This post summarizes the recent paper by R. Macchiarelli and colleagues, on the study of the partial left femur TM 266-01-063 found in July 2001 at Toros-Menalla, Chad. This is the same location where the Sahelanthropus tchadensis holotype was recovered also in 2001: the cranium TM 266-01-060-1.
In many texts, when depicting the human evolutionary tree, the S. tchadensis cranium is often claimed to be consistent with being biped, mainly driven by the foramen magnum position. Habitual bipedalism is a key feature for taxa to be included in the hominin clade, yet some specific characteristics of the bipedal locomotion can differ from those of modern humans. The case of Sahelanthropus is important because the age of its fossils (6-7 million years) approximately matches the time that our branch of the primate family tree diverged from the ancestors of chimpanzees and gorillas. We only know Orrorin tugenensis as another hominin candidate in such chronology. There are three femoral remains of Orrorin, but until now the Sahelanthropus femur had not yet been published, and this is a major skeletal element to understand bipedalism.
What hominin taxa at that time could be a direct ancestor of living humans is a very difficult question to face. The Late Miocene fossil record is really small and makes impossible to sort ancestors from non-ancestral close relatives. In this context, what five key ideas does the new femur bring, according to Macchiarelli et al?
1) Yes, the femur can likely be assigned to S. tchadensis.
We are most confident that the TM 266 femoral shaft belongs to a hominid sensu lato. It could sample a hominid hitherto unrepresented at Toros-Menalla, but a more parsimonious working hypothesis is that it belongs to S. tchadensis.
2) This femur is very different from the Orrorin tugenensis femur, another early hominin normally considered as habitual biped.
The differences between TM 266 and the O. tugenensis partial femur BAR 1002’00 are substantial and are consistent with maintaining at least a species level distinction between S. tchadensis and O. tugenensis.
3) But actually, S. tchadensis may not have been a habitual biped!
If the TM 266 femoral shaft belongs to S. tchadensis, we cannot be confident that the latter was a habitual biped. 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.
4) If the TM 266 femur can be added to the hypodigm of S. tchadensis, the conclusions could be important to actually stop considering S. tchadensis as an early hominin.
The lack of clear evidence that the TM 266 femur is from a hominid that was habitually bipedal further weakens the already weak case for S. tchadensis being a stem hominin.
It is possible that S. tchadensis is a stem hominin with some reduction of the canine and loss of the honing complex, but without the femoral adaptations to terrestrial bipedalism that are seen in A. afarensis and O. tugenensis. 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.
5) A hominin, a panin, or neither? A potential third way for Sahelanthropus.
Being a stem hominin or a stem panin, or their most recent common ancestor, may not be the only options for S. tchadensis. It is probable that during the late Miocene and the early Pliocene, there was a modest adaptive radiation of African hominids that includes taxa that are neither hominins nor panins as defined previously. Any such extinct groups are likely to include taxa with novel morphology or with novel combinations of morphology we also see in hominins or panins. Given the mix of inferred primitive and inferred derived features in S. tchadensis, we suggest it could belong to a group that has no living representative.
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.
Reference: R. Macchiarelli, A. Bergeret-Medina, D. Marchi et al. (2020). Nature and relationships of Sahelanthropus tchadensis. Journal of Human Evolution 149 | Front image: The partial femur TM 266-01-063 (left) in anterior (a), posterior (b), medial (c), and lateral (d) views compared with the CT-based reconstruction of BAR 1002’00 (Puymerail, 2011, 2017, based on a record kindly made available by B. Senut and M. Pickford).
Further information: Toumaï, esperanza de vida | Nutcracker Man