Debrief. You can email him at mj@thedebrief.org or follow him on Twitter @mjbanias.