The Emergence of Kenyanthropus and Australopithecus

Kenyanthropus platyops originates from geological deposits dating to around 3.5 million years ago (M.G. Leakey et al., 2001). Its discovery so recently, with its unique anatomical features so unexpected, sent shockwaves through the anthropological world and started a flurry of speculation. Indeed, recently T.D. White (2003) has suggested that the cranium of the type specimen of Kenyanthropus platyops (specimen KNM-WT 40000) may actually represent a specimen of Praeanthropus, for he suggests that the distortion of the specimen has resulted in a misdiagnosis. While this suggestion deserves serious consideration, some derived features of Kenyanthropus, which are unlikely to be overtly influenced by the type of distortion, suggest otherwise. For example, K. platyops, unlike Pr. afarensis, does not have an occipitomarginal sinus or a compound tem-poronuchal crest; it has reduced incisor heteromorphy, the upper molars are also significantly reduced in size relative to Praeanthropus, and finally its enamel thickness is reduced (see M.G. Leakey et al., 2001). Therefore, the present authors recognize Kenyanthropus as a distinct taxon.

It has recently been suggested by D.E. Lieberman (2001) that Kenyanthropus probably evolved from members of the "anamensis group" or a species very much like it, and that Kenyanthropus is the immediate ancestor to the species called Australopithecus rudolfensis by some (B.A. Wood & Richmond, 2000; B.A. Wood, 2002; Walker & Shipman, 1996; partly Walker, 1976) and Homo rudolfensis by others (D.E. Lieberman et al., 1996). If so, the species rudolfensis should be placed within Kenyanthropus as K. rudolfensis (D.E. Lieberman, 2001; see also partly M.G. Leakey et al., 2001). This scheme would go some way toward helping explain the "problematic" fossils currently allocated to rudolfensis.

As discussed by M.G. Leakey et al. (2001) and D.E. Lieberman (2001), Kenyanthropus is distinct from the proto-australopithecines and more like later hominins in several features, notably its facial skeleton (Figure 4.1).

Figure 4.1 ► Frontal view of distorted Kenyanthropus platyops specimen KNM-WT 40000.

Figure 4.1 ► Frontal view of distorted Kenyanthropus platyops specimen KNM-WT 40000.

The anterior insertion of its cheekbone gives its face a less prognathic appearance, further emphasized by the relatively flat subnasal region, the area in which the upper incisor roots are implanted; it also has a tall cheekbone, like later hominins. The supraorbital torus (browridge) is also less developed and more gracile, as in later hominins. And while it has a small cranial capacity (around 350 cc), its frontal is set high above the face, with no frontal sulcus. Indeed, D.E. Lieberman (2001) suggests that except for its small cranial size, Kenyanthropus is strikingly similar to the famous facial skeleton KNM-ER 1470, which largely defines the anatomical condition of the "rudolfensis group." This would be remarkable because members of the "rudolfensis group" do not appear in the fossil record until almost 1.5 million years later. Kenyanthropus does share some features with some proto-australopithecines, including a small auditory meatus, thick molar enamel, a small brain, and a flat inferior nasal margin (D.E. Lieberman, 2001); though these are primitive features, all of which have been retained from its evolutionary past and are of no significance in terms of working out phylogenetic relationships.

Given the unique combination of anatomical features, primitive and derived, we agree with M.G. Leakey et al. (2001) and D.E. Lieberman (2001) that if we continue to accept the validity of Australopithecus and Paranthropus, then the description of the new genus Kenyanthropus is justified. A genus, unlike a species, is recognized not on the basis of anatomical variability but on its implied evolutionary relationship with existing taxa (see Chapter 1). If a taxon does not share a close sister-group relationship or a recent common ancestor with any previously known genera, then it is a candidate for its own genus. Kenyanthropus cannot easily be accommodated within either Australopithecus, Paranthropus, or Homo, as Cameron (in press b) has shown in his phylogenetic analysis of early hominins (see also the next chapter); in some ways it appears to represent a good ancestral type for both Australopithecus and early Homo.

Recent paleontological and paleobotanical studies at Lake Turkana (Kenya) and within the Omo region (Ethiopia) have examined deposits from the time of the emergence of Kenyanthropus. They indicate that a complex mosaic of woodlands and grasslands stretched over the region, interrupted by gallery forests that occupied and contracted over time. From 4 to 1 Ma, the gallery forests underwent a rapid and dramatic floral change. In addition, at Lake Turkana, around 3.4 Ma, fossil snails typically associated with tropical rainforests have also been found (Potts, 1996; see also Isaac & Behrensmeyer, 1997).

Just when Praeanthropus afarensis and the "bahrelghazali group" were disappearing from the fossil record at the other end of the continent, in South Africa Australopithecus africanus appeared—the first African Pliocene hominin ever discovered and described. The earliest appearance date for this species is around 3.5 Ma (Strait & Wood, 1999; B.A. Wood & Richmond, 2000), which is similar to the proposed dates for Kenyanthropus. As will be discussed in the next chapter, this is the only species that really belongs to the genus Australopithecus. We also believe that, unlike the proto-australopithecines, Australopithecus and Kenyanthropus represent the earliest unchallengeable hominins to date.

In 1925 the Australian Raymond Dart, who held the Chair in Anatomy at the University of Witwatersrand, South Africa, was digging around in a box of fossil specimens supplied to him by the owner of the Rand Mines, when he identified among the numerous baboon skulls, the skull and associated fossilized brain of what became known as the Taung child (Figure 4.2). Realizing their significance, he quickly drafted a paper to the prestigious journal Nature, announcing the discovery of a manlike ape ... and that the first known species of this group be designated Australopithecus africanus, in commemoration, first, of the extreme southern and unexpected horizon of its discovery, and secondly, of the continent in which so many new and important discoveries connected with the early history of man have recently been made, thus vindicating the Darwinian claim that Africa would prove to be the cradle of mankind (Dart, 1925:198).

Dart recognized that this apelike individual walked erect because the foramen magnum, through which the spinal cord connects with the brain, was located directly underneath the cranial base. This means that the head was

Kenya Australopithecus Africanus

Figure 4.2 ► The Taung Child from South Africa, the type specimen of Australopithecus africanus.

Adapted from Dart (1925).

Figure 4.2 ► The Taung Child from South Africa, the type specimen of Australopithecus africanus.

Adapted from Dart (1925).

balanced on top of the vertebral column, unlike other apes, where the foramen magnum is located toward the back of the skull. Unfortunately for Dart, the first decade of the new century had focused paleoanthropological attention onto the English Piltdown specimen, a bizarre forgery, a composite of an orangutan's jaw and a modern human cranium (see F. Spencer, 1990), and this indicated that early hominins had a large brain and apelike teeth, precisely what most researchers at the time predicted. The Taung child, however, had the reverse condition, a small brain and human teeth; so many people considered it a primitive chimpanzee, having little or anything to do with human evolution, and it was argued that the unusual location of the foramen magnum was because it was a young child and by adolescence the foramen magnum would have moved more posteriorly. It would be almost another 30 years before Piltdown was finally shown to be a scientific forgery (Oakley & Hoskins, 1950; F. Spencer, 1990) and thus room made for the australopithecines within the human evolutionary tree. And in the meantime, more and more of them have been discovered in sites in South Africa.

Like most proto-australopithecines, A. africanus was probably not a stone toolmaker, though it was possibly a tool user—after all, chimpanzees use both wood and stone tools and even make some tools of flexible, perishable materials (though they do not make tools of stone); so presumably the australopithecines could do no less. In the 1950s, however, Dart found what seemed to him an unusual sample of fossil antelope and pig bones in the cave site of Makapansgat, which contained specimens of A. africanus. The way in which they had been broken suggested to him that they had been selected and modified to use as tools. Australopithecus africanus, he argued, had chosen suitable bones from carcasses and kept them handy for use in butchering animals they had killed, and some of them perhaps were even weapons for prehistoric warfare, including cannibalism (Dart, 1957, 1959, 1960). Large mammal long bones were clubs, animal jaws with teeth were tools used to help cut material, while other animal parts were the remains of prehistoric feasts. This whole toolkit was given the name osteodontokeratic culture (osteo- meaning "bone," odonto- meaning "teeth," and kerat- meaning "horn"). Dart believed that the australopithecines were marauding killer apes, with a social system based on brutality. It is likely that Dart's experience as an Australian medical officer in the fields of Flanders during the First World War influenced this very pessimistic outlook of human behavioral evolution. We now know, from careful analysis of the way that modern skeletons disintegrate and are taken apart by scavengers, that it was a perfectly natural assemblage;

indeed, rather than doing the eating at Makapansgat, the australopithecines were the prey! The evidence suggests that these bone accumulations at australopithecine sites represented hyena or sabertooth dens and/or leopard kills (Brain, 1981).

Australopithecus africanus (Figure 4.3) was similar in body type and brain size to its often-presumed ancestor Pr. afarensis, though there are many differences between the two: Canine teeth of A. africanus were smaller, and its molars were larger, with even thicker enamel; and its facial skeleton was buttressed with thickened bone alongside the nasal aperture in what Rak (1983) has called "the nasoalveolar triangular frame." Surviving postcranial skeletal parts of A. africanus suggest that, like Pr. afarensis, it was adapted to some form of bipedal locomotion. However, the muscles of the lower limbs may have been arranged in a way unlike those of modern apes and humans (B.A. Wood, 1992), and the arms may have been longer and more powerful—but this is very controversial.

With the arrival of drier climates and the expansion of the savannah around 2.5 Ma, A. africanus disappears from the fossil record. The habitat of A. africanus ranged from a wet forest with high rainfall at Makapansgat

Australopithecus Africanus Habitat
Figure 4.3 ► South African Sterkfontein Australopithecus africanus specimen Sts 5.

to a dry partially wooded savanna at the later sites of Sterkfontein and Taung (Andrews & Humphrey, 1999; McKee, 1999). The drier and more patchy forest cover suggested by the later localities fits in well with the evidence that this is the time of the earliest northern hemisphere glaciation, which started around 2.5 mya and resulted in more arid conditions in Africa (Vrba, 1999). So A. africanus—like Pr. afarensis—experienced a diverse succession of environmental conditions over time (Potts, 1996).

Another genus, Paranthropus, appears in the fossil record at about the same time that A. africanus vanishes and that the "rudolfensis group" and early Homo are first emerging (see Delson, 1988; Grine, 1988; Suwa et al., 1997; Deacon & Deacon, 1999; Dunsworth & Walker, 2002; T.D. White, 2002). In terms of competition and access to available food resources and territorial ranges, Paranthropus are likely to have played some indirect role in the adaptive trends adopted by the earliest humans.

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