In 2009, researchers in Ethiopia came across mysterious fossil foot bones: They were around 3.4 million years old and apparently belonged to a hominin, but were significantly different from the feet of Australopithecus afarensis – the species made world famous by the iconic fossil “Lucy”. But recent fossil discoveries show that Lucy had a more primitive sister species, the Australopithecus deyiremeda. Now researchers have also assigned the fossil foot to this species. Their investigations also provide new insights into the way of life of this ancient prehistoric man – and suggest that our ancestry could look different than expected.
For a long time, scientists assumed that the pre-human species Australopithecus afarensis, to which the famous fossil “Lucy” belongs, was the only one that lived in Ethiopia around 3.4 million years ago. It was believed that all later pre- and early human species, including modern humans, developed from this species. But in 2009, a team led by paleoanthropologist Yohannes Haile-Selassie from Arizona State University discovered parts of a fossil foot that did not fit this picture. The eight foot bones were dated to around 3.4 million years ago and, like Lucy, came from the Afar region in Ethiopia.
Who did the foot belong to?
“When we found the foot in 2009, we knew it was different from Lucy’s species, Australopithecus afarensis,” says Haile-Selassie. “However, it is not common practice in our field to name a species based on postcranial elements – elements below the neck. So we hoped to find something above the neck that is clearly associated with the foot. Skull, jaws and teeth are typically the elements used to identify species.” Haile-Selassie and his colleagues have actually found such fossils. Already in 2015, they described a sister species of Lucy, the Australopithecus deyiremeda, using jaw fossils.
But only now have the researchers been able to assign the foot, discovered in 2009, to this species with the help of other fossils newly discovered in Ethiopia – including teeth and parts of a hip. The old and new fossils show that Australopithecus deyiremeda was significantly more primitive than Lucy and her species. While Lucy’s feet were already shaped similarly to ours, Au. deyiremeda, similar to chimpanzees, also has an opposable big toe. The shape of the foot suggests that this species already walked on two legs on the ground, but was still adapted to climbing trees. “This means that bipedalism appeared in different forms in early human ancestors,” says Haile-Selassie.
Different ecological niches
An analysis of the tooth enamel also revealed that the two Australopithecus species that lived at the same time had different diets: This is how Au ate. deyiremeda especially so-called C3 plants such as herbs, fruits and nuts. Lucy and her peers, on the other hand, also consumed C4 plants, which carry out photosynthesis particularly efficiently when exposed to high levels of sunlight. The C4 plants known today include, for example, corn, sugar cane and millet.
“The simultaneous appearance of feet indicative of tree-climbing behavior and a diet typical of a forested environment contrasts with what is known about the more open-country Australopithecus afarensis,” explains paleontologist Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, who was not involved in the study, in an accompanying commentary also published in Nature. “Importantly, these different ecological niches could explain how two hominid species could coexist in the same area.”
New human family tree?
From Spoor’s perspective, the current study contributes to the definition of Au. deyiremeda as a way of supporting. “Proposals for new hominid species are often met with reluctance,” he explains. “The new dental fossils and the assignment of the foot bones are likely to lead to broader acceptance of Au. deyiremeda as a true species, raising the question of where it fits in the family tree of human evolution.”
Overall, according to the analyses, Australopithecus deyiremeda was more similar to older prehumans such as Ardipithecus ramidus and Australopithecus anamensis, who lived around a million years earlier. But the researchers also found a surprisingly close relationship to a later species, the Australopithecus africanus, which lived in southern Africa around two to three million years ago. The two species may have even been more closely related to each other than to Lucy’s species, Australopithecus afarensis. “If this is true, afarensis loses its iconic status as the ancestor of all later hominids, including probably our own lineage, the genus Homo,” writes Spoor.
Source: Yohannes Haile-Selassie (Arizona State University, Tempe, USA) et al., Nature, doi: 10.1038/s41586-025-09714-4