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Fossil Friday: Sahelanthropus, to Be or Not to Be Bipedal

September 9, 2022, 6:45 AM
Photo: Sahelanthropus, by Didier Descouens, CC BY-SA 4.0 , via Wikimedia Commons.

This week’s Fossil Friday features the iconic hominid fossil Sahelanthropus, which is often considered to be the oldest known representative of the human lineage (Su 2013, Strait et al. 2015, Smithsonian Institution 2022, Wikipedia 2022), but has been subject to fierce scientific controversy ever since its sensational discovery (BBC 2002, Caspari 2002, Hartwig-Scherer 2002, Roach 2002).

In 2018 the prestigious journal Nature (Callaway 2018) and eminent paleoanthropologist John Hawks (2018) reported about and commented on the apparent suppression of scientific evidence that may refute the interpretation of Sahelanthropus as the oldest bipedal hominin. Recently, two new studies (Macchiarelli et al. 2020, Daver et al. 2022) addressed this issue and came to opposite conclusions. A German science journal therefore commented that “the alleged first biped arguably was not” (Schlott 2022). Let’s have a brief look at the history of this issue.

A Look at the History

On the morning of July 19, 2001, French scientist Alain Beauvilain and three Chadian colleagues discovered a fossil cranium in the dunes of the Chadian Sahara Desert, together with an assemblage of other fossils. The sensational discovery was covered by worldwide media and celebrated as shaking the human family tree (Hecht 2002) and having “the impact of a small nuclear bomb” (Radford 2002). The discoverer even wrote a popular science book (Beauvilain 2003) about the story of the find. The specimen received the collection number TM 266 and was nicknamed “Toumaï” (or, “hope of life”) in a widely reported 22-pages press release from the University of Poitiers in July 2002 (Beauvilain & Guellec 2004). It was shortly thereafter described by the head of the research team, Michel Brunet (Brunet et al. 2002), as the fossil hominin Sahelanthropus tchadensis. Geological and paleontological evidence (Vignaud et al. 2002) as well as radiometric dating (Lebatard et al. 2008) suggested that this fossil was between 6.8 and 7.2 million years old and therefore would represent the oldest known ancestor of humans. Sahelanthropus falls right in the time assumed for the human-chimp split by molecular clock studies (Brahic 2012). The smashed fossil thus achieved iconic status.

Of course, the dating is still a bit dubious because the skull was not found in situ in Miocene deposits (Beauvilain 2008). Beauvilain & Watté (2009) even speculated that modern nomads might have assembled the bones and placed them in a grave in the sand dunes, without being aware that they are fossils. Later native Muslim camel herders could have reoriented the exposed skull towards Mecca and reburied the specimen until it was again uncovered by the wind and discovered by science in 2001. Even though the evidence for this speculation is weak (Hawks 2009a), the circumstances of this famous fossil’s discovery were certainly far from optimal.

The fossil skull is also poorly preserved and was totally battered, which made interpretation cumbersome and highly ambiguous. A computer-aided 3D reconstruction by Zollikofer et al. (2005) attempted to establish the crucial position of the foramen magnum as evidence of bipedal locomotion, but this remains controversial (BBC 2005, Hawks 2005, Wolpoff et al. 2006), especially since the crucial angle of the foramen magnum is exquisitely sensitive to the reconstruction (Hawks 2005). Immediately after the original description by Brunet et al. (2002), writing in the journal Nature, a scientific dispute erupted between the describers (Brunet 2002, Guy et al. 2005) and other proponents of a hominin attribution (Begun 2004), on the one side, and critics on the other side (Wolpoff et al. 2002, 2006, Caspari 2002, Wood 2002, Andrews & Harrison 2005, Pickford 2005, Wood & Harrison 2011). These critics strongly doubted the evidence for bipedalism and considered the cranium as belonging, instead, to an extinct ape, most likely a female gorilla. Dr. Brigitte Senut of the Natural History Museum in Paris and discoverer of the “millenium man” Orrorin also supports this identification as female gorilla (BBC 2002). Furthermore, the critics “concluded that the cranium did not sit atop the spine but in front of it, indicating the creature walked on all fours like an ape” and is “not a hominid at all” (Bailey 2006). Various critical authors also remarked that Sahelanthropus is too old to belong in the human lineage, based on molecular clock estimates of the chimp-human split.

A Weak Case

Like several other phylogenetic studies (e.g., Strait & Grine 2004, Hawks 2019, Mongle et al. 2019, Parins-Fukuchi et al. 2019, Martin et al. 2021), a recent cladistic analysis by Parins-Fukuchi et al. (2019) recovered Sahelanthropus as ancestral to later hominins, but the authors admitted that “this result should also be treated cautiously due to the small number of characters recovered for analysis of this portion of the phylogeny” and “we are currently cautious about making strong statements concerning the ancestral position of Sahelanthropus.” Mongle et al. (2019) explicitly emphasized the weak case for Sahelanthropus as hominin. Basically, there were only two characters that allegedly support an attribution of Sahelanthropus to the human lineage.

One is the reduced size of the canine teeth combined with a reduced canine-premolar honing, and the other is the position of the foramen magnum as evidence for an upright posture and bipedal locomotion. However, there are several problems with these characters. Canines are also of reduced size in female apes and some Miocene Eurasian apes (e.g., Oreopithecus, Ouranopithecus, and Giganthopithecus), which have independently evolved reduced canines with reduced canine-premolar honing (Wolpoff et al. 2002, 2006, Andrews & Harrison 2005). Therefore, Wood & Harrison (2011) remarked that “these changes are in fact not unique to hominins and it is conceivable that similar evolutionary responses could have occurred in contemporary African hominids, not just in the hominin lineage.” Concerning the foramen magnum there are three problems. Firstly, the reconstruction of the position of the foramen magnum is highly disputed (Wolpoff et al. 2002, 2006, Andrews & Harrison 2005, Pickford 2005). Wolpoff et al. (2006) rather considered the condition of Sahelanthropus as characteristic for quadrupedal apes.

Secondly, even a hominin-like position is not exclusive to hominins. Wood & Harrison (2011) clarified that

comparisons with other primates, especially gibbons and short-faced monkeys, suggest that this feature is more broadly associated with differences in head carriage and facial length, rather than uniquely with bipedalism. The distinction between bonobos and chimpanzees in this respect, and the overlap between the morphology of bonobos and that of Sahelanthropus and Ardipithecus further support this contention.

Finally, there is more recent evidence that an upright posture and bipedality may have originated independently or earlier in Miocene apes like Danuvius, Graecopithecus, and Rudapithecus (Gierliński et al. 2017, Barras 2019, Böhme et al. 2019, Kivell 2019, University of Missouri 2019, Ward et al. 2019; also see Bechly 2017), so that it would no longer represent a uniquely diagnostic trait of humans.

Sometimes, the low degree of subnasal prognathism (flat face) of Sahelanthropus has also been considered as a human-like character, but this feature is much too early in Sahelanthropus and absent in most of the other early hominins. Cobb (2008) concluded that “it is not possible to determine with any confidence whether the facial morphology of any of the current candidate LCA taxa (Ardipithecus kadabba, Ardipithecus ramidus, Orrorin tugenensis and Sahelanthropus tchadensis) is representative of the LCA, or a stem hominin, or a stem panin or, in some cases, a hominid predating the emergence of the hominin lineage.”

Based on the unpublished PhD thesis of Thibaut Bienvenu (2010), in 2013 the first endocast of Sahelanthropus‘s braincase was introduced in two congress abstracts by Bienvenu et al. (2013a, 2013b). The authors claimed that this endocast reveals “hominin brain shape characteristics, including strongly posteriorly projecting occipital lobes, a tilted brainstem and a laterally expanded prefrontal cortex,” even though the endocranial volume of only 378 ml is comparable to modern chimps (Hawks 2013, Wong 2013, Neubauer 2014). Bower (2013) commented that “an upright posture and two-legged gait stimulated neural reworking in Sahelanthropus, Bienvenu speculated, even though the hominid’s brain was about one-quarter the size of people’s brains today.” That seems like a quite bold speculation considering the dubious bipedal nature of Sahelanthropus. A bigger problem is the fact that this important evidence has not been properly published in the peer-reviewed scientific literature in nine years since the official announcement. This is unacceptable and simply means that this evidence can hardly be seriously considered for the time being.

A DIstinct Genus?

Another controversy concerns the question of whether Sahelanthropus is a distinct genus at all. Haile-Selassie et al. (2004) compared the teeth of early hominins like Sahelanthropus, Orrorin, and Ardipithecus, and suggested that their differences all fall within the expected variation in the same genus Ardipithecus. White et al. (2009a, 2009b) and White (2014) agreed, and Suwa et al. (2009) found further similarities in the skull anatomy of Ardipithecus and Sahelanthropus, which “differed substantially from those of both extant apes and Australopithecus.” Nevertheless, Hawks (2014) rejected a synonymy because he thinks that their proponents have not presented sufficient evidence. Other studies also ignored this alleged synonymy.

As an interesting side note, a few years ago an evolutionist friend of mine pointed out (personal communication by Dr. Erich Weber, University Tübingen) that within a Darwinian paradigm the “Sahelanthropus as gorilla-ancestor hypothesis” would also fit nicely with well-established African biogeographical patterns, as suggested by the zoologist Jonathan Kingdon, because the origin of the gorilla versus chimp plus Homo lineages would align well with a North versus South splitting event, while the origin of the chimp and Homo lineages would follow a West (rainforest) versus East (savanna) pattern. I have never encountered this argument in the published literature on Sahelanthropus, maybe because few paleoanthropologists are well versed in African mammal biogeography. This relates to another controversial issue (Domínguez-Rodrigo 2014, Hawks 2014, King 2014), which is the fact that Sahelanthropus seemed to have lived in woodlands. This has been claimed to have falsified (Brunet 2010) the common “savannah hypothesis” of human origins, which proposed the East African savannah as the cradle of mankind.

A still little-known fact is that a hominid mandible (Brunet et al. 2005) and partial left femur (TM 266-01-063) were found closely associated with the cranium of Sahelanthropus on the day of its discovery. Strangely, in their paper offering further details about fossils attributed to Sahelanthropus, Beauvilain and Le Guellec (2004) did not mention this femur. Beauvilain and Watté (2009) mentioned the mandible and femur, and featured overview photos of the findings at the site, but provided no detailed description, photos, or drawings of the femur. Otherwise, the femur was only mentioned in a blog post by paleoanthropologist John Hawks (2009b) and featured on an anonymous website about Sahelanthropus (Anonymous). When Hawks inquired with the original describer of Sahelanthropus, Professor Michel Brunet, he received the following remarkable reply: “In Chad, we have uncovered thousands of bones, which are in the process of study. Perhaps among them are hominid bones, but I only comment on those that have been published in a scientific review.”

Almost a Crime Story

This is plainly false, because in fact only a few dozen fossils were uncovered together with the cranium of Sahelanthropus, as is clearly visible in the photos from the discovery site (Hawks 2009b). Hawks commented that the femur bone lay unrecognized for three years in the Toros-Menalla faunal collection at the University of Poitiers, but was recognized as a hominid femur in 2004 by Aude Bergeret-Medina and her tutor Roberto Macchiarelli. Later they were unable to find the femur again, so that they drafted a study based on their measurements and photos. What happened then is almost a crime story.

Callaway (2018) reported in Nature, in the words of Hawks (2018), that “two scientists, Roberto Macchiarelli and Aude Bergeret, attempted to present a talk describing this femur at the annual meeting of the Societé d’Anthropologie de Paris this month. The society incredibly rejected their abstract, which has triggered some professional criticism.” Callaway quoted paleoanthropologist Bill Jungers at Stony Brook University as saying that the description of the femur is “long overdue” and “We don’t know why it’s been kept secret. Maybe it’s not even a hominin.” On his blog, John Hawks (2018) spelled out the significance:

If a femoral specimen was found with the Sahelanthropus cranial remains, it is important evidence. If the femur preserves anatomy that would test the hypothesis of upright posture or bipedal locomotion, no one should pretend that the evidence does not exist. Secrecy was ridiculous from the start, now it is inexcusable.

Hawks (2018) also said: “What amazes me is just how long this has all gone on. All of the critics could be silenced within hours by data and evidence. Instead, silence about these key fossils has reigned for fifteen years.” Hawks mentioned that David Pilbeam, who was a co-author of the original description, had already seen the femur a decade ago. Hawks found this “very troubling” and calls for “Nature‘s editors to investigate.”

Indeed, I think the reasons for this secrecy were pretty obvious, and I personally consider this case a genuine scientific scandal (see Constans 2018 for a history of this scandal around the femur of Toumaï). Of course, the discovery of the oldest fossil hominin makes for better publicity and yields higher funding than the discovery of just another fossil ape. The association of the skull with a hominid femur that does not seem to be hominin, but, instead, more similar to the femur of a quadrupedal ape (Macchiarelli and Bergeret quoted in Callaway 2018 and Hawks 2018), was certainly inconvenient. Scientists are only human, though, and what moves them may not always be a purely unbiased pursuit of the truth. In an idolized discipline such as paleoanthropology, a great booster of scientific careers and fame, the incentive to see the evidence as one wishes may be even greater than in other, less celebrated fields. Indeed, Michel Brunet, the describer of Sahelanthropus, became famous in France and even had a street named after him in Poitiers.

More Strange Obstacles

Anyway, after many more strange obstacles, the study of the femur was finally published two years ago (Macchiarelli et al. 2020). This study indeed came to the conclusion that Sahelanthropus was not bipedal and likely not a hominin but rather “could belong to a group that has no living representative” (Marshall 2020, Yirka 2020; also see Luskin 2021). The authors also cautioned that “if we treat the hominin status of S. tchadensis, or any other enigmatic taxon, as a given and not a hypothesis, we run the risk of adding further confusion to a picture that is already “complicated and less easy to resolve.” Yirka (2020) made the implications of this study even more clear and emphasized that the evidence “suggests Sahelanthropus tchadensis was not a hominin, and thus was not the earliest known human ancestor.” Dr. Madelaine Böhme from the University of Tübingen remarked that “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” (Böhme quoted in Marshall 2020). Just two years later, a new study by Daver et al. (2022) totally disagreed and, on the basis of exactly the same femur bone, came to the opposite conclusion that Sahelanthropus was a habitual bipedal hominin, but also found evidence that its arms were adapted for arboreal climbing (CNRS 2022). How convenient, now Sahelanthropus could be celebrated again as our oldest ancestor. Honi soit qui mal y pense. Of course, this most recent study by Daver et al. (2022) was immediately overhyped in the media reports (Burakoff 2022, Callaway 2022, Handwerk 2022, Lazaro 2022, Liebermann 2022, Pare & Davis 2022, Wilson 2022) and also Wikipedia (2022) uncritically accepted the new study as a refutation of the previous study by Machiarelli and colleagues.

However, this is not the case at all (Schlott 2022). Liebermann (2022) admitted in his expert analysis, published in the journal Nature, that “the Sahelanthropus femur doesn’t have ‘smoking-gun’ traces of bipedalism.” Bernard Wood commented that Daver et al. “cherry-pick what they think is information which is consistent with the femur shaft being a biped, and they studiously ignore information to the contrary.” His colleague Roberto Macchiarelli still “thinks Sahelanthropus is more likely to be an ape than a hominin” (Callaway 2022), also because the angle the femur makes with the pelvis would be “mechanically unstable for a vertical stance” (Macchiarrelli quoted in Wilson 2022). A further important reason why the conclusion in favor of bipedality by Daver et al. (2022) has to be considered as very poorly founded is another new study by Cazenave et al. (2022), who unequivocally proved that the crucial character of the calcar femorale cannot be considered as diagnostic for bipedality (also see Schlott 2022).

John Hawks commented that “the two teams who have collected data from the femur seem to disagree entirely about what the femur shows … They’re looking at the same piece of bone. I don’t understand how they disagree about this. If either group could just release (surface 3-D and internal CT scan) data so that we can all examine it, there would be no reason for this disagreement” (Hawks quoted in Handwerk 2022). If even one of the world’s leading paleoanthropologists cannot decide, who is right in this controversy, then we can safely conclude that the question is still unresolved. Overhyping and overinterpretation of dubious and highly controversial evidence is commonplace in paleoanthropology. If nothing else, the case of Sahelanthropus at least shows that in evaluating alleged proofs of human evolution, a healthy dose of skepticism is called for.

Author’s note: This article is a significantly expanded and updated version of a previous article of mine at Evolution News (Bechly 2018).

References