Ana Reif
A new morphological analysis suggests Sahelanthropus tchadensis may have been bipedal after all.
Sahelanthropus tchadensis is the name given to a 7-million-year-old fossil primate species discovered in Chad in 2001. It has garnered great interest due to its temporal proximity to the most recent common ancestor between modern humans and chimpanzees. Initially, one nearly complete cranium and a fragmentary lower jaw were attributed to the taxon. It was judged to be relatively chimp-like, though it had several features exhibited by later hominins that are absent in extant great apes or earlier ape ancestors (Read more: https://www.nature.com/articles/nature00879). Despite the lack of postcranial remains at the time, Brunet et al. (2002) suggested S. tchadensis could have been partially bipedal because of its relative facial and basicranial resemblance to later, habitually bipedal hominins. This assessment remains controversial, and the ape’s tentative bipedalism has become closely tied to its status as the oldest known hominin (ancestor of humans, but not other apes).
TM 266-01-060-1, the Sahelanthropus tchadensis holotype (source: humanorigins.si.edu)
Overview: 20 years of research
Ahern (2005) analyzed the position of the foramen magnum as an indicator of bipedalism. In apes, an anteriorly placed foramen magnum is known to be associated with bipedalism: humans are bipedal and have a foramen magnum almost in the exact middle of the basicranium, where non-bipedal apes have a more posteriorly placed foramen magnum. Ahern (2005) concluded that the position of the foramen magnum on the skull, now called TM 266-01-060-1, was not different enough from extant great apes to suggest bipedalism on its own, casting doubt on Brunet et al. (2002)’s opinions, though not ruling out bipedalism altogether– not much more could be done without postcranial fossils.
In 2004, a partial femur (TM 266-01-063) found at the same time and location as the skull of S. tchadensis, previously unidentified, was reevaluated and determined to have come from a primate. Because of the lack of evidence of other large-bodied primate species in the same geotemporal location, Machiarelli et al. (2020) have since attributed the femur to S. tchadensis. They then compared several measurements between TM 266-01-063 and femurs from modern humans, chimpanzees, and several known bipedal hominins.
Source: Daver et al. (2022)
Cross-sections of the Sahelanthropus femur compared to other apes’ femurs
Source: Machiarelli et al. (2020)
Unfortunately, TM 266-01-063 is incomplete, and some of the regions most helpful for determining functional morphology are missing. As a result, Machiarelli et al. (2020) concluded that the femur lacks significant evidence of bipedalism, taking Sahelanthropus out of the running for earliest bipedal ape, and insinuating that it may not be a direct ancestor of Homo.
Most recent developments
Daver et al. (2022) recently submitted an analysis of Sahelanthropus’s postcranial remains that contradicts Machiarelli et al. (2020). They judged that the femur’s cross-sections and cortical bone distributions look more like modern humans’ than extant nonhuman apes, and that the presence of the calcar femorale, a feature of the hip joint, indicates the bone would have been able to distribute the heavy loads associated with walking upright. Additionally, Daver et al. (2022) determined that features associated with enhanced hip flexion– including a lateral gluteal tuberosity and a defined protea linea aspera– also would have enabled Sahelanthropus to walk upright. While these adaptations for hip flexion are not necessarily derived, they are consistent with the overall functional morphology of habitual bipeds.
They also asserted that two ulnae discovered at the same site can be safely attributed to Sahelanthropus, and that these ulnae are clearly adapted for arboreal locomotion. As a result, they conclude that Sahelanthropus represents a snapshot of hominin evolution right after the split between human and chimpanzee lineages because it has a combination of traits of both late Miocene apes and early, partially bipedal hominins.
Accepting this fossil as a hominin may mean that the earliest hominin evolution occurred over a greater geographical range than previously thought, as the Chad site is much farther West than other sites yielding early hominins. Additionally, it would push the split between human and chimpanzee lineages significantly earlier than some molecular analyses have indicated (Brunet et al. 2002).
Source: humanorigins.si.org
References:
Ahern, J. C. M. (2005). Foramen magnum position variation In Pan troglodytes, Plio-Pleistocene hominids, and Recent Homo sapiens: Implications for recognizing the earliest hominids. American Journal of Physical Anthropology, 127(3), 267–276. https://doi.org/10.1002/ajpa.20082
Brunet, M., Guy, F., Pilbeam, D. et al. (2002). A new hominid from the Upper Miocene of Chad, Central Africa. Nature 418, 145–151. https://doi.org/10.1038/nature00879
Daver, G., Guy, F., Mackaye, H.T. et al. (2022) Postcranial evidence of late Miocene hominin bipedalism in Chad. Nature 609, 94–100. https://doi.org/10.1038/s41586-022-04901-z
Macchiarelli, R., Bergeret-Medina, A., Marchi, D., & Wood, B. (2020). Nature and relationships of Sahelanthropus tchadensis. Journal of Human Evolution, 149, 102898. https://doi.org/10.1016/j.jhevol.2020.102898