What about bipedalism in Sahelanthropus?

No evidence in Sahelanthropus postcranial remains supporting habitual bipedalism

Researchers led by Marine Cazenave from the Max Planck Institute for evolutionary Anthropology in Leipzig, Germany, re-examined the fossil evidence for possible habitual bipedalism of Sahelanthropus who lived about seven million years ago. The new analysis demonstrate that both the external morphology and the endostructural signal are not consistent with habitual bipedalism.

The seven million year old Sahelanthropus tchadensis was discovered in 2001 in Chad and announced in 2002 as a habitual bipedal hominin species based on an adult distorted cranium. Thus, any additional information from postcranial elements is essential to clarify its nature and taxonomic status.

In 2020, researchers described a partial femur from the same fossiliferous spot and suggested that its morphology was inconsistent with habitual bipedalism. In 2022, researchers conducted a more in-depth study of the specimen and also considered two previously unreported partial ulnae. They suggested that the femur exhibits several hallmarks of selection for bipedalism. An international collaboration of researchers re-examined these findings in a new study including now several Miocene ape remains.

No evidence of habitual bipedalism

The team found no features of the S. tchadensis partial femur that are exclusively seen in the habitual terrestrial bipeds, such as the australopiths and modern humans. As indicated by Marine Cazenave, “some attributes of the partial femur are even not present at all in hominins but observed in non-hominin hominoids or even non-primates”. “There are even some anatomical similarities with the carnivorans” adds Nikolai Spassov, Professor at the Department of Paleontology and Mineralogy of the National Museum of Natural History Bulgarian Academy of Sciences who co-authored the study.

The authors considered additional features not investigated so far, notably limb proportions, that are compatible with the typical condition of the African great apes but depart from that of habitual bipeds. The researchers conclude that reconstructing the locomotor behavior of S. tchadensis from postcranial remains should take into account all functionally-related morphological features rather than only counting similarities with hominins.

Implications for the taxonomic status of Sahelanthropus

The debate has called into question its status as a putative hominin. How S. tchadensis actually moved is still unknown. But what if Sahelanthropus was a another African Late Miocene fossil ape?