Fossil Analysis Sheds Light on Early Human Walking Evolution: Expanding the Debate

The long-standing debate regarding whether our earliest ancestors walked on knuckles like chimpanzees or stood upright like modern humans may be closer to resolution, yet skepticism remains.

Scott Williams and researchers at New York University recently reanalyzed fossil remains of Sahelanthropus tchadensis, indicating that this species possessed at least three anatomical features suggesting it was our earliest known bipedal ancestor.

The journey to this conclusion has been extensive.

Fossilized remains of a skull, teeth, and jawbone from approximately 7 million years ago were first identified in 2002 in Chad, north-central Africa. The distinctive features of this ancient species, including its prominent brow ridge and smaller canine teeth, were quickly acknowledged as diverging from ape characteristics.

Analyzing the skull’s anatomy suggests it was positioned directly over the vertebrae, analogous to other upright, bipedal hominins.

In 2004, French scientists uncovered the femur and ulna associated with the Sahelanthropus skull from Chad. However, it wasn’t until 2020 that researchers claimed the femur exhibited curvature similar to that of non-bipedal great apes.

Since then, scholarly debate has fluctuated. For instance, in 2022, researchers Frank Guy and Guillaume Daver of the University of Poitiers argued for anatomical features of the femur that indicate bipedalism. In 2024, Clement Zanoli and colleagues from the University of Bordeaux countered, suggesting Guy and Daver’s assertions were flawed, as the anatomical characteristics of bipedalism may also appear in non-bipedal great apes.

Lead study author Williams started with a “fairly ambivalent” stance on Sahelanthropus.

His team investigated the femur’s attachment point for the gluteus maximus muscle, finding similarities to human femur anatomy.

They also compared the femur and ulna size and shape; while similar in size to chimpanzee bones, they aligned more closely with human proportions.

Additionally, they identified the “femoral tuberosity,” a previously overlooked feature of Sahelanthropus.

“We initially identified it by touch, later confirming it with 3D scans of the fossil,” Williams shared. “This bump, present only in species with a femoral tubercle, contrasts smooth areas found in great apes and plays a critical role in mobility.”

This area serves as an attachment point for the iliofemoral ligament, the strongest ligament in the human body. While relaxed when seated, it tightens during standing or walking, securing the femoral head in the hip joint and preventing the torso from tilting backward or sideways.

However, Williams expressed doubts about whether this study would fully end the conversation about how Sahelanthropus moved.

“We are confident Sahelanthropus was an early bipedal hominin, but we must recognize that the debate is ongoing,” Williams noted.

In response to a recent paper, Guy and Daver issued a joint statement asserting that humans likely began walking on two legs by 2022: “This reaffirms our earlier interpretations about Sahelanthropus adaptations and locomotion, suggesting habitual bipedalism despite its ape-like morphology.”

They acknowledged that only new fossil discoveries could unequivocally conclude the matter.

John Hawkes, a professor at the University of Wisconsin-Madison, also endorsed the new findings, noting their implications for understanding the complex origins of the hominin lineage.

“It may be deceptive to perceive Sahelanthropus as part of a gradual evolution towards an upright posture. It reveals crucial insights into these transformative changes,” Hawkes commented.

However, Zanoli contended, stating, “Most of the evidence aligns Sahelanthropus with traits seen in African great apes, suggesting its behavior was likely a mix between chimpanzees and gorillas, distinct from the habitual bipedalism of Australopithecus and Homo.

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