Qs people asked me on science and my replies to them: Part 274: Our ape ancestor

Q: People say we have descended from apes. Which one?

Krishna: This is thought to be our ape ancestor: Sahelanthropus tchadensis.

It is one of the oldest known species in the human family tree. This species lived sometime between 7 and 6 million years ago in West-Central Africa (Chad).

This is is an extinct species of the Homininae (African apes) dated to about 7 million years ago, during the Miocene epoch. The species, and its genus Sahelanthropus, was announced in 2002, based mainly on a partial cranium, nicknamed Toumaï, discovered in northern Chad.

Sahelanthropus tchadensis lived close to the time of the chimpanzee–human divergence, possibly related to Orrorin, a species of Homininae that lived about one million years later. It may have been ancestral to both humans and chimpanzees (which would place it in the tribe Hominini), or alternatively an early member of the tribe Gorillini. In 2020, the femur was analyzed, and it was found that Sahelanthropus was not bipedal, casting some doubt on its position as a human ancestor, but this was refuted in 2022 (1).

It looked somewhat like this:

Cast of the holotype Sahelanthropus tchadensis cranium TM 266-01-060-1, dubbed Toumai, in facio-lateral view.

Some of the oldest evidence of a humanlike species moving about in an upright position comes from Sahelanthropus. The foramen magnum (the large opening where the spinal cord exits out of the cranium from the brain) is located further forward (on the underside of the cranium) than in apes or any other primate except humans. This feature indicates that the head of Sahelanthropus was held on an upright body, probably associated with walking on two legs.

In addition to the cranium, nicknamed Toumaï, and fragments of jaws and teeth that have already been published, the locality of Toros-Menalla 266 (TM 266) yielded two ulnae (forearm bone) and a femur (thigh bone). These bones were also attributed to Sahelanthropus because no other large primate was found at the site; however, it is impossible to know if they belong to the same individual as the cranium. 

The acquisition of bipedalism is considered to be a decisive step in human evolution. Nevertheless, there is no consensus on its modalities and age, notably due to the lack of fossil remains. A research team, involving researchers from the CNRS, the University of Poitiers and their Chadian partners, examined three limb bones from the oldest human representative currently identified, Sahelanthropus tchadensis. Published in Nature on August 24, 2022 (3), this study reinforces the idea of bipedalism being acquired very early in our history, at a time still associated with the ability to move on four limbs in trees (2).

The description of three limb bones of Sahelanthropus confirms habitual bipedalism, but not exclusively.

The first early humans, or hominins, diverged from apes sometime between 6 and 7 million years ago in Africa. Sahelanthropus tchadensis has two defining human anatomical traits: 1) small canine teeth, and 2) walking upright on two legs instead of on four legs.

We don’t know everything about our early ancestors—but we keep learning more! Paleoanthropologists are constantly in the field, excavating new areas with groundbreaking technology, and continually filling in some of the gaps about our understanding of human evolution.

At 7 million years old, Sahelanthropus tchadensis is considered the oldest representative species of humanity.

Until something else is found, this one remains to be our ape ancestor! 

Footnotes:

1. Daver, G.; Guy, F.; Mackaye, H. T.; Likius, A.; Boisserie, J. -R.; Moussa, A.; Pallas, L.; Vignaud, P.; Clarisse, N. D. (2022-08-24). "Postcranial evidence of late Miocene hominin bipedalism in Chad" (PDF). Nature. Springer Science and Business Media LLC. 609 (7925): 94–100. doi:10.1038/s41586-022-04901-z. ISSN 0028-0836. PMID 36002567

2. https://www.sciencedaily.com/releases/2022/08/220824120832.htm#:~:t....

3. Two articles: a) A new hominid from the Upper Miocene of Chad, Central Africa, Michel Brunet et al., Nature, 11 July 2002. DOI: 10.1038/nature00879. b) Virtual cranial reconstruction of Sahelanthropus tchadensis, Christoph P.E. Zollikofer et al., Nature, 7 April 2005. DOI: 10.1038/nature03397