The Emergence of Sahelanthropus Orrorin and the Lothagam Hominids

The recent significant discovery and description of Sahelanthropus tchadensis from Chad by a joint French and Chadian paleoanthropological team, dating to between 6 and 7 million years ago (Brunet et al., 2002; Vignaud et al., 2002; see also B.A. Wood, 2002), has done much to refocus our attention on the divergences of the hominins from other hominids. One of the key differences of the hominins from most other hominids is the development of a primitive form of bipedal locomotion, which would likely postdate the development of increased ability for upright posture (see Wood, 2002). Around 5-6 million years ago, the common human-chimpanzee ancestor had what we now think is the primitive hominid locomotion pattern, a quadrupedal, knuckle-walking ability (Richmond & Strait, 2000). Shortly after the split from the last common ancestor with the chimpanzee, the hominins must have developed a more upright gait, with some form of primitive bipedal walking. If Sahel-anthropus was bipedal, as suggested by Brunet et al. (2002), either it was a hominin or, if it represents a common ancestor to the chimpanzee-human lineages, knuckle-walking must have developed independently in both Pan and Gorilla. For reasons to be discussed presently, we think that the latter is most unlikely.

Associated with the development of these patterns of locomotion and positional behavior would be the development of numerous features of the skull and teeth. In the proto-chimpanzee, we would see much the same condition observed in the last common ancestor with the proto-gorilla, which presumably, like extant Gorilla and Pan, would include the following: large male canines, a moderately small premolar and molar complex, lower third premolar unicuspid, thin molar enamel, prognathic premaxilla (snout), marked postorbital constriction, developed supraorbital torus, a braincase pushed well back from the face, a low frontal, large masticatory muscles and corresponding skeletal attachment sites for these muscles, and, finally, a relatively small brain (Cameron, in press a). Some of these features were of course retained in the earliest hominins, after the split from the chimpanzee, as primitive features.

In the earliest hominins, we see, along with the derived form of bipedal locomotion, some reduction in male canine size, development of a bicuspid lower third premolar, and an increase in molar enamel thickness. Somewhat later, hominins developed a number of uniquely shared features not seen in the other clades: an absolute increase in brain size, a further increase in molar enamel thickness, further reduction in male canine size, reduced supraorbital torus, an absolute reduction in postorbital constriction, development of a relatively high frontal, and a nonprog-nathic premaxilla. The significance of Sahelanthropus is that, while we know very little about its pattern of locomotion or positional behavior, we finally have evidence of the facial and dental anatomy of a species at the point of divergence from the chimpanzee. Does Sahelanthropus follow the proto-chimpanzee condition or the basal hominin condition?

Not surprisingly, Sahelanthropus retains a number of primitive hominid features, including a small brain, developed postorbital constriction, and a low frontal. All of these features can be considered as part of the primitive condition called neuro-orbital disjunction (the frontal lobes of the brain are placed well back from the face). What might tend to refute a close relationship with the chimpanzee lineage, however, are a number of claimed hominin features, including its proposed bipedal locomotion, reduced canine size, increased molar enamel thickness, and absolutely reduced facial prognathism. A strict cladistic interpretation would thus place Sahelanthropus within the hominins, though at this point we are hesitant to do this, for several reasons. First is that the published illustrations (Brunet et al., 2002) (Figure 3.1) show a very long basicranium; though it is distorted, there is no doubt that in this respect, it is outside the hominin range and thus comparable to a gorilla. The claim of bipedalism rests on the interpretation of the basicranium as short; but this is absolutely not so! Second, the enamel thickness is uncertain; it actually matters rather little, because the earliest hominin of which most are fairly confident, Ardipithecus, had thin enamel (though Cameron doubts the hominin status of Ardipithecus). Third, the short face of Sahelanthropus is duplicated in some modern gorilla skulls.

One feature of Sahelanthropus that is surprising is the very small canines. The describers maintained that the almost complete Sahelanthropus skull TM 266-01-060-1 is a male, but there seems no convincing reason for this. If we contemplate the alternative, that it might be a female, then the canine size suddenly becomes comparable to that of the late Miocene hominid Graecopithecus from Greece dating to about 9.5 million years ago, which has been argued by some to be a "proto-gorilla" (Dean & Delson, 1992; Cameron, 1997a). The canines of Graecopithecus, though still sexually dimorphic, were unexpectedly small. Perhaps in this instance parsimony fails: The common ancestor had rather small canines, and in their subsequent evolution, both gorilla and chimpanzees, independently, evolved large canines.

Farther east of Chad by almost 2,500 km, and dating to somewhat later than Sahelanthropus, is Orrorin tugenensis from Lukeino in Kenya, dated to the latest Miocene. Senut et al. (2001) claim that it represents a direct human ancestor, largely because of certain features of the femur; one commentator

Sahelanthropus And Orrorin

Figure 3.1 ► Professor Michael Brunet with the now famous "Toumai" skull representative of the late Miocene hominid Sahelanthropus tchadensis from Chad.

Adapted from a photograph by Patrick Robert in Time Magazine (July 22, 2002).

Figure 3.1 ► Professor Michael Brunet with the now famous "Toumai" skull representative of the late Miocene hominid Sahelanthropus tchadensis from Chad.

Adapted from a photograph by Patrick Robert in Time Magazine (July 22, 2002).

has strongly criticized the original arguments (Haile-Selassie, 2001), but more detailed arguments have recently been put forward (Pickford et al., 2002). Specifically, in Orrorin the shaft of the femur is anteriorly convex and the pilaster is vertical, and it has a distinct intertrochanteric line, an elongated and strongly anteroposteriorly compressed femoral neck, and a developed obturator externus groove. In the neck of the femur, the cortex is thick inferiorly and thin superiorly, as in bipeds (but less markedly so than in Homo), rather than equally thick all round, as in nonhuman apes. Though Pickford et al. (2002) acknowledge that in some respects the bipedal condition of the femur is distinctly less developed than in later hominins, it is clear that the argument that Orrorin is a hominin has been much strengthened by this analysis. Less clear is the significance of some features that appear to be even more human-like than in australopithecines, such as the larger size of the femoral head and its anterior (instead of posterior) twist, and the more medially (less posteriorly) oriented lesser trochanter. The authors suggest that the australopithecine type of bipedalism may not be intermediate between Orrorin and Homo, and either the evolution of human locomotion took an indirect route (a view that was in fact already implicit in the work of such authors as McHenry [1986]), or else, more controversially, all known species of australopithecines are to be excluded from the human line altogether.

Further material of Orrorin, especially craniodental, is needed. Already, geological and geophysical work have refined the age of the deposits; early reporting gave a date of "about 6 million" years (Senut et al., 2001); the Lukeino Formation is now dated by K/Ar, backed up by paleomagne-tism, to between 5.7 and 6.0 Ma, and the most significant specimens (from Kapsomin) are 5.8-5.9 Ma (Sawada et al., 2002).

If both authors are readier than formerly to accept a basal hominin status for Orrorin, the two of us differ in our interpretation of Sahelanthropus. Cameron believes that it probably does represent a very early hominin. For example, orthognathy in gorillas, though it does exist, is rare; it is more parsimonious to accept that the last common ancestor of Gorilla and Pan had not only a prognathic face but also thin molar enamel and that the reduced prognathism and thick molar enamel observed in Sahelanthropus, and most hominins, developed after their split from the last common ancestor with the chimpanzee. This view, however, is at odds with the molecular evidence; if we maintain a divergence time of 4.6-6.2 million years ago, as calculated by Chen and Li (2001) in what seems much the most thorough survey to date, then Sahelanthropus is too early to be a hominin (or else there is something a great deal wrong with Chen and Li's molecular clock). Likewise, Ardipithecus, which has thin molar enamel, would represent the survivor of a lineage more primitive than Sahelanthropus. Groves, however, is convinced that the long basicranium of Sahelanthropus, the more sagittal orientation of the petrous bone, and the implied steep angle of the nuchal plane exclude it from the Hominini, except perhaps at its very base. In essence, there is no evidence that it has any hominin character states at all. Only more detailed studies and publications on this most important specimen will resolve these and other issues.

An important hominid, which spans the divide between Sahelanthropus and Orrorin and the australopithecines, is the mandibular fragment from Lothagam in Kenya, dated only as "older than 4.2 Ma and younger than 5.0 Ma" (McDougall & Feibel, 1999). Kramer (1986) and T.D. White

(1986), writing at a time when Australopithecus afarensis rated as the earliest member of the human lineage, both argued that the Lothagam mandible has detectable australopithecine affinities, though with ple-siomorphic features. Most recently, M.G. Leakey and Walker (2003) have suggested a close relationship to Australopithecus anamensis, though they stress that until additional specimens are discovered, it is not possible to allocate it to any known hominin species.

It matters whether Sahelanthropus is the earliest hominin or whether this position goes to Orrorin, Ardipithecus kadabba (see next section), or the Lothagam mandible, because if one or more of them is hominin, then it will give us a minimum date for the separation of the human and chimpanzee lineages. But even if none proves to be hominin, their significance will be undiminished, because we will have a glimpse of the morphology of immediate "presplit" apes.

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