Lead
A reanalysis led by Dr Scott Williams of New York University, published in Science Advances, examines thigh and forearm bones attributed to Sahelanthropus tchadensis from the Djurab desert in Chad. The team finds anatomical features consistent with upright walking in specimens dated to about 7 million years ago, strengthening the species’ claim as an early hominin after the human–chimpanzee split. The results revive a long-running debate: some specialists interpret the same evidence as ambiguous or ape-like, not definitively bipedal. All parties agree the question cannot be settled without more fossils from the original locality.
Key takeaways
- Sahelanthropus tchadensis fossils are dated to roughly 7 million years ago and were recovered from the Djurab desert in Chad; fragments were first publicized in 2001.
- The new study re-examined a partial femur and forearm bones using 3D contour analysis and comparative metrics against apes and known hominins, published in Science Advances (2026).
- Researchers identified a femoral tubercle on the thigh bone and other traits—femoral twist and muscle attachment patterns—that the team links to bipedal mechanics.
- Lead author Dr Scott Williams interprets the suite of features as evidence Sahelanthropus adopted terrestrial bipedalism while still using trees for some behaviors.
- Critics at the Max Planck Institute and elsewhere call the femoral evidence faint and the bone preservation poor, arguing the traits are equivocal or ape-like.
- Prof Michel Brunet and colleagues at the University of Poitiers, who originally reported the fossils, welcome the new analysis but emphasize more fieldwork at the site this year is essential.
- Consensus is lacking: the result tightens the case but does not conclusively resolve whether Sahelanthropus was a habitual ground biped or an ape with some bipedal capabilities.
Background
The search for the earliest members of the human lineage focuses on the period around the estimated human–chimpanzee split, commonly placed near 6–7 million years ago. Sahelanthropus tchadensis, named from material recovered in 2001 at a Chadian site in the Djurab desert, has been central to that search because its cranium displayed a mix of ape-like and human-like features. When Prof Michel Brunet and his team first reported the fossils, they argued the way the skull sat on the spine suggested upright posture and called the species a candidate for the ancestor of later hominins.
Subsequent debate has turned on the scarcity of postcranial remains: without well-preserved lower-limb bones, locomotor interpretations remained speculative. Partial limb bones later attributed to Sahelanthropus were described but did not produce consensus; different teams reached different inferences depending on which traits they emphasised and how they interpreted preservation damage. Over two decades, the species has marked the tension between scarce fossil evidence and high stakes for models of early human evolution.
Main event
Williams and colleagues revisited the disputed femur fragment and associated forearm elements with updated imaging and comparative geometry. They measured proportions, surface topography and three-dimensional contours, and compared those features to samples of extant African apes and established hominins such as Australopithecus. A specific prominence on the upper femur—reported as a femoral tubercle—drew attention for being an attachment point for a major ligament and muscle system that stabilises the trunk during bipedal stance.
The team also highlights a torsion, or twist, in the femoral shaft that orients the knee and foot forward, plus muscle attachment sites consistent with enlarged gluteal stabilisers. Taken together, the authors argue, these traits better match a bipedal gait than habitual quadrupedalism. They characterise Sahelanthropus as an ape-like animal that had adopted a terrestrial bipedal repertoire while still using trees for foraging and safety.
Not all reviewers were persuaded. Dr Marine Cazenave at the Max Planck Institute noted the femoral region is damaged and called the prominence “very faint,” arguing that it is not a definitive marker of habitual upright walking and resembles variation seen in fossil and living apes. Dr Rhianna Drummond-Clarke said some anatomical signals look promising but stressed remaining ambiguity about whether two-footed locomotion was primarily terrestrial or arboreal in function.
At the University of Poitiers, Dr Guillaume Daver and Dr Franck Guy said the new analysis reinforces their longstanding interpretation but acknowledged the limits of current material and urged renewed excavation at the Djurab locality to recover more conclusive postcranial remains.
Analysis & implications
If Sahelanthropus did practice terrestrial bipedalism around 7 million years ago, that pushes the emergence of ground-based walking very close to the human–chimpanzee split and implies that some hominin locomotor adaptations arose earlier than many models assume. Early acquisition of bipedal traits would reshape reconstructions of the environments and selective pressures acting on our earliest ancestors, with mosaic adaptations for both trees and open ground.
However, morphological traits can be homoplastic—similar forms evolving independently—so a femoral bump or torsion pattern does not automatically indicate direct ancestry. Convergence among apes adapting to particular ecological niches is a well-documented complication in paleoanthropology; distinguishing shared ancestry from parallel evolution requires larger samples and careful phylogenetic analysis.
Methodological advances—high-resolution 3D surface mapping and expanded comparative datasets—increase confidence in subtle anatomical signals, but they do not substitute for additional specimens that demonstrate consistent patterns across individuals. For a species known from very few bones, a single interpreted trait can swing the narrative disproportionately, which helps explain the intensity of the debate.
Practically, the study underscores the urgency of renewed fieldwork in Chad and better stratigraphic documentation so future finds can be confidently dated and associated. It also illustrates how reinterpretation of existing material with new techniques can change the balance of evidence without necessarily closing a scientific question.
Comparison & data
| Taxon | Approx. age | Femoral tubercle | Femoral torsion | Locomotor inference |
|---|---|---|---|---|
| Chimpanzee (Pan) | Extant | absent/variable | low/ape-like | knuckle-walking, occasional bipedalism |
| Sahelanthropus tchadensis | ~7 Ma | reported present (faint) | reported moderate torsion | possible terrestrial bipedism, arboreal use |
| Australopithecus (e.g., afarensis) | 4–2 Ma | present | higher, human-like | habitual bipedalism |
The table summarises broad comparisons used by the authors and critics. Sahelanthropus sits temporally ahead of Australopithecus and is closer to the estimated split from chimpanzees; its reported features are intermediate and, according to the new analysis, more similar to later hominins than to living apes in key respects. However, descriptions such as “reported present (faint)” reflect preservation limits and differing interpretations among specialists.
Reactions & quotes
Supporters and sceptics each framed the new work within existing positions. Williams emphasised the functional interpretation; critics highlighted preservation and alternative explanations.
“We think the earliest hominins were adapting to terrestrial bipedalism,”
Dr Scott Williams, New York University (lead author)
Opponents stressed the fragmentary nature of the evidence and the potential for ape-like variation to explain the features.
“Most of the results point to similarities with African great apes; the evidence for upright walking is weak,”
Dr Marine Cazenave, Max Planck Institute for Evolutionary Anthropology
Researchers tied to the Chadian field programme welcomed renewed attention but urged more fossils before revising broad evolutionary models.
“This is welcome additional analysis, but the debate will not be settled until more material is found in the Djurab deposits,”
Dr Guillaume Daver and Dr Franck Guy, University of Poitiers (field team)
Unconfirmed
- Whether the femoral tubercle identified is unique to habitual bipeds; critics argue similar morphology can appear in apes or result from preservation artefacts.
- Whether the limb fragments are from the same individual or population, and if they are representative of typical Sahelanthropus anatomy.
- Whether Sahelanthropus was primarily terrestrial or used bipedalism mainly in arboreal contexts—this remains unresolved by current material.
Bottom line
The new reanalysis strengthens the argument that Sahelanthropus possessed anatomical features consistent with at least occasional terrestrial bipedalism about 7 million years ago, bringing it closer to being a plausible early hominin. Yet the evidence is incremental rather than definitive: preservation issues, small sample size and alternative functional interpretations mean that scepticism is scientifically warranted.
The immediate priority is palaeontological: additional, well-preserved postcranial material from the Djurab site and clear stratigraphic context are essential to move from intriguing suggestion to consensus. Until such finds appear, Sahelanthropus will remain a pivotal but contested specimen in narratives about how and when walking on two feet became a defining human trait.