7 million years ago, present-day northern Chad lay near woodlands, lakes, rivers, and open grasslands. The famous fossils of Sahelanthropus tchadensis, discovered at Toros-Menalla, were not part of today’s dry Sahara. Researchers believe Sahelanthropus tchadensis spent much of its time in wooded habitats near water rather than in open grasslands. It may have climbed trees to hide from predators in dense vegetation or moved on the ground in search of food and fresh water. Its varied habitat is one factor in the debates about the evolution of early upright walking. Archaeologists ask whether bipedalism may have emerged before humans fully adapted to open savannas. Africa’s climate was gradually drying. Forests were changing to grasslands. Versatile primates that could move easily in both trees and on the ground would have a greater chance of survival.
A new analysis of the 7-million-year-old fossils of Sahelanthropus tchadensis, one of the oldest known candidates for the human lineage (discovered in 2001 by a team led by Michel Brunet), has renewed scientific debate over when humans first began walking upright. Researchers led by Scott Williams believe they have the answer to the key question: Was Sahelanthropus a habitual biped, always walking upright, or an ape that sometimes stood on two legs?
- Human Ancestors Were Already Bipeds 7-Million Years Ago
- This Ancient Fossil Could Rewrite the Story of Human Origins
Supporters of the New Analysis Maintain that Sahelanthropus Was an Upright Walker
Composite comparison of early hominin fossils and evidence for bipedalism. (Bioanthropologist1/CC BY-SA 4.0)
Williams and his colleagues argue that reanalyzed fossils provide strong evidence that Sahelanthropus walked habitually on two legs. The research team reexamined a partial femur and two forearm bones using geometric morphometrics, a technology commonly used for landmark identification in 3D imaging. The imagery selects specific points on a biological structure to capture shape information and determine differences. Used in evolutionary interpretation, it relates shape variation to evolutionary processes and ecological adaptations.
Museum exhibit comparing fossils of Sahelanthropus tchadensis, Orrorin tugenensis, and Ardipithecus ramidus, three of the earliest known hominins, used by researchers to investigate the origins of upright walking and the evolution of the human lineage. (Emoke Denes/CC BY-SA 4.0)
Williams’ team identified several features associated with bipedalism. First, they found a femoral tubercle, a bony attachment for the iliofemoral ligament. It is one of the body’s strongest ligaments, and it is found only in hominins that walk upright consistently. This is what “really sold the case for bipedalism,” said evolutionary morphologist Williams. “It’s a subtle feature, so other research groups didn’t recognize it.” Williams also noted an inward twist of the femur, as seen in later human ancestors. The muscle attachment patterns around the hip are consistent with maintaining balance as the body moves on one leg at a time during walking. The femoral shaft seems adapted to transmit weight while standing upright. The forearm bones show that Sahelanthropus was also an excellent climber. Williams’ team concluded that constant bipedalism evolved about 7 million years ago at close to the same time as the split between humans and chimpanzees. Large brains evolved millions of years later. Therefore, bipedalism came first in human evolution.
- The Faces of Ancient Hominids Brought to Life in Remarkable Detail
- Controversial Footprint Suggests Human-like Creatures May Have Roamed Crete Nearly 6 Million Years Ago
Other Researchers Do Not Believe that the Fossils Demonstrate Habitual Bipedalism
Two prominent paleontologists, Marine Cazenave and Roberto Macchiarelli, concluded that the femur lacks the distinctive characteristics expected in an animal that regularly walked upright. Cazenave is known for her research on ancient ecosystems and currently works with the Max Planck Institute for Evolutionary Anthropology. She specializes in studying fossil records to understand the impact of climate change. Macchiarelli is a palaeoanthropologist who formerly worked at the National Museum of Prehistory and Ethnography in Rome. He is currently Professor of Human Paleontology at the University of Poitiers, France. In a 2020 article in theJournal of Human Evolution, he concluded that the femur lacks the distinctive characteristics expected in an animal that regularly walked upright. First, Cazenave and Macchiarelli argue that the shaft is similar to that of African apes. The internal bone structure does not match bipeds - in fact, the proposed bipedal features reflect climbing adaptations. They point out that the fossil is incomplete and damaged, creating more uncertainty. Sahelanthropus may have occasionally walked upright, but moved mainly like a modern chimpanzee. Both researchers suggest that it may not belong to human ancestry at all. Both believe that Sahelanthropus fossils could be those of an extinct ape that lived near the time humans and chimpanzees diverged, not a human ancestor.
- "Neanderthal-Human Hybrid" Skull Turns Out To Be Modern Human
- New Revelations When a 3,000-Year-Old Prosthetic Toe is Examined with Cutting-Edge Technology
Why the Debate Matters
Map of major African fossil localities associated with early ape and hominin evolution. (Kameraad Pjotr & Sting/CC BY-SA 3.0)
The disagreement is about the timeline of human evolution. If Sahelanthropus was a true biped, then bipedalism began roughly 7 million years ago. Walking upright may be humanity’s earliest defining characteristic and the earliest human ancestors evolved in central Africa and eastern Africa.
However, if Sahelanthropus was not habitually bipedal, then the earliest confirmed upright walkers are younger species such as Orrorin tugenensis (about 6 million years old) or Ardipithecus ramidus (about 4.4 million years old). The timing of the origin of habitual bipedalism shifts forward by hundreds of thousands to more than a million years, and the identity of the first human ancestor remains unresolved.
Points of Agreement
Despite their disagreement, researchers agree that Sahelanthropus lived approximately 7 million years ago in what is now Chad. It had a chimpanzee-sized brain and retained strong climbing abilities. However, the available fossils are extremely limited and more complete skeletons are needed before the question can be settled.
The Argument Continues
The latest fossil analysis strengthens the argument that habitual upright walking may have appeared just after the human–chimpanzee split, and it had some identifying anatomical features that its authors think are associated with hominin bipedalism. However, critics maintain that the same fossils are too fragmentary and have too many ape-like characteristics to support that conclusion. As a result, Sahelanthropus tchadensis remains one of the most debated fossils in paleoanthropology—and one of the most important for understanding when human ancestors first stood up and began walking on two legs.
Top Image: Fossil skull of Sahelanthropus tchadensis ("Toumaï"), discovered at Toros-Menalla in Chad and dated to approximately seven million years ago. Source:(Oryctes/CC BY-SA 3.0)
By Ramsey Hardin
References
Bassi, Margherita. 2026. “New Fossil Analysis Suggests This Seven-Million-Year-Old Primate Walked on Two Legs, Potentially Making It the Oldest Known Human Ancestor.” Smithsonian Magazine (January). https://www.smithsonianmag.com/smart-news/new-fossil-analysis-suggests-this-seven-million-year-old-primate-walked-on-two-legs-potentially-making-it-the-oldest-known-human-ancestor-180987947/
Devitt, James. 2026. “Anthropologists Offer New Evidence of Bipedalism in Long-Debated Fossil Discovery.” NYU News.https://www.nyu.edu/about/news-publications/news/2026/january/anthropologists-offer-new-evidence-of-bipedalism-in-long-debated.html?challenge=d06e90d7-4d8f-4b88-9d8c-10b73beb60f
Eureka Alert. 2026. “Anthropologists offer new evidence of bipedalism in long-debated fossil discovery.” EurekaAlert.https://www.eurekalert.org/news-releases/1109959.
Macchiarelli, Roberto & Bergeret-Medina, Aude & Marchi, Damiano & Wood, Bernard.
(2020). “Nature and relationships of Sahelanthropus tchadensis.” Journal of Human Evolution. 149. 10.1016/J.JHEVOL.2020.102898.
Mathey, Laura, and Stephen Ousley, “Geometric Morphometrics” (Chap, 5.3), pp. 289-298, in
Obertová, Zuzana, Alistair Stewart, and Cristina Cattaneo, eds. 2020. Statistics and Probability in Forensic Anthropology. N.p.: Elsevier Science, https://doi.org/10.1016/B978-0-12-815764-0.00023-X.
https://www.sciencedirect.com/science/article/pii/B978012815764000023X
Science Daily. 2026. “This ancient fossil could rewrite the story of human origins.” ScienceDaily. https://www.sciencedaily.com/releases/2026/01/260103155024.htm.
Scott, Anastasia. 2026. “This 7-Million-Year-Old Fossil May Reveal When Ancient Humans Started Walking Upright.” Discover Magazine. https://www.discovermagazine.com/this-7-million-year-old-fossil-may-reveal-when-ancient-humans-started-walking-upright-48484
Williams, Scott, Xue Wang, Isabella Araiza, Jordan Guerra, Marc Meyer, and Jeffrey Spear. 2026. “Earliest evidence of hominin bipedalism in Sahelanthropus tchadensis.” Science Advances 12, no. 1 (January). DOI: 10.1126/sciadv.adv0130
.svg){kind=link}
