The Emergence of Paranthropus

With the discovery by Alan Walker of KNM-WT 17000 (nicknamed the "Black Skull" due to its color; see Figure 4.4) in Kenya during the mid-1980s, another major problem in paleoanthropology was largely settled, the origins of the unique robust hominins of the genus Paranthropus (see Walker et al., 1986; Walker & Leakey, 1988). Until the discovery of this specimen, dated to 2.5 Ma, there was no fossil evidence for their origins: in East Africa, P boisei appears around 2.3 Ma and disappears around 1.4 Ma; in South Africa, P robustus appears around 1.8 Ma and disappears around 1.6 Ma (Keyser, 2000; T.D. White, 2002). The Black Skull is anatomically intermediate between Pr. afarensis and the later species of Paranthropus. The species that traditionally make up species of Paranthropus are monophyletic, and thus their generic distinction from other hominins is clearly justified (see Groves, 1989a; Strait et al., 1997; Strait & Grine, 2000; Cameron, in press b; and the appendix); as long as we are going to split them up into different genera at all, of course.

To what species should we allocate the Black Skull? Some authorities see it as a simple lineal ancestor of Paranthropus boisei and so place it in that species. Yet it does fall outside the range of P boisei, with its smaller cranial capacity, its larger front teeth (as suggested by its surviving incisor roots, which are well developed), and its very prominent, prognathic lower face. Besides, it is probably ancestral to the South African species

Figure 4.4 ► Cranial specimen of Paranthropus walkeri (KNM-WT 17000), found by Alan Walker in West Turkana, Kenya, dated to around 2.5 million years ago.

P. robustus as well, though not all specialists agree about this (see Walker et al., 1986; Clarke, 1988; Kimbel et al., 1988; Walker & Leakey, 1988; B.A. Wood, 1988; B.A. Wood & Richmond, 2000).

In 1968, Arambourg and Coppens discovered a large and strange-looking jaw (Omo-18) in 2.6-million-year-old deposits in the Omo valley and placed it in a new genus and species, Paraustralopithecus aethiopicus (see Arambourg & Coppens, 1968; Howell & Coppens, 1976). Walker et al. (1986), when describing the Black Skull, recommended that studies should be made to determine whether it might represent the same species as the Omo-18 jaw and, if so, it should be called Paranthropus aethiopicus. No one seems to have taken up this sensible suggestion; instead, people have rushed right in and ascribed the Black Skull to Paranthropus (or even Australopithecus) aethiopicus without any special study at all. We believe that it is best to avoid this designation, for two main reasons: (1) because Omo-18 and the Black Skull might not, after all, be the same animal (and we now know of another big-toothed species that was living at the same time as the individual defined by the "Black Skull," the so-called A. garhi);

(2) because the name Australopithecus aethiopicus had previously been used by a few anthropologists for "Lucy" (see Tobias, 1980) and, the rules of nomenclature being very tortuous, this would restrict the use of the name aethiopicus. Ferguson (1989) gave the Black Skull its own species name, walkeri, and all in all we think it safest to call the species represented by the Black Skull Paranthropus walkeri (Groves, 1999), until it is demonstrated that the large robust Omo mandible and the Black Skull do indeed belong to the same species.

Recent archaeological interpretations at Swartkrans in South Africa suggest that P. robustus may have used animal long bone shafts as digging "sticks." There is also the suggestion of burned bone, but the geological deposits containing these fossils also overlap with the presence of early Homo fossils (B.A. Wood, 1992). Thus, we cannot tell whether it was Paranthropus or early Homo who burnt these bones (Pickering, 2001), or whether it was either of them, because the burns could even have been the result of natural processes such as bushfires.

Later Paranthropus species have a large flat face, and the brain was relatively small — somewhat bigger than a chimpanzee's and much bigger than that of the Black Skull (McHenry, 1988) (Figure 4.5). They also had a large powerful jaw, the back teeth (premolar and molars) being extremely large, acting as "grinding stones" for the preparation of hard food objects prior to digestion; yet the front teeth (incisors and canines) were tiny, smaller even than in modern humans (Robinson, 1954, 1972; Tobias, 1967). While their masticatory architecture was well developed, body size was only a little larger than that of most proto-australopithecines, and the body proportions were also similar to that of most proto-australopithecines (McHenry, 1988). This primitive yet specialized hominin, like Australopithecus, was a bipedal walker (Susman, 1988, contra Robinson, 1972). It appears to have specialized in consuming large amounts of tough plant foods typical of parched environments, unlike contemporary Homo, which was more of a generalized opportunistic feeder (see Kay & Grine, 1988; Susman, 1988).

Both Homo and Paranthropus lived during a period of drier climate and more open vegetation, compared to the wetter conditions existing at the time of Australopithecus (Reed, 1997). It is likely that Paranthropus overspecial-ized and, with the ever-changing climatic conditions associated with the northern glaciation cycles, failed to adapt to the more generalized dietary regime that was required given the increasing aridity (see Foley, 2002; Pickering, 2001). This overspecialization ultimately led to its extinction.

Figure 4.5 ► Type specimen of Paranthropus boisei, Olduvai Gorge, Tanzania, found by Mary Leakey in 1959 and dated to around 1.8 million years ago.

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