Analysis of 4.4-million-year-old ankle reveals how early ancestors moved and evolved

For more than a century, scientists have been piecing together the puzzle of human evolution, examining fossil evidence to understand the transition from our earliest ancestors to modern humans.

A new study from Washington University in St. Louis, published in Communication Biologyprovides compelling evidence to support the hypothesis that humans evolved from an African ape-like ancestor. With this discovery, which challenges previous findings, researchers are able to narrow the range of explanations for the origin of the human lineage. In doing so, scientists are one step closer to answering life’s biggest question, “Where did we come from?”

The study, led by Thomas (Cody) Prang, assistant professor of biological anthropology in Arts and Sciences at Washoe, revisited the discovery of the 4.4-million-year-old Ardipithecus—nicknamed “Ardi”—discovered in 1994.

Ardi is one of the oldest and most complete skeletons ever discovered. According to Prang, another known early human ancestor skeleton, about 1 million years older than “Lassi,” represents the first stage of human evolution.

“One of the surprises in this discovery was that Ardi walked upright, yet retained many ape-like features, including one foot,” Prang said.

“Apes, like chimpanzees and gorillas, have a large toe that is different, allowing them to grasp tree branches as part of a climbing lifestyle. Yet it also had features that are associated with our lineage. That makes Ardipithecus a truly transitional species.”

Prang said the researchers initially proposed that Ardi exhibited a general form of locomotion rather than behavior specific to African apes, leading them to conclude that this early human ancestor was not ape-like after all. This came as a big surprise to the paleoanthropology community.

“Based on their analysis, they concluded that living African apes, like chimpanzees and gorillas, are dead ends or evolutionary cul de sacs rather than stages of human emergence,” Prang said. “Instead, they believed Ardi provided evidence for a more common ancestor that was not like chimps or gorillas.”

Reconsidering the Ord

By studying chimpanzees’ and gorillas – the large bone in the ankle that joins the tibia of the leg and the calcaneus (heel) of the foot. This important bone also offers insight into how early species evolved to bipedal (two-legged) locomotion.

For the study, Prang and colleagues compared Ardi’s ankles to those of monkeys, apes and early humans. Their analysis shows that Ardi’s ankle is the only one in the primate fossil record that shares similarities with African apes.

According to Prang, this monkey is known for its adaptations of vertical climbing and terrestrial plant quadrudalism. This is a form where an animal walks on all fours with the soles of its feet on the ground, including its heels, and this is how Ardi used its feet.

In addition to these primitive features, Ardi’s talus also exhibited features suggesting an improved push-off mechanism in the foot. This complexity indicates a combination of climbing and walking behaviors in this early hominin species, which is important in understanding the evolution of bipedalism.

“This finding is both controversial and consistent with what people originally thought,” Prang said.

“Of course no one disputes the importance of this discovery (Ardi), but many in the field would say that the initial interpretation was probably flawed. And so, this paper is a correction of the initial idea that separated Ardi from chimps and gorillas.”

It is important to note that this thesis does not imply that humans evolved from chimpanzees. However, the research adds further evidence to the hypothesis that the common ancestor humans share with chimpanzees was probably very similar to chimpanzees living today.