This tiny bump on an ancient thigh bone has become a big deal for anyone trying to pin down when the human story truly began. Anthropologists working with New York University have used advanced 3D analysis to revisit fossils attributed to Sahelanthropus tchadensis, a roughly 7-million-year-old ape from central Africa. The new anatomical reading strengthens a long-running argument that this species could walk on two legs-an ability that, if present this early, places bipedalism close to the split between the human lineage and the line leading to chimpanzees and bonobos. Instead of treating upright walking as a late upgrade added after other “human” traits appeared, the fossils force attention onto locomotion as an early design constraint that shaped everything else that followed.
The key update comes from the femur. Applying 3D geometric morphometric and detailed comparisons of individual features, researchers identified a femoral tubercle, where the iliofemoral ligament attaches. That ligament is critical for stable upright stance and gait in modern humans because it prevents the trunk from pitching backward when the hips are extended. The study also reaffirmed two other features associated with bipedal locomotion: a human range twist in the femur that helps to aim the legs forward, and patterns of muscle attachment consistent with a gluteal complex that stabilizes the pelvis at each step. Put together, these signals place mechanical emphasis where engineers would predict it: at the hip, where posture is managed as a load-management problem with every step.
“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 Scott Williams, an associate professor in New York University’s Department of Anthropology, who headed the study.
That combination-ground walking without abandoning climbing-matters because it agrees with an increasingly common picture of early hominins as hybrid movers, not cleanly “ape” or “human.” Educational summaries of early hominin locomotion describe a long interval in which ancestors combined tree use with regular terrestrial travel, rather than switching habitats in a single leap. In that framing, early bipedalism looks less like a finish line and more like a platform: a stable way to hold the trunk upright while still keeping the arms and hands available for climbing, carrying, and feeding.
From an anatomy-and-mechanics standpoint, bipedal striding is unusual not because it exists-many animals manage two-legged movement-but because humans are adapted to it as a default mode. The human stance leans heavily on the geometry of joints and the placement of ligaments, which reduces the muscular effort just to stand. Background work on the anatomy of bipedalism stresses how pelvic shape, hip musculature, and femoral angling cooperate to keep the center of mass controlled over a narrow base of support. The advantage for paleoanthropologists is practical: bones involved in those constraints often preserve diagnostic signatures, especially in the pelvis and lower limbs. That makes a thigh bone more than a fossilized fragment-it is a record of how forces were routed through a body.
The fossil record surrounding the human–chimpanzee divergence is, however, sparse and structurally incomplete. For decades, most candidate species from approximately 8 to 4.4 million years have been found within only a few regions of Chad, Kenya, and Ethiopia; they also include Sahelanthropus, Orrorin, and Ardipithecus. In a generally used overview of the earliest hominins, one finds that these taxa indeed share some hints at hominin-like biology but also retain many ape-like traits, making family-tree placement complicated. For instance, more extensive postcranial material is present in Ardipithecus ramidus, showing strong climbing adaptations alongside more limited evidence for upright posture. Orrorin tugenensis is often discussed based on its femoral traits, which are consistent with bipedal weight support but still show signs of significant arboreal activity. An emerging theme is that early hominins were diverse experiments in movement, rather than one blueprint that marched toward modern form.
This debate persists for Sahelanthropus in particular, because different skeletal elements can be read in different ways and preservation can blur fine anatomical signals. Critics have argued that deformation and damage limit what can be inferred from the femur, while supporters see the newly documented ligament attachment as precisely the kind of discrete feature that is hard to explain away. The same tension appears in many early-hominin discussions: small morphological cues have to be weighed against uncertainty in fossil distortion, incomplete comparative samples, and the fact that early lineages may not map neatly onto later categories.
Of the many reasons this round of analysis is drawing attention, one is methodological: high-resolution 3D tools are improving the ability to interrogate subtle topography on fossil surfaces and to compare shapes quantitatively across samples, even when specimens are fragile or dispersed. That trend extends well beyond the studies themselves: NYU engineers have also built portable, lower-cost digitization systems that can scan fossils at scale for broader access. In one deployment, a device assembled for under $3,500 digitized more than 200 fossils in weeks and was designed to help museums with limited resources create usable 3D archives without shipping delicate specimens to distant facilities.
The better digitization does not automatically produce agreement on the human-origins question, but it does change what disagreement can be about. Where anatomical claims can be tested against shared 3D models and repeatable measurements, arguments shift away from who had access to which originals and toward whether specific features do or do not consistently track with known locomotor patterns.
“Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.”
If that assessment holds, the engineering story of human evolution becomes less about a sudden invention of upright walking, more about early, incremental solutions to stability, load transfer, and balance, solutions built into hips, ligaments, and muscle attachments long before the more obvious signatures of humanity appeared
