The Seven Million-Year-Old Skull That Could Rewrite Human Evolution What If Our First Ancestor Was Already Walking Upright?
Around seven million years ago, what is now northern Chad looked very different from today’s Sahara Desert. Instead of endless sand, the region contained woodlands, lakes, rivers, and patches of open grassland. The famous fossils of Sahelanthropus tchadensis, discovered in 2001 at Toros-Menalla by a research team led by Michel Brunet, came from this rich and varied environment.
Most researchers believe Sahelanthropus spent much of its time in wooded habitats close to water. It likely climbed trees to escape predators while also traveling across the ground in search of food and freshwater. This mixed lifestyle has become central to one of the biggest questions in human evolution: Did habitual upright walking evolve before our ancestors fully adapted to life on open savannas?
As Africa’s climate gradually became drier and forests slowly gave way to expanding grasslands, primates capable of moving efficiently both in trees and on the ground may have gained an important survival advantage.
New Fossil Analysis Revives an Old Debate
A new analysis of the seven-million-year-old fossils has reignited debate over when humans first evolved habitual bipedalism. Researchers led by Scott Williams sought to answer a long-standing question: Was Sahelanthropus a habitual upright walker, or was it an ape that occasionally stood on two legs?
Using geometric morphometrics—a three-dimensional imaging technique that compares precise anatomical landmarks—the team reexamined a partial femur and two forearm bones. This method allows scientists to analyze subtle differences in bone shape and relate them to evolutionary adaptations and patterns of movement.
Evidence Supporting Habitual Bipedalism
Williams and his colleagues argue that the fossils contain several anatomical features strongly associated with habitual upright walking.
One of their most important findings is the presence of a femoral tubercle, a bony attachment for the iliofemoral ligament, one of the strongest ligaments in the human body. According to Williams, this feature is found only in hominins that consistently walk upright.
“It’s a subtle feature, so other research groups didn’t recognize it,” Williams explained, describing it as the observation that “really sold the case for bipedalism.”
The researchers also identified an inward twist of the femur similar to that seen in later human ancestors. In addition, the muscle attachment patterns around the hip appear well suited for maintaining balance while walking on one leg at a time, and the femoral shaft seems adapted to support body weight during upright posture.
At the same time, the forearm bones indicate that Sahelanthropus remained an excellent climber. According to the research team, this combination suggests an animal capable of both efficient tree climbing and habitual bipedal walking.
If this interpretation is correct, habitual bipedalism evolved approximately seven million years ago, close to the evolutionary split between humans and chimpanzees. Large human brains would not appear until several million years later, implying that walking upright preceded brain expansion in human evolution.
Critics Remain Unconvinced
Not all paleoanthropologists accept these conclusions.
Marine Cazenave and Roberto Macchiarelli argue that the available fossils do not provide convincing evidence for habitual bipedalism. In their earlier analysis published in the Journal of Human Evolution, they concluded that the femur lacks the distinctive characteristics expected in an animal that regularly walked upright.
According to their interpretation, the femoral shaft more closely resembles that of modern African apes. They also argue that the internal bone structure does not match that of known bipeds and that the proposed bipedal features may instead reflect climbing adaptations.
Another concern is the condition of the fossil itself. Because the femur is incomplete and damaged, critics believe that drawing firm conclusions from it is difficult.
Cazenave and Macchiarelli suggest that Sahelanthropus may have occasionally walked on two legs but probably moved primarily like a modern chimpanzee. They also question whether it belongs within the human lineage at all, proposing instead that it may represent an extinct ape species living near the time humans and chimpanzees diverged.
Why This Debate Matters
The disagreement goes far beyond the anatomy of a single fossil.
If Sahelanthropus was a true habitual biped, then upright walking emerged approximately seven million years ago and may represent the earliest defining characteristic of the human lineage. It would also strengthen the idea that some of the earliest human ancestors evolved in Central Africa as well as Eastern Africa.
However, if Sahelanthropus was not a habitual upright walker, then the earliest widely accepted bipeds would instead be younger species such as Orrorin tugenensis (about six million years old) or Ardipithecus ramidus (about 4.4 million years old). This would shift the origin of habitual bipedalism forward by hundreds of thousands—or even more than a million—years, leaving the identity of the first human ancestor unresolved.
Points of Agreement
Despite their differences, researchers agree on several key facts.
Sahelanthropus tchadensis lived approximately seven million years ago in what is now Chad. It possessed a brain roughly the size of that of a modern chimpanzee and retained strong adaptations for climbing.
Scientists also agree that the available fossil evidence remains extremely limited. More complete skeletons will be needed before the question of its locomotion—and its place in human evolution—can be resolved with confidence.
The Debate Continues
The latest analysis strengthens the argument that habitual bipedalism may have appeared shortly after the human–chimpanzee evolutionary split. Supporters believe the fossils preserve subtle anatomical features consistent with upright walking, while critics argue that the remains are too fragmentary and retain too many ape-like characteristics to support such a conclusion.
For now, Sahelanthropus tchadensis remains one of the most controversial fossils in paleoanthropology—and one of the most significant discoveries for understanding when our earliest ancestors first stood upright and began walking on two legs.
Source: The Seven Million-Year-Old Skull That Could Rewrite Human Evolution What If Our First Ancestor Was Already Walking Upright?
Sources
Journal of Human Evolution
Max Planck Institute for Evolutionary Anthropology
University of Poitiers Human Paleontology Research
Michel Brunet and Toros-Menalla Fossil Research
Research by Scott Williams and colleagues on the morphology of Sahelanthropus
Current scientific publications in the fields of Paleoanthropology and Human Evolution
The Seven Million-Year-Old Skull That Could Rewrite Human Evolution What If Our First Ancestor Was Already Walking Upright?