Sahelanthropus tchadensis is one of the oldest known species in the human family tree, living approximately 6 to 7 million years ago. Its discovery in West-Central Africa shifted the recognized geographic distribution of early humans, which had previously been limited to East and South Africa. The fossils reveal a combination of ape-like and human-like features, sparking scientific discussion about its exact relationship to modern humans. The emergence of this species so close to the estimated divergence of human and chimpanzee lineages makes it a subject of intense study.
The Discovery of Toumaï
The story of Sahelanthropus begins in 2001 with a Franco-Chadian expedition led by French paleontologist Michel Brunet. His team’s persistence paid off in the Djurab Desert of northern Chad, a place far from the traditional fossil-hunting grounds of Africa’s Great Rift Valley. This discovery challenged the “East Side Story” hypothesis, which suggested that the earliest hominins would be found exclusively in eastern Africa.
The expedition unearthed several fossils, but the most significant was a nearly complete, though distorted, cranium. This specimen was nicknamed “Toumaï” by the team, a name from the local Dazaga language that means “hope of life.” Along with the cranium, the team found pieces of the jaw and several teeth. Their age was determined not by radiometric dating, but through faunal analysis, which compares the surrounding animal fossils to those dated elsewhere.
Physical Characteristics
The anatomy of Sahelanthropus tchadensis presents a fascinating mosaic of features, some reminiscent of apes and others more aligned with humans. The most prominent ape-like characteristic is its small braincase, which housed a brain with a volume of about 360 to 378 cubic centimeters, comparable to that of a modern chimpanzee. The skull also features a very heavy and pronounced brow ridge, and the back of the skull retains an ape-like appearance.
In contrast, Sahelanthropus displays several traits that hint at its place within the human family. Its face was relatively flat compared to living apes, and its canine teeth were smaller and less sharp, with wear on the tips rather than the sides. This reduction in canine size is a notable departure from the large, projecting canines used for threat displays in most other male primates. These features suggest a potential shift in social behaviors away from aggression-based competition.
Perhaps the most discussed detail is the position of the foramen magnum, the opening at the base of the skull. In Sahelanthropus, this opening is situated further forward than in quadrupedal apes. This placement suggests an upright posture and is a primary indicator of bipedalism, a topic of intense debate.
The Bipedalism Debate
How Sahelanthropus moved is a subject of intense scientific debate, centered on conflicting evidence. The primary argument for bipedalism, as mentioned, comes from the forward-positioned foramen magnum of the Toumaï skull. This placement suggests the creature held its head upright, a characteristic of species that walk on two legs. A digital reconstruction of the crushed cranium confirmed this forward position.
This conclusion was challenged by the analysis of a partial femur, or thigh bone, found near the Toumaï cranium. An initial study of this femur, which was not identified as potentially belonging to a hominin until 2004, concluded that its shape was not consistent with habitual bipedalism. Some researchers argued that its features were more aligned with a creature that moved on all fours.
Further complicating the picture, a subsequent, more detailed analysis of the femur and two forearm bones (ulnae) proposed the opposite. This study suggested the femur’s characteristics were consistent with walking on two legs, while the forearm bones indicated it was also a capable tree climber. Because of these conflicting interpretations, the exact locomotion of Sahelanthropus remains unresolved. It may have practiced a form of early or occasional bipedalism while retaining the ability to move through trees.
Sahelanthropus’s Place in Human Evolution
The unique mix of anatomical features and the uncertainty surrounding its locomotion mean the precise position of Sahelanthropus in the human evolutionary tree is still debated. One leading hypothesis, favored by its discoverers, is that it is a direct human ancestor, a hominin that lived shortly after our lineage diverged from the one that led to chimpanzees.
An alternative interpretation suggests Sahelanthropus may not be a direct human ancestor at all. Some scientists propose it could be an early ancestor of a different ape lineage, such as gorillas. Another possibility is that it represents a common ancestor to both humans and chimpanzees, existing before the final split between the two lineages occurred. This would place it close to a branching point in primate evolution.
A third hypothesis posits that while related to both humans and chimpanzees, it belongs to an extinct side branch that is not a direct ancestor to either. The debate is complicated by the limited fossil evidence available. Until more post-cranial fossils are discovered to clarify how it moved and what its body looked like, its classification remains uncertain. The discovery of Sahelanthropus has shown that the early stages of human evolution were more complex and geographically widespread than previously understood.