Controversial Fossil May Not Have Been An Upright Human Ancestor After All

A cast of the Sahelanthropus tchadensis holotype cranium TM 266-01-060-1, dubbed Toumaï. Didier Descouens (CC BY-SA 4.0)

Just under 20 years ago, the fossilized remains of a mysterious creature were unearthed in the Djurab Desert of Chad. It was found to be around 7 million years old and appeared to represent a species of upright walking hominin, suggesting it could be the earliest human ancestor ever discovered.

However, over the years, doubts and controversy have surrounded this tantalizing claim. In a new study published in the Journal of Human Evolution, four anthropologists argue it was not an upright hominin and most likely an ape-like creature that walked on all-fours. As such, the species may not be as "human" as once thought.

The species is known as Sahelanthropus tchadensis, named after the Sahel region of northern Chad where it was unearthed. The individual was dubbed Toumaï, which means "hope of life" in the Chadian Daza language. Its original discovery was centered around the fossilized remains of a near-complete cranium and a lower jaw. Early analysis of the skull's foramen magnum – the hole in the base of the skull through which the spinal cord passes – suggests this species had an erect posture and, therefore, walked upright on two legs. This would indicate the species was a bipedal hominid and could be considered as a direct human ancestor. 

They also discovered a left femur bone nearby, suggesting it belonged to it too. However, the specimen has become somewhat of an enigma. Most unusually, very few researchers have been allowed to physically examine it, and the team that discovered it has scarcely published any work about it. 

But it was an unusual discovery for a few other reasons. The remains were found in north-central Africa, whereas most other early hominin fossils are from Eastern and Southern Africa. The dating of the fossils, around 7 million years, is also surprisingly close to human-chimpanzee divergence when the ancestors of these two species split ways on the evolutionary tree. Other researchers have also doubted whether the skull truly shows the animal was upright bipedal. In fact, they suggest that the specimen has some notable similarities with other ape lineages. Of course, the "disappearance" of the femur bone has also muddied the waters further. 

This new study throws further doubt on the upright hominin claim by paying special attention to the femur bone, referred to as TM 266. Through studying images and measurements of the bone, they conclude: “The lack of clear evidence that the TM 266 femur is from a hominid that was habitually bipedal further weakens the already weak case.” As just one example, the bone shows a distinct curve, which you wouldn’t expect with an upright animal.

However, there’s more to the tale. New Scientist points out that an upcoming study due to be published in a Nature journal, which has not yet been peer-reviewed, maintains that Sahelanthropus tchadensis is bipedal. This paper concludes: “These new findings confirm that hominins were already terrestrial biped relatively soon after the human-chimpanzee divergence but also suggest that careful climbing arboreal behaviors was still a significant part of their locomotor repertoire.” 

The debate is unlikely to be settled anytime soon. Like many chapters of the human story, there are vast gaps in our knowledge. For example, a number of different studies have identified that some modern-day people have genetic ancestry derived from an unknown hominin. While scientists know this species existed — and our ancestors mated with it — because of the imprint left on our DNA, there is no physical record of the hominin ever existing. 

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