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Sahelanthropus Tchadensis: an Examination of Its Hominin Affinities

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Sahelanthropus Tchadensis: an Examination of Its Hominin Affinities nature of bipedalism is a holdover from a gradistic approach to phylogeny. Future discoveries of infra-cranial material to determine locomotor abilities are of utmost importance. If S. tchadensis is shown to be a biped, then bipedalism arose soon after the divergence point, and much earlier than anticipated. Some caution should be taken, as the mosaic features of S. tchadensis could be evidence of a complicated evolutionary history during the supposed divergence period. Genetic evidence now points to a period of hybridization, which would make sorting out the species affiliation of purported hominin fossils problematic. For the present, it appears that S. tchadensis can be classified as the first known hominin. Sahelanthropus tchadensis: AN Even if future finds cause a re-evaluation of EXAMINATION OF ITS HOMININ its phylogenetic status, S. tchadensis is a AFFINITIES AND POSSIBLE valuable addition to the fossil record. PHYLOGENETIC PLACEMENT Knowledge of early panin morphology is also essential to phylogenetic relationships of early hominins. Abstract WHAT IS A HOMININ? An examination of the Sahelanthropus tchadensis is a Sahelanthropus tchadensis material supports candidate for the first known hominin.1 Its its putative hominin status. The cranial age, at nearly seven million-years-old morphology of S. tchadensis exhibits a (Vignaud et al. 2002: 155), places it very mosaic of primitive and derived close to the supposed divergence point of characteristics. This is to be expected from a the chimpanzee and human lineages (Wood species so near to the assumed divergence 2002: 133). A hominin is a member of the point of the chimpanzee and human Tribe Hominini, which include any species lineages. However, the natures of the that are found upon the clade that ultimately derived characteristics align it closer to the lead to Homo sapiens (Wood 2005: 23). hominin clade than to the panin clade. While Similarly panins are members of the Tribe it cannot be shown conclusively that S. tchadensis was an obligate biped, evidence from a recent virtual-reconstruction is Note that all the literature cited still uses the indicative of an upright posture. However, term hominid, which is reflective of a more the status of the earliest hominin should not traditional taxonomy. For the purpose of rest entirely on the presence of an upright this paper the meaning are equivalent, as form of locomotion. It should not be Brunet et aI., refer to a hominid as "any expected that the first members of the member of that group more closely related hominin clade would necessarily have a to extant humans than to the extant completely bipedal mode of locomotion. chimpanzees," (2002: 753). Attributing hominin status solely on the TOTEM: vo1.l6 2007-2008 Panini, which include any species that are of Chororapithecus abyssinicus, so far found upon the clade that leads to Pan represented only by nine individual teeth, troglodytes (Wood 2005: 23). This there were no known ape fossils from this represents current taxonomical thinking, time period (Suwa et al. 2007: 921). To which stresses cladistic taxonomy, or sorting ascertain the appearance of the first out evolutionary relationships based on hominin, it will be necessary to know the derived characteristics. Unfortunately little morphology of early members of the panin is currently known about Upper Miocene lineage as well. This will allow the apes in general, which are ancestral to these determination of which traits are primitive two lineages (Suwa et al. 2007:921).This Upper Miocene ape characteristics, versus compounds the problem of identifying the which are derived hominin characteristics. It earliest hominin in the fossil record. As the must be remembered that the last common divergence point is approached, it becomes ancestor of the hominin and panin lineages increasingly more difficult to identify to would not resemble an extant chimpanzee. which of these clades a fossil should be Using extant chimpanzees to sort out assigned (Fleagle 2000: 96). It may be primitive versus derived traits is ultimately impossible to ever identify the problematic, as chimpanzees have been last common ancestor of the chimpanzee and evolving ever since the divergence point. human lineages with any certainty. With the problems of homoplasies and Evolution is mosaic in nature. Traits reversals in evolution, fossils of early panins do not necessarily evolve at the same rate in are sorely needed. a lineage. Among the hominins, the evolution of bipedalism preceded brain Sahelanthropus tchadensis CRANIAL encephalization by at least two million MORPHOLOGY years, and given recent possible hominin The holotype for S. tchadensis is TM discoveries such as S. tchadensis, up to five 266-01-060-1, a 95% complete million years. One must thus be cautious in cranium(Brunet et al. 2002a: 146). TM 266 attributing hominin status based on single is the Toros-Menalla fossil locality, located traits or fragmentary fossils. The story of in the Djurab Desert of Chad, 2500 km west Ramapithecus and its past hominin status, from the supposed origin of previously one that was based mostly on thick molar known hominins in east Africa (Brunet et al. enamel and supposedly reduced canine teeth 2002a: 146). While the cranium is virtually highlight this problem (pilbeam 1966: 1). complete, it suffers from severe However unlike Ramapithecus, S. deformation. It was found in one piece, with tchadensis is known from a virtually matrix filled cracks. The cranium features a complete cranium as well as multiple mosaic of primitive and derived traits. The mandibular fragments (Brunet et al. 2005: face in particular is derived from that of 752). With more information available, one Upper Miocene apes since it is short in should be able to make a more accurate length and presents with a marked reduction attribution of S. tchadensis's phylogenetic in facial prognathism. The cranium placement. however, is very primitive and ape like. It is The problem of recognizing the first long and has a small cranial capacity, hominin is compounded by the dearth of initially estimated to be between 320-380cc fossils in the Upper Miocene of Africa, based on the original distorted cranium especially the period between five and seven (Brunet et al. 2002a: 146), and revised to million years ago. Until the recent discovery 370-380cc based on CT scans and a virtual TOTEM: vo1.16 2007-2008 Copyright © 2008 TOTEM: The U.W.O. Journal of Anthropology reconstruction of the cranium (Zollikofer et cannot be obtained through faunal al. 2005: 758). correlation due to differences in faunal The cranium is best described as migration rates, there appears to be no derived anteriorly and primitive posteriorly. controversy in the literature regarding the According to Wood, "put simply, from the dating of TM 266 locality. Thus, S. back it looks like a chimpanzee, whereas tchadensis is currently the oldest candidate from the front it could pass for a 1.75- for hominin status. million-year-old advanced australopith" (2002: 134). While it shares its derived face Sex Determination with that of later hominins, S. tchadensis Determining the sex of the S. presents with a supraorbital torus that is tchadensis cranium is of great importance, unlike any seen in early hominins. It has a as its hominin status may vary depending on thick and flat continuous brow similar to whether one ascribes it as a male or a female those found in Homo erectus. This of course of its species. In many apes, males exhibit does not imply any phylogenetic substantial sexual dimorphism in body size, relationship (Wolpoff et al. 2002: 581). The robusticity, and musculature, especially of brows are relatively larger, and absolutely the mastication apparatus. Even species thicker, than those of male gorillas, yet the exhibiting less body dimorphism between cranium is diminutive in size, similar to that the sexes still possess marked sexual of a female chimpanzee (Brunet et al. dimorphism in the canines, such as the 2002a: 148). The canines feature apical wear extant bonobos. Male apes in particular have or are worn down on the tips (Brunet et al. a marked canine/pre-molar honing complex, 2002a: 150). The holotype was found where the distal edge of the maxillary canine without a mandible, but other mandibular hones against the mesial edge of the fragments, as well as numerous teeth, have mandibular third pre-molar. This results in a been recovered from adjacent areas. cut back, or angled mesial surface to the pre- Unfortunately no lower third pre-molar has molar tooth. Female apes are conversely yet been recovered. This tooth is of more gracile in body size and robusticity, importance because it forms part of the have reduced canines and often lack a robust canine/pre-molar honing complex, a honing complex (Wolpoff et al. 2002: 581). primitive feature of Upper Miocene apes, Interpretation of the sex of S. but one that is lacking in known hominins tchadensis is complicated by the mosaic (Brunet et al. 2005: 753). nature of the cranial features. It simply does not fit neatly among known apes analogues. Dating The cranium is very robust. It has a massive The fossils have been dated to supraorbital torus. It presents with a large between six and seven million-years-old nuchal crest and a large and rough nuchal (Vignaud et al. 2002: 155) with recent plane, indicative of large neck musculature. analysis favouring a date closer to seven Based on robustness alone, the S. tchadensis million (Brunet et al. 2005: 753). This cranium could be sexed as a male (Brunet et makes S. tchadensis the oldest of the al. 2002a: 148). potential hominins uncovered thus far. Based on the canines, and indirectly, Dating of the TM 266 locality was done based on cranial size, S. tchadensis could through faunal correlation with two sites in instead be sexed as a female.
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