How one of the oldest candidates for human ancestors moved through its environment remains an open question. A seven-million-year-old fossil from central Africa has been interpreted as either an early upright walker or an ape that was still quadrupedal.
New research now adds weight to this question. Published in Achievements of sciencestudy provides the most compelling anatomical evidence yet that Sahelanthropus chadensis – a species discovered in Chad in the early 2000s – was capable of walking on two legs. Using high-resolution 3D analysis, the team identified a femur structure previously found only in bipedal hominins, as well as other features that support upright posture and movement. Because bipedal walking is a defining feature of human ancestry, evidence of upright walking at this age places the species at or close to the earliest known point of human ancestry.
“Our analysis of these fossils offers a direct [evidence] What Sahelanthropus chadensis could walk on two legs, demonstrating that upright walking evolved early in our lineage and from an ancestor that was most similar to today's chimpanzees and bonobos,” study author Scott Williams said in the study. press release.
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Why this ancient human fossil is difficult to classify
Comparison of the skull and femurs of chimpanzees, Sahelanthropus and Australopithecus (from left to right).
(Image credit: Scott Williams/NYU and Jason Heaton/University of Alabama at Birmingham)
Researchers discovered the fossil in Chad's Jourab Desert in the early 2000s, but early research focused primarily on its skull. Some features of the base of the skull suggested that the head may have been in an upright position, but the anatomy of the skull alone provided limited insight into how the animal actually moved.
Interpretations became more complex as other bones from the original discovery were examined, including parts of the forearm and femur. Analysis of these limb bones has varied, with some researchers emphasizing features associated with climbing and others identifying features consistent with bipedalism.
If this species could walk on two legs, the implications go beyond just one fossil. Upright walking may have evolved shortly before humans and chimpanzees diverged, long before later evolutionary changes such as brain enlargement or tool use.
The new findings also challenge the idea that upright posture emerged only after major changes in body size or brain anatomy, suggesting that upright posture evolved in species that still retained many ape-like characteristics.
Identifying early signs of upright walking
In the new study, the researchers reexamined the forearm and femur using detailed comparisons with living apes and fossil hominins, as well as high-resolution 3D shape analysis. Together, these approaches allowed the team to distinguish traits associated with upright walking from traits associated with climbing or more general movements.
The most distinctive feature they identified was the femoral tubercle, a small protrusion on the femur that serves as the attachment point for the iliofemoral ligament. This ligament, the strongest in the human body, plays a key role in stabilizing the hip during standing and walking. Until now, the femoral tubercle has only been found in bipedal hominins.
The analysis also confirmed two additional features associated with bipedal locomotion: a natural rotation of the femur that helps orient the legs forward, and a pattern of gluteal muscle attachment comparable to that observed in early human ancestors such as Australopithecus. Together, these features contribute to hip stability during standing and walking.
Although the species probably still spent time in trees and had much shorter legs than modern humans, the proportions of its limbs differ from those of apes. The relatively long femur compared to the forearm indicates an early transition to bipedalism, suggesting that bipedalism was already part of its behavioral repertoire.
“Sahelanthropus chadensis was essentially a bipedal ape with a brain the size of a chimpanzee and likely spent much of its time in trees. foraging and looking for safety,” Williams said. – Despite appearances, Sahelanthropus “has been adapted to use a bipedal posture and locomotion on land.”
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