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Newly Published Analysis Refutes Claims that Sahelanthropus tchadensis Was Human Ancestor

Photo: Toumaï skull, by Didier Descouens, CC BY-SA 4.0 , via Wikimedia Commons.

In 2002, Michael Brunet of the Centre National de la Recherche Scientifique in France announced the discovery of a major hominin fossil find dubbed the “Toumai Skull,” representing the species Sahelanthropus tchadensis. Although at that time the fossil was known only from a skull and some other bone and jaw fragments, he called Toumaï “the earliest known hominid, [which] could be consider[ed] as the ancestor of all later hominids, i.e. as the ancestor of the human lineage.” 

Articles by Brunet and colleagues in the journal Nature called it “the earliest known hominid ancestor,” or more cautiously proposed it as “close to the last common ancestor of humans and chimpanzees.” Although Brunet’s technical paper at the time admitted that “There is not yet sufficient information to infer reliably whether Sahelanthropus was a habitual biped,” he and his team proposed that “such an inference would not be unreasonable given the skull’s other basicranial and facial similarities to later fossil hominids that were clearly bipedal.” To this day, the Smithsonian Institution calls it “one of the oldest known species in the human family tree.” 

These claims have led to much disagreement in the paleoanthropology community. Brigitte Senut, of the Natural History Museum in Paris, called Toumai “the skull of a female gorilla,” and co-wrote in Nature, along with Milford H. Wolpoff, Martin Pickford, and John Hawks, that “Sahelanthropus was an ape,” not bipedal, and that many features “link the specimen with chimpanzees, gorillas or both, to the exclusion of hominids.” This debate has continued.

More Discovered than Reported

It turns out that there was more of Sahelanthropus discovered than was initially reported. At the end of 2020, nearly two decades after the fossil was first reported, the debate was seemingly settled when the femur of Sahelanthropus was finally described. The technical paper, “Nature and relationships of Sahelanthropus tchadensis,” published in the Journal of Human Evolution, confirmed that Sahelanthropus was a quadruped with a chimp-like body plan. New Scientist explained the implications of the new study:

The leg bone suggests that Sahelanthropus tchadensis, the earliest species generally regarded as an early human, or hominin, didnʼt walk on two legs, and therefore may not have been a hominin at all, but rather was more closely related to other apes like chimps.

As the technical paper put it:

A partial left femur (TM 266-01-063) was recovered in July 2001 at Toros-Menalla, Chad, at the same fossiliferous location as the late Miocene holotype of Sahelanthropus tchadensis (the cranium TM 266-01-060-1). … The results of our preliminary functional analysis suggest the TM 266 femoral shaft belongs to an individual that was not habitually bipedal, something that should be taken into account when considering the relationships of S. tchadensis. … In terms of size and shape, the external morphology of the shaft is closer to that of the common chimpanzee than to modern humans, gorillas, or orangutans. … Likewise, the cross-sectional morphology of the TM 266 distal shaft is most similar to that of Pan [chimpanzees]. … Given the results of the comparative analyses in the previous section, the overall morphology of TM 266 appears to be closer to that of common chimpanzees than to that of habitually bipedal modern humans. … Given the broader comparative context of the morphology of the TM 266 femur, there is no compelling evidence that it belongs to a habitual biped, something that would strengthen the case for S. tchadensis being a hominin. 

Since “the bone is curved, not straight, typical of apes like chimps,” New Scientist quoted the lead author Roberto Macchiarelli as saying, “There are a lot of indicators which deeply discourage bipedal gait.” Madelaine Böhme at the University of Tübingen in Germany said: “I saw the pictures 10 or 12 years ago, and it was clear to me that itʼs more similar to a chimp than to any other hominin.” put the implications bluntly: Sahelanthropus “was not a hominin, and thus was not the earliest known human ancestor.” 

This evidence forced the researchers to suggest that if Sahelanthropus were a human ancestor then that would mean bipedality is no longer a necessary qualification for status as a hominid — an unorthodox view that would cause great complications for the primate tree. As the recent article in the Journal of Human Evolution concluded:

Based on our analyses, the TM 266 partial femur lacks any feature consistent with regular bouts of terrestrial bipedal travel; instead, its gross morphology suggests a derived Pan-like bauplan. Thus, if there is compelling evidence that S. tchadensis is a stem hominin, then bipedalism can no longer be seen as a requirement for inclusion in the hominin clade.

Did Rivals Stonewall Publication?

New Scientist told one last part of this story that is potentially disturbing, most especially for those who think that the scientific community is always objective. First, New Scientist commented on the circumstances under which it took nearly 20 years for the femur — which apparently contradicts Brunet’s initial view that Sahalenthropus was a bipedal human ancestor — to be described: 

The researchers found a femur, or thigh bone, along with two ulnas, or forearm bones, that would help clarify the matter, but they published nothing about them for almost two decades, prompting criticism from colleagues. Brunet didnʼt respond to a request for comment from New Scientist.

Why did it take so long for the femur to be described? As New Scientist explained, after the femur was discovered in 2004 it was brought to the University of Poitiers. Collaborators wanted to study the femur, but the two lead authors of the present study, Macchiarelli and Aude Bergeret-Medina, opted not to do this “until this could be checked with Brunet and his team.” What happened next was quite strange:

Later, Bergeret-Medina was unable to find the femur. Neither she nor Macchiarelli ever saw it again. However, when Brunetʼs team didnʼt describe the femur, she and Macchiarelli prepared a study using her photos and measurements.

What happened to the femur? Did the original discoverers hold on to the bones to stonewall an analysis with a conclusion they didn’t like? This seems to be hinted by the New Scientist story, but the situation isn’t entirely clear. 

What is clear is that New Scientist further reported that the present authors “first tried to present their findings at a 2018 conference in Poitiers, but the presentation was rejected by the organisers.” Meanwhile, apparently one member of Brunet’s team is preparing a manuscript which proposes a different view — that the femur is consistent with bipedality. This is good. Perhaps if they have the actual bones their study will shed more light on the facts. Perhaps if they share the bones with other researchers even more light will be shed. One hopes that time will tell. 

So where does all of this leave us? New Scientist put the answer plainly: “It remains unclear when and where bipedalism first evolved.”