Lead: A new multi-method study published in Science Advances presents strong anatomical evidence that Sahelanthropus tchadensis, a 7-million-year-old species unearthed in Chad’s Djurab Desert in the early 2000s, had adaptations for walking on two legs. Researchers used 3D geometric morphometrics and comparative trait analysis to identify a femoral tubercle and other features linked to upright posture. The team reports a relatively long femur compared with the ulna and hominin-like femoral rotation and gluteal attachments, arguing these traits align Sahelanthropus closer to early hominins than to living apes. If accepted, the finding places bipedal locomotion very near the root of the human lineage.
Key Takeaways
- Sahelanthropus tchadensis is dated to about 7 million years ago and was discovered in Chad’s Djurab Desert by paleontologists from the University of Poitiers in the early 2000s.
- The study, published in Science Advances, used 3D geometric morphometrics and multi-trait comparisons with living and fossil taxa to analyze ulnae and femora.
- Researchers identified a femoral tubercle—an attachment point for the iliofemoral ligament—previously observed only in hominins, and interpreted it as a key bipedal marker.
- Measured relatively, Sahelanthropus had a longer femur relative to its ulna than extant apes; its relative femur length approached that of Australopithecus but remained shorter than modern humans.
- The study affirms earlier observations of femoral antetorsion (a forward twist of the femur) and a gluteal complex consistent with weight-bearing hip stabilization in early hominins.
- The research team includes scientists from New York University, the University of Washington, Chaffey College, and the University of Chicago, with Scott Williams (NYU) quoted as leading the analysis.
- Authors argue these combined traits support the interpretation that bipedalism emerged early in the hominin lineage, though debate over direct ancestry vs. a side branch persists.
Background
Sahelanthropus tchadensis was first described from cranial remains recovered in the early 2000s in Chad’s Djurab Desert, a discovery notable for its age near the Miocene–Pliocene boundary. Initial publications concentrated on the skull, which showed a mix of ape-like and potentially hominin features, prompting controversy over whether the species was part of the human lineage. More recently, additional elements from the same locality—forearm (ulna) and thigh (femur) fragments—have been reanalyzed, giving researchers new anatomical angles to test locomotor hypotheses. Because locomotion is a core criterion for classifying hominins, bone features tied to upright walking have decisive interpretive weight in debates about early human ancestry.
Functional traits related to bipedalism (limb proportions, muscle attachment sites, and bone torsion) are commonly compared across extant apes, modern humans, and fossil hominins such as Australopithecus. Australopithecus, known from specimens like “Lucy,” lived roughly 4–2 million years ago and shows unequivocal bipedal adaptations despite retained arboreal abilities. If Sahelanthropus—several million years older—shows homologous bipedal features, it would push the origin of habitual or obligate bipedal mechanics deeper into time. That possibility has evolutionary implications for reconstructing the last common ancestor of humans and African apes and for modeling habitat and behavioral shifts in the late Miocene.
Main Event
The new analysis applied two principal methods: (1) a multi-fold trait comparison against bones of living apes, modern humans, and fossil taxa, and (2) 3D geometric morphometrics to quantify shape differences and highlight focal features. The authors report that the femur fragment preserves a distinct femoral tubercle—the insertion area associated with the iliofemoral ligament—which in living primates is strongly associated with bipedal stance and gait. That structure, the team argues, has so far only been documented in hominins and is functionally significant for resisting hip hyperextension during upright walking.
In addition to the tubercle, the femur shows antetorsion within the range seen in early hominins: a forward twist that orients the knee and foot more directly under the body during strides. The 3D models also indicate gluteal muscle attachment morphology consistent with a weight-bearing hip stabilizing complex, which aids balance in single-leg support phases of bipedal walking. Together these anatomical signals strengthen the interpretation of terrestrial locomotion beyond occasional bipedal posturing.
Quantitative comparisons revealed that Sahelanthropus had a relatively long femur compared with its ulna—shorter than in modern humans but distinct from extant apes and approaching Australopithecus proportions. The research team emphasizes the combination of traits rather than any single character as the basis for their conclusion: limb proportions, tubercle presence, antetorsion, and gluteal morphology form a coherent functional suite. The authors acknowledge that the fossil record is fragmentary and that reconstructing behavior from isolated elements requires caution, but they maintain that convergent evidence across methods reduces the likelihood of misinterpretation.
Analysis & Implications
If Sahelanthropus genuinely possessed habitual or well-developed bipedal adaptations, the timing of the origin of bipedalism shifts closer to 7 million years ago. That timing implies that the last common ancestor of humans and chimpanzees may already have exhibited more diverse locomotor behaviors than previously thought, including partial commitment to upright posture. This challenges reconstructions that place the emergence of hominin bipedalism only after a major split from an exclusively arboreal ancestor and suggests mosaic evolution of locomotor and cranial traits.
Evolutionary models must now reconcile how bipedal adaptations coexisted with apelike cranial dimensions and likely arboreal habits in Sahelanthropus. The combination of a chimpanzee-sized brain, tree-climbing behaviors, and terrestrial walking implies a facultative biped: an animal capable of efficient ground locomotion without abandoning arboreal competence. That mixed locomotor repertoire would have adaptive advantages in patchy, mosaic environments of the late Miocene, where foraging on the ground and in trees both mattered.
On taxonomy and ancestry, the findings strengthen the case for placing Sahelanthropus among early hominins, but they do not definitively resolve whether it is a direct ancestor of later Homo/Australopithecus or a close side branch. Paleobiogeography is also affected: the Djurab Desert locality in Chad demonstrates that early hominin diversification and experimentation with bipedal mechanics were not limited to East Africa. Future discoveries, especially more complete postcranial material, will be necessary to test scenarios about locomotor evolution, population structure, and ecological drivers.
Comparison & Data
| Taxon | Relative femur:ulna | Key bipedal markers |
|---|---|---|
| Extant African apes | Long arms, short legs | No femoral tubercle; low antetorsion; arboreal adaptations |
| Sahelanthropus tchadensis (7 Ma) | Femur longer than ulna; shorter than modern humans | Femoral tubercle present; antetorsion in hominin range; gluteal attachments similar to early hominins |
| Australopithecus (4–2 Ma) | Legs relatively longer, bipedally adapted | Hominin-like femoral features and gluteal complex |
| Modern humans | Long legs, short arms | Clear bipedal suite; strong iliofemoral ligament attachment |
The table summarizes relative proportions and trait presence rather than absolute metric values, reflecting the fragmentary nature of the Sahelanthropus postcranial sample. The authors used shape-space plots from 3D morphometrics to place the Sahelanthropus elements closer to Australopithecus than to extant apes on key dimensions. While exact measurements vary with reconstruction choices, the qualitative pattern—distinct from ape morphology and trending toward hominin anatomy—remains robust across analytic permutations reported in the paper.
Reactions & Quotes
Researchers directly involved in the work frame the discovery as a meaningful step toward resolving long-standing debates about the origin of bipedalism.
“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,”
Scott Williams, Associate Professor, NYU Department of Anthropology (lead analyst)
Williams characterizes Sahelanthropus as mosaic in habit, combining arboreal behavior with adaptations for upright movement on the ground. He emphasizes that the femoral tubercle and other features are functionally consistent with habitual bipedal mechanics, rather than occasional upright postures.
“Our analysis of these fossils offers direct evidence that Sahelanthropus tchadensis could walk on two legs, demonstrating that bipedalism evolved early in our lineage,”
Science Advances study authors (collective)
The study authors collectively argue that multiple independent lines of evidence—morphometric fits, trait comparisons, and functional interpretation—converge on bipedality. Outside experts not involved in the study have welcomed the data while noting the need for additional material to confirm broader behavioral inferences.
Unconfirmed
- Whether Sahelanthropus is a direct ancestor of Homo and Australopithecus or a closely related side branch remains unresolved pending more complete fossil evidence.
- The proportion of time Sahelanthropus spent on the ground versus in trees is inferred from morphology but not directly measurable from the current skeletal sample.
- Precise locomotor kinematics (e.g., gait patterns and speed) cannot be reconstructed from the available fragments and remain speculative.
Bottom Line
The new 3D and comparative analysis of Sahelanthropus postcranial fragments adds persuasive anatomical evidence for bipedal adaptations at about 7 million years ago, shifting the temporal window for upright walking nearer to the human–ape divergence. Key features—the femoral tubercle, antetorsion within the hominin range, and gluteal attachment morphology—form a coherent functional suite that aligns Sahelanthropus more closely with early hominins than with extant apes.
Nevertheless, important uncertainties remain about whether these traits indicate habitual obligate bipedalism or a facultative, mixed locomotor strategy, and whether Sahelanthropus sits on the direct line to later Homo. Additional postcranial discoveries, independent replications of the morphometric results, and broader sampling across late Miocene sites will be essential to test and refine the evolutionary scenarios this study advances.
Sources
- Interesting Engineering — news outlet reporting on the Science Advances study
- Science Advances — peer-reviewed journal where the study is published