Anthropologists offer new evidence of bipedalism in long-debated fossil discovery

In recent decades, scientists have debated whether a seven-million-year-old fossil was bipedal—a trait that would make it the oldest human ancestor. A new analysis by a team of anthropologists offers powerful evidence that Sahelanthropus tchadensis—a species discovered in the early 2000s—was indeed bipedal by uncovering a feature found only in bipedal hominins.

Using 3D technology and other methods, the team identified Sahelanthropus’s femoral tubercle, which is the point of attachment for the largest and most powerful ligament in the human body—the iliofemoral ligament—and vital for walking upright. The analysis also confirmed the presence of other traits in Sahelanthropus that are linked to bipedalism. 

“Sahelanthropus tchadensis was essentially a bipedal ape that possessed a chimpanzee-sized brain and likely spent a significant portion of its time in trees, foraging and seeking safety,” says Scott Williams, an associate professor in New York University’s Department of Anthropology who led the research. “Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.”

The study, which included researchers from the University of Washington, Chaffey College, and the University of Chicago, appears in the journal Science Advances. 

Sahelanthropus was discovered in Chad’s Djurab desert by University of Poitiers’ palaeontologists in the early 2000s, with initial analyses focusing on its skull. Two decades later, studies on other parts of that discovery—its forearms, or ulnae, and thigh bone, or femur—were reported. This prompted debate over whether the species was bipedal or not, leaving open the question on its status: Is Sahelanthropus a hominin (a human ancestor)? 

In the Science Advances study, the scientists took a closer look at the ulnae and femur using two primary methods: a multi-fold trait comparison with the same bones of living and fossil species and 3D geometric morphometrics—a standard method for analyzing shapes in greater detail in order to illuminate areas of particular interest. Among the compared fossil species was Australopithecus—an early human ancestor, well-known through the discovery of the “Lucy” skeleton in the early 1970s, who lived an estimated four to two million years ago. 

The analysis revealed three features that point to bipedalism in Sahelanthropus:

• The presence of a femoral tubercle, which provides attachment for the iliofemoral ligament linking the pelvis to the femur and has so far been identified only in hominins

• A natural twist, specifically within the range of hominins, in the femur—or femoral antetorsion—that helps legs to point forward, thereby aiding walking 

• The presence, drawn from the 3D analysis, of gluteal, or butt, muscles similar to those in early hominins that keep hips stable and aid in standing, walking, and running

The latter two traits—femoral antetorsion and gluteal complex—had previously been identified by other scientists; the Science Advances study affirmed their presence. 

The authors also found that Sahelanthropus had a relatively long femur relative to its ulna—additional evidence of bipedalism. The researchers note that apes have long arms and short legs, whereas hominins have relatively long legs. And while Sahelanthropus had much shorter legs than do modern humans, these were distinct from apes and approached Australopithecus in relative femur length, suggesting another adaptation to bipedalism. 

“Our analysis of these fossils offers direct evident that Sahelanthropus tchadensis could walk on two legs, demonstrating that bipedalism evolved early in our lineage and from an ancestor that looked most similar to today’s chimpanzees and bonobos,” concludes Williams. 

The paper’s other authors were Xue Wang and Jordan Guerra, both NYU doctoral students, Isabella Araiza, an NYU graduate student at the time of the study and now a doctoral candidate at the University of Washington, Marc Meyer, an anthropology professor at Chaffey College, and Jeffery Spear, an NYU graduate student at the time of the study and now a researcher at the University of Chicago.

This research was supported by a grant from the National Science Foundation (BCS-2041700).

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