A recent analysis of fossil limb bones from Sahelanthropus tchadensis, an ancient primate that lived approximately 7 million years ago in North Africa, has identified anatomical features consistent with upright walking. The study, led by Dr. Scott Williams of New York University and published in Science Advances, suggests the species was bipedal and represents a potential early hominin, contributing to ongoing scientific discussion regarding its locomotion and evolutionary classification.
The Enigmatic Sahelanthropus tchadensis
Sahelanthropus tchadensis is a primate species whose fossils were discovered in 2001 in Chad's Djurab desert. Dating back approximately 7 million years, its existence aligns with the estimated divergence time of the Homo (human) and Pan (chimpanzee) lineages, making it a subject of inquiry regarding early human ancestry. The fossil record for this species is limited, consisting of a distorted skull nicknamed Toumaï, several teeth, jaw fragments, two arm bones (ulnae), and a partial femur.
Following the initial discovery, Professor Michel Brunet of the University of Poitiers, who led the original research team, proposed in 2002 that Sahelanthropus walked upright. This hypothesis was based on the position of the foramen magnum (the opening in the skull where the spinal cord connects), which he suggested indicated an upright posture. This interpretation positioned Sahelanthropus as a potential ancestor within the human lineage, but the limited fossil evidence, particularly from the lower body, led to scientific debate regarding its conclusive mode of locomotion.
New Analysis of Limb Bones
In new research published in Science Advances, Dr. Scott Williams, an associate professor at New York University, and his colleagues conducted a re-examination of the Sahelanthropus ulnae and femur. The research team, which included contributors from the University of Washington, Chaffey College, and the University of Chicago, employed advanced methodologies. These included tactile examination, 3D technology, 3D geometric morphometrics (a method for detailed shape analysis), and a multi-fold trait comparison with the same bones from living and fossil species, including Australopithecus, an early human ancestor.
Key Anatomical Features Identified
The analysis by Williams' team identified and re-affirmed several anatomical features in the Sahelanthropus fossils that are associated with bipedalism:
- Femoral Tubercle: A subtle bump was identified on the femoral bulb, located on the top front of the femur where it connects to the hip joint. This feature is an attachment point for the iliofemoral ligament, a major ligament in the human body that tightens when standing and helps prevent torso sway during bipedal walking. Williams noted that this feature has been primarily observed in bipedal hominins and was not found in numerous fossil apes from Africa and Asia dating back 20 to 7 million years.
- Femoral Antetorsion: The study confirmed the presence of a natural twist in the thigh bone (femoral antetorsion), which falls within the range observed in hominins. This twist facilitates the forward pointing of the leg during walking and contributes to the positioning of the knees closer to each other than the hips, similar to modern humans.
- Gluteal Muscle Attachments: Indications of buttock muscle attachments designed to stabilize the hips during upright standing, walking, and running were observed. These structures were noted as similar to those found in early hominins, differing from those in living nonhuman apes.
- Limb Proportions: The research revealed that Sahelanthropus possessed a relatively long femur in proportion to its ulna. Hominins typically exhibit relatively longer legs compared to apes, which have longer arms. While Sahelanthropus's legs were shorter than those of modern humans, their relative femur length was distinct from apes and approached that of Australopithecus.
The analysis also indicated that Sahelanthropus's curved arm bones show adaptations for tree climbing and arboreal activity, akin to modern chimpanzees and bonobos.
Interpretations and Scientific Implications
Based on these anatomical features, Williams and his team suggest that Sahelanthropus tchadensis was an ape-like animal that walked upright on the ground, likely while also utilizing trees for activities such as foraging and seeking safety. This interpretation aligns with a 2022 paper in Nature by Guillaume Daver and colleagues, which also concluded that this 7-million-year-old hominin was bipedal while engaging in significant arboreal activity.
The findings support the hypothesis that bipedalism emerged early in the human lineage from an ancestor resembling present-day chimpanzees and bonobos. This positions Sahelanthropus as a strong candidate for the earliest known hominin, situated around the evolutionary divergence point between humans and chimpanzees.
Diverse Scientific Perspectives and Ongoing Debate
The findings have elicited varied responses from the scientific community:
- Support: Dr. Guillaume Daver and Dr. Franck Guy of the University of Poitiers, who have historically supported the bipedal nature of Sahelanthropus, welcomed the new evidence. Dr. Jeremy DeSilva, a biological anthropologist at Dartmouth College not involved in the research, described the evidence as convincing after reviewing the 3D scans.
- Skepticism and Questions: Dr. Marine Cazenave of the Max Planck Institute for Evolutionary Anthropology noted that most results indicate similarities with African great apes or extinct apes and characterized the evidence for upright walking as "weak." She also questioned the significance of the femoral tubercle, describing it as "very faint" in a "highly damaged" region of the thigh bone and not directly related to upright walking. Dr. Rhianna Drummond-Clarke, also from the Max Planck Institute, found some evidence compelling but raised questions regarding whether the bipedalism was arboreal or terrestrial, with terrestrial bipedalism being considered a defining characteristic of the human lineage. She suggested the evidence could also indicate Sahelanthropus was an early chimpanzee that evolved towards less upright posture and knuckle-walking. Dr. Williams himself had initially expressed skepticism regarding Sahelanthropus's hominin classification due to its age.
Despite the new evidence, a universal consensus on Sahelanthropus's classification as a hominin rather than an ape has not yet been reached due to the limited fossil record. Researchers generally agree that more fossil discoveries are necessary to definitively resolve the debate surrounding the locomotion and precise evolutionary placement of Sahelanthropus tchadensis.