A seven-million-year-old skull unearthed in Chad has long been at the center of a heated debate concerning human origins. The species, known as Sahelanthropus tchadensis, was first introduced in the early 2000s as a potential early hominin. This classification is significant because it suggests that upright walking emerged close to the evolutionary divergence between the human lineage and that of chimpanzees and bonobos.
Now, a research team led by Scott Williams, an associate professor in New York University’s Department of Anthropology, claims to have found the strongest evidence yet from a different part of the anatomy: the thigh bone. The group, which also includes researchers from the University of Washington, Chaffey College, and the University of Chicago, has published their findings in Science Advances.
The crux of the debate is bipedalism. If Sahelanthropus regularly walked on two legs, it aligns more closely with the human lineage. If not, it may be more akin to other ancient apes or represent a complex mix that defies easy classification.
“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,” said Williams. “Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.”
Reading Bones in 3D
The discovery of Sahelanthropus in Chad’s Djurab Desert by University of Poitiers’ paleontologists initially focused on the cranium. Subsequent reports described limb bones found with the material, including parts of the femur and ulnae. These later publications fueled skepticism, with some researchers arguing that the limb bones appeared ape-like or even questioned whether they belonged to the same species.
The new study employs two approaches. First, the team compared various traits in the ulnae and femur to those in living primates and fossil species. Second, they used 3D geometric morphometrics, a method that measures shape in fine detail. The comparisons included Australopithecus, a well-known early human ancestor that lived roughly four to two million years ago, which helped establish the idea that upright walking preceded the development of large brains.
“In overall shape, the results may sound surprising. Both the ulna and the femur look most similar to Pan, the genus that includes chimpanzees and bonobos. Yet we believe that broad resemblance is not the full story. In our view, a lower limb can still preserve clear signals tied to upright posture, even when the forelimb reflects climbing and other tree-based behavior,” Williams explained.
The team also assessed whether the ulnae and femur likely “match” in size. Using breadth measures from each bone, they found a strong correlation between ulna and femur size. This pattern supports the notion that the bones could originate from the same individual or similarly sized individuals.
Why One Small Bump Matters
The central claim rests on three features in the femur that the authors argue are difficult to explain without bipedal walking.
- Femoral Tubercle: A small raised area linked to the iliofemoral ligament, the strongest in the human body, which stabilizes the hip during upright standing and walking. The study reveals that this tubercle has so far been identified only in hominins. In Sahelanthropus, the researchers report a preserved tubercle that extends from the greater trochanter region and fits the expected attachment pattern.
- Femoral Antetorsion: A natural twist in the femur that helps the legs point forward. The fossil does not preserve the ends of the bone used in many clinical measurements, so the team calculated a torsion angle using planes on the shaft surface. The result places the specimen with hominins rather than great apes, which often show the opposite pattern.
- Gluteal Complex: The study reported butt muscle attachment features that resemble early hominins and support hip stability during standing and walking. The team also noted the absence of some structures common in chimpanzees, while a gluteal ridge and a feature interpreted as a gluteal tuberosity do appear.
The researchers added a fourth line of support from proportions. Apes tend to have long arms and short legs, while hominins trend toward longer legs. Sahelanthropus still looks closer to Pan than to humans, but it shows a relatively long femur compared with its ulna. The team argued this pushes the animal away from a purely ape-like body plan and toward a “mosaic” suited to both trees and ground travel.
“Our analysis of these fossils offers direct evidence 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,” concluded Williams.
Practical Implications of the Research
These findings reshape how we think about the earliest steps toward being human. If Sahelanthropus truly combined chimpanzee-like climbing with meaningful adaptations for upright walking, then bipedalism may have started as a flexible strategy, not a sudden switch.
This matters for future fossil work because it shifts what researchers look for. Instead of expecting a clean, human-like walking package right away, teams can test for smaller, targeted features that reveal how hips and knees handled body weight.
The study also offers predictions that can guide field searches. If future discoveries uncover a pelvis from this species, the authors expect an intermediate design between chimpanzees and later hominins. Clear, testable predictions like that help move debates from opinion toward evidence.
Over time, this kind of work can refine timelines for when key traits emerged, and it can sharpen how museums, textbooks, and educators explain human origins to the public.
Research findings are available online in the journal Science Advances.
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