In new research, paleoanthropologists from the United States and Canada analyzed the morphology of a hominin talus (large bone in the ankle that joins with the tibia of the leg and the calcaneus of the foot) attributed to Ardipithecus ramidus, a species of hominid that lived in the east of the African continent around 4.4 million years ago. Their results demonstrate that the fossil bears similarities to the tali of chimpanzees and gorillas, who are adapted to vertical climbing and terrestrial plantigrade quadrupedalism (form of locomotion where an animal moves on four limbs on the ground with the entire soles of its feet, including the heel, touching the surface). Additionally, the authors identified the presence of derived features in the specimen consistent with previous suggestions of an enhanced push-off mechanism in the foot of Ardipithecus ramidus.
Ardipithecus ramidus, a hominid that lived in Africa more then 4 million years ago. Illustration by Arturo Asensio, via Quo.es.
The 4.4 million-year-old partial skeleton of Ardipithecus ramidus — nicknamed ‘Ardi’ — was discovered in 1994.
This species had an ape-size brain and a grasping big toe used for clambering in the trees.
It walked on two feet and had diamond-shaped upper canines, not the v-shaped ones chimpanzees use to chomp.
“Ardi is one of the oldest and most complete skeletons to ever be discovered,” said Dr. Thomas (Cody) Prang, a researcher at Washington University in St. Louis.
“Approximately 1 million years older than ‘Lucy,’ another well-known early human ancestor skeleton, Ardi represents an earlier stage of human evolution.”
“One of the surprises in this discovery was that Ardi walked upright, yet retained a lot of ape-like characteristics, including a grasping foot.”
“Apes, like chimpanzees and gorillas, have a big toe that’s divergent, which allows them to grip tree branches as part of a climbing lifestyle.”
“Yet it also had features that align with our lineage. That makes Ardipithecus a true transitional species.”
Scientists initially proposed that Ardi demonstrated a generalized form of locomotion rather than behavior typical of African apes, leading them to conclude that this very early human ancestor was not similar to apes after all. That came as a big surprise to the paleoanthropology community.
“Based on their analysis, they concluded that living African apes — like chimpanzees and gorillas — are like dead ends or cul-de-sacs of evolution, rather than stages of human emergence,” Dr. Prang said.
“Instead, they thought that Ardi provided evidence for a more generalized ancestor that wasn’t similar to chimps or gorillas.”
By studying chimpanzees’ and gorillas’ tali, researchers can decipher how they move — specifically, how they climb trees vertically.
This important bone also offers insight into how early species transitioned to bipedal (two-legged) locomotion.
For the new study, Dr. Prang and colleagues compared Ardi’s ankle to the ankles of apes, monkeys and early humans.
Their analysis showed that Ardi’s ankle is the only one in the primate fossil record that shares similarities with African apes.
These apes are known for their adaptations to vertical climbing and terrestrial plantigrade quadrupedalism, hinting that Ardi might have used its feet similarly.
In addition to these primitive features, Ardi’s talus also exhibited characteristics suggesting an enhanced push-off mechanism in the foot.
This complexity indicates a blend of climbing and walking behaviors in this early hominin species, which is pivotal in understanding the evolution of bipedalism.
“The finding is both controversial and also aligned with what people thought originally,”Dr. Prang said.
“Nobody disputes the importance of the discovery of Ardi, of course, but many people in the field would say the initial interpretation was probably flawed.”
“And so, this paper is a correction of that initial idea that distanced Ardi from chimpanzees and gorillas.”
“It’s important to note that our paper does not imply that humans evolved from chimpanzees.”
“However, the research adds more evidence to the hypothesis that the common ancestor humans share with chimpanzees was probably quite similar to the chimpanzees living today.”
The paper was published in the journal Communications Biology.
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T.C. Prang et al. 2025. Ardipithecus ramidus ankle provides evidence for African ape-like vertical climbing in the earliest hominins. Commun Biol 8, 1454; doi: 10.1038/s42003-025-08711-7
