New analysis of 7-million-year-old remains from Chad strengthens the case that Sahelanthropus tchadensis—first found in 2001—was a bipedal hominin rather than an ape. Researchers led by Scott Williams (New York University) report in Science Advances that a small but distinctive bump on the preserved femur is a femoral tubercle associated with an iliofemoral ligament arrangement that supports an upright, vertical posture. The femoral feature aligns with a 2022 Nature study by Guillaume Daver and colleagues that interpreted associated limb bones as reflecting habitual bipedalism. While the evidence remains fragmentary, the new interpretation makes Sahelanthropus a stronger candidate for a lineage on the Homo side of the Pan–Homo split about 7 million years ago.
Key takeaways
- Sahelanthropus tchadensis lived about 7 million years ago; its limited remains were recovered in Chad in 2001 and include a distorted skull (TM 266), some jaw fragments, two arm bones and a partial femur.
- The new paper (Science Advances) identifies a femoral tubercle on the partial femur—an attachment site for the superior band of the iliofemoral ligament—interpreted as supportive of a vertical bipedal posture.
- Williams and colleagues compared the femoral morphology with many fossil and living primates and report that similar tubercles are absent in the large sample of fossil apes dated 20–7 million years ago.
- The finding reinforces a 2022 Nature analysis by Daver et al. that assigned limb elements to Sahelanthropus and argued for habitual bipedality with retained arboreal capabilities.
- Despite the new claim, the sample remains tiny: the total known Sahelanthropus material is a handful of bones, so some classification uncertainty persists.
- If confirmed, Sahelanthropus would be among the earliest creatures with the distinctive hip–femur adaptations linked to hominin vertical bipedalism.
Background
Sahelanthropus was described from material recovered in 2001 in central Chad; the reconstructed skull nicknamed Toumaï and scant postcranial fragments placed the species at roughly the same time that molecular and fossil estimates place the Pan–Homo split, near 7 million years ago. Because the remains are few and partly distorted, researchers have long debated whether Sahelanthropus belonged on the hominin branch (ancestral to humans) or instead represents an early ape lineage. The reconstruction of head position on the spine and some cranial traits led early describers to suggest possible bipedality, but alternative readings have emphasized ape-like features and raised the prospect of misassigned postcranial bones.
In 2022 Guillaume Daver and collaborators published in Nature an analysis that placed limb elements with Sahelanthropus and argued for habitual bipedalism combined with arboreal behavior. That work reignited debate: locomotor interpretations hinge on a few bones and on how to compare them with other early Miocene and Pliocene primates. The broader fossil record of African and Asian fossil apes spanning roughly 20–7 million years is large, yet none of those specimens have been reported with the same femoral protrusion now highlighted by Williams and colleagues.
Main event
The new analysis, published in Science Advances, focuses on a partial femur long associated with the Chad finds. Using detailed morphological inspection and high-resolution imaging, Scott Williams and the team identified a subtle posterolateral bump near the femoral head connection to the pelvis. Tactile examination—Williams has described running his fingers over the bone—helped confirm the raised insertion consistent with a femoral tubercle for the superior band of the iliofemoral ligament.
The authors argue that this tubercle would have altered ligament mechanics so that standing straight tightened the ligament and stabilized the hip in a vertical posture—an arrangement characteristic of hominins and absent in knuckle-walking apes. They note that while some primates can intermittently rear onto the hind legs, habitual vertical bipedalism with the head aligned over the feet is distinct among hominins.
The paper explicitly links the femoral morphology to the 2022 Daver et al. interpretation: together, the cranial orientation, limb-level anatomy and the tubercle provide convergent evidence for early bipedal adaptations. At the same time, Williams and coauthors emphasize caution: the femur is partial, preservation is imperfect, and more fossils would materially improve confidence in taxonomic placement.
Analysis & implications
If the femoral tubercle interpretation holds, it moves the emergence of vertical-support bipedal mechanics closer to the Pan–Homo split itself, implying that at least some hominin traits evolved very soon after divergence. This challenges models in which early hominins remained largely arboreal with only later shifts toward terrestrial bipedality. Instead, the evidence supports a mosaic scenario: pelvic and proximal femur changes toward upright stance accompanied continued tree use.
The result also bears on the long-running debate about the evolution of knuckle-walking in African apes. If Sahelanthropus shows early hominin-like hip mechanics, it weakens arguments that the Pan–Gorilla common ancestor was a knuckle-walker ancestral to both humans and chimps; alternatively, knuckle-walking may have evolved independently in chimp and gorilla lineages. Resolving that point affects reconstructions of the last common ancestor shared by humans and African apes.
Practically, the finding underscores how small osteological details can shift phylogenetic interpretation. A tiny tubercle on a femur—missed or dismissed for 25 years—now becomes a central piece of evidence. The paper is likely to prompt reexamination of other fragmentary femora and renewed field efforts in regions of late Miocene deposits to search for associated material that can confirm or disconfirm the interpretation.
Comparison & data
| Taxon | Age (Ma) | Locomotor signal | Femoral tubercle | Representative material |
|---|---|---|---|---|
| Sahelanthropus tchadensis | ~7.0 | Proposed habitual biped + arboreal | Reported (Williams et al., Sci Adv) | TM 266 skull, partial femur, limb fragments |
| Australopithecus spp. | 4–2 | Derived bipedal gait with arboreal retention | Present in hominins | Multiple skeletons (A. afarensis, etc.) |
| Pan (chimpanzees) | extant | Knuckle-walking, occasional bipedalism | Absent in fossil apes sampled | Modern comparative sample |
The table summarizes the contrast between the newly emphasized proximal femur anatomy in Sahelanthropus and later australopithecines. While australopithecines show more derived femoral shapes closer to Homo, the Sahelanthropus signal—if the tubercle is homologous with later hominins—would indicate an earlier reconfiguration of pelvis–hip mechanics focused at the femoral head and proximal shaft.
Reactions & quotes
“It’s a really important little bump because it is indicative of hominins and not apes,”
Scott Williams, New York University (quoted in Science Advances report)
Williams framed the discovery as the product of detailed, patient inspection: weeks of examining the bone in comparison with many fossil and modern primates. He also acknowledged limits—more specimens are necessary to move from plausible to definitive.
“The morphology meshes with earlier work that saw a mix of tree use and habitual bipedalism,”
Guillaume Daver, lead author of the 2022 Nature paper (paraphrased)
Daver’s 2022 study had already suggested habitual bipedality in limb elements associated with Sahelanthropus; the femoral tubercle strengthens that multi-line argument but does not by itself close the debate.
Unconfirmed
- Whether Sahelanthropus is a direct ancestor of Homo rather than a side branch or an early chimp/gorilla relative remains unresolved due to limited material.
- Whether femoral tubercles of this form are strictly unique to hominins is not proven; Williams concedes fossil coverage is incomplete and exceptions could exist.
- Assignment of all limb fragments to Sahelanthropus is debated—some elements may be from contemporaneous primates in the same deposits.
- The precise gait mechanics (stride length, pelvic rotation) cannot be reconstructed reliably from the existing partial femur alone.
Bottom line
The Science Advances reappraisal of the Chad femur makes a credible case that Sahelanthropus possessed a proximal femoral morphology consistent with early hominin vertical bipedalism, reinforcing earlier work that the species showed a mosaic of tree use and upright walking. This interpretation, if sustained by additional finds, would place key hip adaptations at or very near the time of the Pan–Homo split about 7 million years ago and would reshape models of how and when upright posture evolved.
Crucially, the claim rests on scant material: the next steps are targeted fieldwork for more specimens and renewed comparison of existing early Miocene and late Miocene femora. Until broader corroboration appears, Sahelanthropus stands as a strong candidate for an early hominin role—one that highlights how small anatomical details can change big-picture narratives of human origins.
Sources
- Haaretz (news report of the Science Advances paper and interview)
- Science Advances (academic journal; original Williams et al. article published there)
- Nature (academic journal; Daver et al., 2022 paper reanalyzing Sahelanthropus limb elements)