My Cousin Sahelanthropus: How the discovery of Sahelanthropus tchadensis established a lineage

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I just never actually “seen” a grit before.

Toumai is our first cousin Vinney. Discovered in 2001 by French paleontologist Michel Brunet and his team of four at Toros-Menalla in Chad, West Africa, Toumai dates around 7 million years (MYA), making him the oldest possible hominin ancestor to date. The discovery of Toumai (TM 226) led to the establishment of a new species known as Sahelanthropus tchadensis. Unfortunately, as is often the case with extremely old material, limited fossil remains are found. This, combined with the fact that many early hominids show both chimpanzee and human traits, complicates any assessment done on a specimen, often resulting in disagreements between researchers. Like many others, S. tchadensis falls victim to these restraints. Since its discovery in 2001, only nine cranial specimens, one of which is Toumai, have been recovered. Due to the limited nature of the assemblage, many researchers are wary of classifying directly under the hominin lineage. Although limited, strong evidence for hominin traits can be seen, resulting in S. tchadensis being placed as a possible hominin within the lineage.

Generally speaking, the cranium of Sahelanthropus is orthognathic in the facial region with a small ape-size braincase (320-380cc), and a long and narrow basicranium (Bruent et al. 2002 & Zollikofer et al. 2004). Additionally, evidence for a thick and continuous supraorbital torus, a small, posteriorly located sagittal crest and large nuchal crest (in presumed males), a large mastoid process can be seen. When observing the craniodental morphology of Sahelanthropus, evidence shows a narrow U-shaped dental arch, relatively small incisors, mesiodistally longer upper canines with abundant apical wear, and no lower diastema between the canine and first premolar. Also, molars show low rounded cusps with bulbous lingual faces with an enamel thickness that is intermediate between chimpanzees and later Australopithecus (Bruent et al. 2005).

S. tchadensis also shows a more anteriorly placed foramen magnum suggesting bipedalism, a trait that is considered to be the prime indicator of a hominin. However, due to the limited amount of fossil material and a lack of postcranial remains, the bipedality of S. tchadensis has been hotly debated. Additionally, a very large supraorbital torus (brow ridge) and a more orthognathic face is noted on Toumai. This is unexpected since these features do not appear until the establishment of the genus Homo at around 2MYA. While all these traits have helped to place S. tchadensis on the hominin lineage, other more chimpanzee-like traits have helped remove S. tchadensis from the lineage. Traits such as an enlarged premolar complex, thin enamel on the cheek teeth, a premolar 3 not resembling at premolar 4 and a small cranial capacity (about 360ml) have pushed S. tchadensis away from the hominin lineage.

to be human or to be chimp, what will be folks?

Chimpanzees are our closest cousins within the Primate Order. In fact, we shared a common ancestor for millions of years before eventually splitting into two separate families and developing our own derived traits. These differing traits have allowed us and our chimp cousins to each best exploit the environments in which we have resided for millions of years. For chimpanzees, the traits are often associated with more arboreal forested environments, whereas for us, these traits are better adapted to more open arid environments. By linking these traits with geological and paleoecological studies, researchers can approximate where the split between chimps and us occurred. It has been estimated that the split occurred around 8-6 MYA, thus, it is not surprising to see both human and chimpanzee traits within S. tchadensis. Since S. tchadensis falls close to the initial split, and since the process of adaptation can take millions of years, it is expected to see both human and chimpanzee traits. So, while Toumai never bailed us out of a crime, he did expand our understanding of the split between our chimp relatives and us.

 

Citations:

  1. Brunet, M., Guy, F., Pilbeam, D. et al. 2002. A new hominid from the Upper Miocene of Chad, Central Africa. Nature. Vol. 418: pp 145-151
  2. Wood, B. 2002. Hominid revelations from Chad. Nature. Vol. 418: pp 133-135
  3. Zollikofer, P. E. C., Ponce de Leon, S. M., Lieberman E. D. et al. 2004. Virtual cranial reconstructions of Sahelanthropus tchadensis. Nature. Vol 434: pp 755-759
  4. Brunet, M., Guy, F., Pilbeam, D. et al. 2004. New material of the earliest hominid from the Upper Miocene of Chad. Nature. Vol. 434: pp 752-755
  5. Brunet M., Guy F., Pilbeam D., Mackaye H. T. et al. 2002. A New Hominid from the Upper Miocene of Chad, Central Africa. Nature. Vol418: pp 145-151
  6. Bruent M., Guy F., Pilbean D., Lieberman D. et al. 2005. New Material of the Earliest Hominid from the Upper Miocene of Chad. Nature. Vol. 434: pp 752-755
  7. Wood B. 2002. Hominid Revelations from Chad. Nature. Vol.418: pp 133-135
  8. Zollikofer C., Ponce de Leon M., Liberman D., Pilbeam D. et al. 2004. Virtual Cranial Reconstruction of Sahelanthropus tchadensis. Nature. Vol. 434: pp 755-759

 

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