Toumai (Sahelanthropus tchadensis): The Seven-Million-Year Skull

Toumai is the nickname of a fossil cranium found in the Djurab Desert of northern Chad in July 2001 and named Sahelanthropus tchadensis in 2002. At about 7 million years old, it is one of the oldest fossils ever proposed as a member of the human lineage. Its discovery pushed the search for human origins far west of East Africa’s Rift Valley and opened a debate, still unresolved, about whether this creature walked on two legs.

Background: the search for the earliest hominins

Scientists use the word hominin for humans and all extinct species more closely related to us than to chimpanzees. Comparisons of human and chimpanzee DNA suggest the two lineages separated somewhere between about 5 and 10 million years ago, depending on the assumed rate of genetic change. Until the late 1990s, almost no hominin fossils older than about 4.5 million years were known.

Most earlier discoveries, such as Lucy in Ethiopia, came from the East African Rift. Many researchers assumed that the human line began there. The French palaeontologist Michel Brunet doubted this. In 1995 his team had already found a 3.5-million-year-old jaw in Chad, later named Australopithecus bahrelghazali and nicknamed Abel, about 2,500 kilometres west of the Rift.

Discovery in the Djurab Desert

The Franco-Chadian Palaeoanthropological Mission (known by its French initials, MPFT) worked in extremely harsh conditions among the shifting dunes of the Djurab, a part of the Sahara where wind constantly exposes ancient sediments. On 19 July 2001, at a locality called Toros-Menalla 266, the Chadian student Ahounta Djimdoumalbaye spotted a nearly complete but badly crushed cranium lying on the surface. The field team that season was led by Alain Beauvilain.

The specimen received the catalogue number TM 266-01-060-1. Chad’s president at the time, Idriss Déby, gave it the name Toumai, a word in the local Daza (Goran) language used for children born just before the dry season and often translated as “hope of life”. Brunet and an international team of colleagues published the find in the journal Nature on 11 July 2002.

What the skull shows

Toumai combines very old and surprisingly modern features. Its braincase is small, with an estimated volume of roughly 360 to 370 cubic centimetres, about the same as a chimpanzee’s. Yet its face is relatively short and flat, and it has a very thick brow ridge.

Its canine teeth are small and worn at the tip, unlike the large, sharpened canines of male apes. Reduced canines are one of the traits used to identify hominins. The tooth enamel is also intermediate in thickness between that of chimpanzees and later hominins.

Because the cranium was distorted when found, a team led by Christoph Zollikofer produced a virtual reconstruction from CT scans, published in 2005. They argued that the foramen magnum, the opening through which the spinal cord leaves the skull, sat further forward than in apes. In living species, a forward foramen magnum is associated with a head balanced on top of an upright spine.

A green Sahara

Today the Djurab is one of the driest places on Earth, but 7 million years ago the landscape was very different. The animal and plant fossils found with Toumai point to a lakeside environment with open grassland, patches of woodland and gallery forest along rivers.

The Toros-Menalla sites have yielded fossil fish, crocodiles, turtles, hippopotamus-like animals, early elephants, horses, pigs and monkeys. This mosaic of habitats matters for theories of human origins. An older idea held that our ancestors began walking upright after leaving the forest for open savanna; Toumai’s world suggests that early hominins, if that is what they were, lived where trees and open ground met.

Dating the fossil

There are no volcanic layers at Toros-Menalla, so the standard radiometric methods used in East Africa cannot be applied. The original estimate of 6 to 7 million years came from biochronology: comparing the extinct animals found with Toumai, such as early elephants, pigs and horses, with dated faunas elsewhere.

In 2008 a team led by Anne-Elisabeth Lebatard used cosmogenic beryllium isotopes in the sediments and obtained an age of roughly 6.8 to 7.2 million years. Some geologists have questioned whether that method is reliable in a desert where surface material is constantly reworked. Nonetheless, an age of about 7 million years is the figure most often cited.

The debate over upright walking

Soon after the 2002 announcement, the palaeoanthropologists Milford Wolpoff, Brigitte Senut, Martin Pickford and John Hawks argued that Toumai might be an ape rather than a hominin. They suggested the small canines could belong to a female ape and that the skull’s features did not prove upright posture. Brunet’s team replied that the combination of traits pointed firmly to the human side of the split.

The dispute widened over a thigh bone (femur) collected at the same site in 2001 but not described for many years. In 2020 Roberto Macchiarelli and colleagues published an analysis concluding that the femur did not belong to a habitually bipedal creature. In 2022 a team led by Guillaume Daver and Franck Guy, including members of the original mission, described the femur and two forearm bones (ulnae) in Nature and argued that Sahelanthropus walked upright on the ground while still climbing trees skilfully.

Part of the problem is that the limb bones were not found attached to the skull, so it cannot be proven that they belong to the same individual or even the same species. The scientific community remains divided, and the question is likely to be settled only by further finds.

Key people

• Michel Brunet, French palaeontologist at the University of Poitiers, who led the Franco-Chadian mission and the description of the species.
• Ahounta Djimdoumalbaye, the Chadian student who found the cranium.
• Alain Beauvilain, the geographer who led the field team in 2001 and later wrote about the circumstances of the find.
• Christoph Zollikofer and Marcia Ponce de León, who created the 2005 virtual reconstruction.

By the numbers

Timeline

• 1995: Brunet’s team finds the jaw of Abel (Australopithecus bahrelghazali) in Chad.
• 2001, 19 July: Ahounta Djimdoumalbaye finds the cranium TM 266-01-060-1 at Toros-Menalla.
• 2002, 11 July: Brunet and colleagues name Sahelanthropus tchadensis in Nature.
• 2005, April: Additional jaws and teeth and a virtual reconstruction of the skull are published.
• 2008: Cosmogenic dating gives an age of roughly 6.8 to 7.2 million years.
• 2020: An analysis of the femur concludes it is not from a habitual biped.
• 2022, August: A Nature paper argues the femur and ulnae show upright walking combined with climbing.

Why it matters

Toumai showed that early hominins, or creatures very close to them, lived in Central Africa as well as East Africa, and much earlier than the famous australopithecines. It also showed how difficult it is to recognise the very first members of our lineage, which may have looked more like apes than like us. Together with other early finds such as Ardi, it continues to shape debates about when and why our ancestors began to walk upright.