Sahelanthropus tchadensis

The species in brief

Sahelanthropus tchadensis is a single, battered skull, plus some jaws and teeth, from the desert of northern Chad. It is about 7 million years old, which makes it among the oldest fossils ever claimed as part of the human family. It was found far from the East African rift valley, where most early human fossils come from, and that alone changed ideas about where our line began.

The name means “Sahel man from Chad”, after the dry belt south of the Sahara. The skull is nicknamed Toumai.

Discovery

The Franco-Chadian Paleoanthropological Mission, led by the French paleontologist Michel Brunet, had been searching the Djurab Desert in northern Chad since the 1990s. In 1995 it had already found a jaw of a much younger australopith there, nicknamed Abel, showing that early humans had lived in central Africa, 2,500 kilometres west of the Rift Valley.

On 19 July 2001, at a site called Toros-Menalla, Ahounta Djimdoumalbaye, a Chadian student on the team, spotted a skull on the surface of the sand. Along with it, the team found five jaw pieces and teeth. The skull was crushed from top to bottom and had been squashed sideways, but was mostly complete. Brunet’s team published it in Nature in July 2002 and named it Sahelanthropus tchadensis. Its nickname, Toumai, means “hope of life” in the local Goran language, and is given to children who are born close to the start of the dry season.

The sediments have no volcanic ash layers, so the usual dating methods could not be used. The age came from comparing the fossil animals in the same layers with those from East African sites, which suggested 6 to 7 million years. A later study using beryllium isotopes in the sediments gave an age of about 7 million years, though that is disputed because the skull was lying on the surface.

What it looks like

• Brain. About 360 to 380 cubic centimetres, similar to a chimpanzee’s.
• Face and brow. A heavy, continuous brow ridge over the eyes, which gives the skull a look of a gorilla’s, and a flatter face than a chimpanzee’s.
• Teeth. Small canine teeth with a worn tip instead of the long sharp canines of an ape. This is one of the features that its discoverers used to place it on the human side.
• Skull base. The opening where the spinal cord enters the skull appears to sit under the skull rather than at the back, suggesting a head that sat on an upright spine. This evidence is based on a crushed skull, and different reconstructions give different results.

In 2022 a team led by Franck Guy and Guillaume Daver reported that a thigh bone and two forearm bones, which they say were found at the same site in 2001 and kept in storage for years, show that Sahelanthropus walked upright and also climbed. Other researchers have argued that the thigh bone looks more like an ape’s, and that these bones could belong to a different animal. The argument is not resolved.

A challenge to the old picture

When the skull was announced in 2002 the discoverers said Sahelanthropus was close to the point where humans and chimpanzees split. This was surprising for several reasons. It was found in Chad. It is old, about 2.5 million years older than Ardipithecus ramidus, and it mixed features that seemed advanced, such as small canines, with primitive ones, such as a small brain. And it lived in a mosaic of lake edges, forest and desert, not on open grassland.

Not everyone accepted it as an early human. The paleoanthropologist Milford Wolpoff and colleagues argued that it is a female ape, because the same features can be seen in female gorillas, and that Toumai therefore proves nothing about walking. The debate has continued for two decades.

Where it fits

If Sahelanthropus is a human relative, it is one of the earliest. Two other claimants of similar age are Orrorin tugenensis from Kenya, about 6 million years old, and Ardipithecus kadabba from Ethiopia. None of the three is known well enough for researchers to agree how they relate to each other or to later species such as Ardipithecus ramidus and the australopiths.

Where to see it

The Toumai skull belongs to Chad. It is kept in N’Djamena, the capital, in the care of the Chadian authorities, and has rarely been shown outside the country.

Sources

• Smithsonian National Museum of Natural History, Human Origins Program. Sahelanthropus tchadensis.
• Brunet, M. and others (2002). A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418: 145 to 151.
• Lebatard, A.-E. and others (2008). Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad. Proceedings of the National Academy of Sciences 105: 3226 to 3231.
• Wolpoff, M. H. and others (2002). Palaeoanthropology: Sahelanthropus or ‘Sahelpithecus’? Nature 419: 581 to 582.
• Daver, G. and others (2022). Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature 609: 94 to 100.