The Face of Toumaï
In 2001, a team led by French paleontologist Michel Brunet discovered a nearly complete, though distorted, cranium in the Djurab Desert of northern Chad. The fossil, catalogued as TM 266-01-060-1, was given the nickname Toumaï, which means "hope of life" in the local Daza language. Dating to between 6 and 7 million years ago, Sahelanthropus tchadensis is one of the oldest known species in the human family tree. Its discovery was important in paleoanthropology. Before Toumaï, early hominin fossils had only been found in East Africa's Great Rift Valley and in South Africa. Finding this specimen 2,500 kilometers to the west radically expanded the known geographic range of our earliest relatives.
The skull itself has a mix of features. Ape-like characteristics include its small braincase—estimated at 320 to 380 cubic centimeters, similar to a modern chimpanzee's—a sloping face, and a prominent brow ridge. Other features appear more human-like. Toumaï has small canine teeth, a feature that distinguishes hominins from other apes whose canines are large and sharp. The face is also relatively flat compared to living apes. This combination of traits has led to an ongoing debate about where Sahelanthropus fits on the evolutionary tree. Some scientists argue it is a direct human ancestor, while others suggest it could be an ancestor to gorillas or a species that existed before the human and chimpanzee lineages diverged.
The Question of Two Feet
A central debate surrounding Toumaï revolves around how it moved. One of the defining traits of the human lineage is bipedalism—walking upright on two legs. Evidence for this posture can be found in the skull, specifically in the position of the foramen magnum. This is the large opening at the base of the skull where the spinal cord connects to the brain. In quadrupedal apes, this opening is positioned towards the back of the skull. In humans, it is located further forward, underneath the cranium, to balance the head atop an upright spine.
The discoverers of Toumaï argue that its foramen magnum is positioned in a way that suggests it walked upright. After the initial discovery, a virtual reconstruction of the crushed skull was performed using 3D X-ray imaging, which the research team claimed confirmed a forward position consistent with bipedalism. However, this interpretation is not universally accepted. Other researchers, analyzing the same features, argue the skull's anatomy is more consistent with a quadrupedal ape. The discovery of a femur bone near the skull, which was not analyzed until years later, has added to the controversy, with some studies concluding it is inconsistent with habitual bipedalism. The environment Toumaï lived in was a mix of forests, wooded savannas, and lakes, a diverse habitat where the ability to move both in trees and on the ground would have been advantageous.