Scientists from Washington University in St. Louis (WashU) studying a 4.4-million-year-old ankle bone fossil say the find represents “compelling evidence” to support the hypothesis that the earliest humans evolved from an ape-like creature in Africa.
According to a statement announcing the scientific team’s research, several aspects of the ancient bone have helped to “narrow the range of explanations for the origin of human lineage,” and move us closer to answering one of life’s greatest questions: where do we come from?
Researchers first discovered a nearly complete skeleton of Ardipithecus in 1994, and knew almost immediately it represented an early stage of human evolution. More analysis ultimately determined that the specimen, nicknamed Ardi, was one of the oldest skeletons ever found, dating to approximately 4.4 million years ago. For comparison, the WashU researchers note that Ardi is almost 2 million years older than the well-known Australopithecus skeleton ‘Lucy’ discovered in Africa in 1974.
Study leader Thomas (Cody) Prang, an assistant professor of biological anthropology in WashU’s school of Arts & Sciences, said the analysis of Ardi included several surprises, including the discovery that he walked upright, but still maintained a lot of Ape-like characteristics, such as 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,” Prang explained. “Yet it also had features that align with our lineage. That makes Ardipithecus a true transitional species.”
During earlier analyses, scientists initially proposed that Arid probably did not move like African apes and was not a missing link between humans and an ancient ape-like ancestor. Instead, Prang’s team says those earlier works suggested Ardi demonstrated a “more generalized” form of locomotion that was not necessarily Ape-like. Prang’s team said this early conclusion led scientists to determine Ardi’s skeletal construction was “not similar to ape after all,” which came as a “big surprise” to the broader paleoanthropology community, who were still looking for an evolutionary missing link between humans and other primates.
“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,” Prang said. “Instead, they thought that Ardi provided evidence for a more generalized ancestor that wasn’t similar to chimps or gorillas.”
To conduct a new analysis that would either confirm the original determination that ARdi was not an evolutionary missing link, Prang teamed up with Matthew W. Tocheri at Lakehead University in Thunder Bay, Canada, Biren A. Patel at the University of Southern California, Scott A. Williams at New York University, and Caley M. Orr at the University of Colorado Anschutz.
The team’s initial analysis focused on comparing Ardi’s ankle bones to the ankle bones of chimpanzees and gorillas. This began with studying the large talus bone found in gorilla and chimpanzee ankles, which joins the foot with the tibia of the leg and the calcaneus (heel). This bone is critical, the team notes, because it offers insight into how early hominin species “transitioned” to bipedal (two-footed) locomotion.
When comparing the bone in Ardi with the same bone found in gorillas and chimpanzees, the research team found a surprising similarity. According to their statement, the comparison found that Ardi’s ankle bone “is the only one in the primate fossil record that shares similarities with African apes.”
For example, gorillas and chimpanzees have feet and ankles that are adapted to vertical tree climbing and what Prang called “terrestrial plantigrade quadrupedalism.” The researcher described this form of locomotion as movement on four limbs on the ground “with the entire soles of its feet, including the heel, touching the surface.” Prang said the structure of Ardi’s ankle suggests the human predecessor “might have used its feet similarly.”
Notably, the team found that the construction of Ardi’s ankle “exhibited characteristics” hinting at an enhanced “push-off” mechanism in the foot. The team believes this design complexity suggests a blend of climbing and walking behaviors, “which is pivotal in understanding the evolution of bipedalism.”
Prang acknowledges that the team’s analysis, which suggests that humans evolved from an ape-like ancestor, is controversial. However, he also notes that the suggestion is “aligned with what people thought originally,” before the controversial analysis suggesting Ardi did not evolve from apes.
“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,” Prang said. “And so, this paper is a correction of that initial idea that distanced Ardi from chimpanzees and gorillas.”
In the statement’s conclusion, Prang said it is important to note that their study “does not imply that humans evolved from chimpanzees.” Instead, he believes their paper “adds more evidence” that whatever common ancestor Ardi, and therefore modern humans, shared with chimpanzees “was probably quite similar to the chimpanzees living today.”
The study “Ardipithecus ramidus ankle provides evidence for African ape-like vertical climbing in the earliest hominins” was published in Communications Biology.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.