During the twentieth century palaeoanthropologists searching for the origin of the Hominins became convinced that the group has an East African origin. However, in 2001 scientists from the working on Upper Miocene deposits in the Toros Menalla region of Chad uncovered a group of possible Hominin fossils which they assigned to a new species, Sahelanthropus tchadensis, and which included a nearly complete, is distorted, cranium as well as a mandible and some isolated teeth.
If correctly interpreted as a Hominin, then Sahelanthropus tchadensis would be the oldest known member of the group. As such the discovery has been subjected to a considerable amount of scrutiny, particularly as the original specimens had been subject to considerable taphonomic (post-mortem) alteration. One of the problems with the original material was that it was had to interpret the position of the foramen magnum (the whole in the skull through which the brain is connected to the central nervous system) because of distortion of the skull. This is important, because in (upright) Hominins it is located roughly in the centre of the bottom of the skull, while in Apes it is typically towards the back of the skull. A virtual reconstruction of the cranium of Sahelanthropus tchadensis has suggested that the foramen magnum would have been on the base.
Sahelanthropus tchadensis also has a shorter, more vertical face, reduced canines and a related lack of a honing complex (a gap between the lower canine and the first premolar, into which the upper canine fits, allowing the two canines to rub together and sharpen, or 'hone' one-another), and a downward facing lip on the nuchal crest at the rear of the skull, all of which are Hominin traits. However, it also has a small, Ape-like neurocranium, as well as having a size and number of tooth-roots consistent with an Ape, presenting a mosaic of features consistant with an very early Hominin, or possibly an Ape closely related to the earliest Hominins.
Different analyses of a femur and two ulnae found at the same location as the cranium have suggested both that Sahelanthropus tchadensis was either habitually bipedal, or not habitually bipedal, and that it probably spent at least some of its time in the trees. Since a cladistic analysis has recovered Sahelanthropus tchadensis as a Hominin, this raises at least the possibility that Hominins were habitually walking on two legs 7 million years ago.
One aspect of Sahelanthropus tchadensis which has not been studied extensively is its dentition, despite a number of teeth and tooth fragments being available. In a paper published in the South African Journal of Science on 31 July 2024, Walter Neves, Leticia Valota, and Clovis Monteiro of the Institute of Advanced Studies at the University of São Paulo, present the results of a morphometric analysis which compared the upper posterior dentition of Sahelanthropus tchadensis to that of living Apes and fossil Plio-Pleistocene Hominins.
The upper posterior teeth were chosen as these were the teeth for which the mesiodistal and buccolingual diameters could be determined in Sahelanthropus tchadensis. These were compared to a selection of teeth from Pan troglodytes (Chimpanzees), as well as the Hominins Orrorin tugenensis, Ardipithecus ramidus, Australopithecus afarensis, Australopithecus africanus, Paranthropus boisei, Paranthropus robustus, Homo habilis, and Homo erectus.
For each tooth the ratio between the mesiodistal and buccolingual diameters (length and width) was calculated, and these were plotted on a distribution map. The teeth fell into approximately three groups on this map, one comprising modern Chimpanzees, one comprising the 'Robust Autralopithecines' Paranthropus boisei and Paranthropus robustus, and one containing all other Hominins.
The teeth of Sahelanthropus tchadensis fell within the non-Robust Hominin cluster, plotting closest to Ardipithecus ramidus, a Miocene-Pliocene species with a large number of known specimens, which again has some Ape-like characteristics, but which is generally accepted as a Hominin by palaeoanthropologists today. Neves et al. interpret this as supporting the hypothesis that Sahelanthropus tchadensis is a Hominin rather than an Ape.
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