Australopithecus bahrelghazali from 3.5 million years ago. Found 2,500 km west of the Rift Valley; destroying the theory that humans evolved in a single corridor.
Clumsystiggy, CC BY 4.0, via Wikimedia Commons
The discovery at Koro Toro
In 1995, a French-Chadian team led by paleontologist Michel Brunet made a discovery in the Djurab Desert of Chad that changed the map of human evolution. At a site designated KT 12, near Koro Toro, they unearthed a fossilized mandible fragment. This initial find was soon followed by an upper premolar and, at nearby sites, two other jawbone fragments. The most complete specimen, KT-12/H1, became the holotype for a new species: Australopithecus bahrelghazali. The name honors the Bahr el Ghazal (River of the Gazelles) region where the fossils were found.
The fossils consist of a partial mandible with its lower second incisor, both canines, and all four premolars intact. Morphologically, the jaw shows a flatter front surface at the chin compared to its East African contemporary, Australopithecus afarensis. The premolar is also distinct, featuring three separate roots. While some researchers argue these differences fall within the range of variation for A. afarensis, the discovery's location alone presented a radical departure from previous findings. Until this point, every known australopithecine had been found exclusively in Southern or Eastern Africa.
Radiometric dating of the surrounding sediments provided a firm age for the fossils. Using a method based on the ratio of beryllium isotopes (10Be/9Be), scientists dated the deposits to approximately 3.58 million years ago, placing A. bahrelghazali squarely in the Pliocene epoch.
Beyond the Rift Valley
The existence of an australopithecine in Central Africa directly challenged a prominent hypothesis known as the "East Side Story". Proposed by paleontologist Yves Coppens, the theory suggested that the formation of the Great Rift Valley around 8 million years ago created a climatic barrier. To the west, in the heavily forested regions, apes continued to evolve. To the east, a drier savanna environment drove the adaptation of bipedalism and the evolution of hominins. For years, the fossil record supported this model, as all early hominin discoveries occurred east of the Rift.
Australopithecus bahrelghazali, found 2,500 kilometers (1,500 miles) west of the Rift Valley, invalidated this geographic restriction. It demonstrated that early hominins were not confined to a single evolutionary corridor in East Africa but were widely distributed across the continent. The discovery implies that our early ancestors were adaptable to a wider range of habitats than previously understood.
Analysis of associated animal fossils and sediments at Koro Toro shows its ancient environment. Around 3.5 million years ago, this part of the Sahara was a lakeside setting with a mix of grasslands, woodlands, and wetlands, similar to the modern Okavango Delta. Isotope analysis of the hominin's tooth enamel suggests a diet primarily based on C4 savanna plants like grasses and sedges, supplemented by C3 forest foods such as fruits or insects.
💡Fun Facts
The main fossil mandible, KT-12/H1, is nicknamed "Abel" as a tribute to Michel Brunet's colleague, Abel Brillanceau, who had passed away.
The discovery was made by the Mission Paléoanthropologique Franco-Tchadienne (MPFT), a scientific collaboration between the University of Poitiers and the University of N'Djamena.
The name of the region, Bahr el Ghazal, translates from classical Arabic to "River of the Gazelles".
Later discoveries in Chad by Brunet's team would include the much older *Sahelanthropus tchadensis*, nicknamed "Toumaï", which dates back approximately 7 million years.
