rial m T
Figure 6.5 ► Photograph of original Gongwangling (Lantian) cranium from China.
an orangutan-like ape, not a hominin at all (Wu, 2000). The evidence from Lantian (Wu & Poirier, 1995) and stone tool evidence from Hebei (Jia, 1985) both suggest a hominin entry date into China around 1 Ma.
Larick and Ciochon (1996) suggest that middle Pliocene cooling and drying encouraged a South Asian dispersal by proto-human groups as a band of open tropical and subtropical habitats appeared in present-day Arabia, continuing on through into Southeast Asia. With the lowering of the sea level as a result of increased glacial ice sheets, movement between Africa, Arabia, and Asia was made easier and the route considerably shorter with the emergence of land bridges between these regions. For example, rather than having to move north into the Levant and then traverse a northern route into Asia, hominins could cross from present-day Eritrea into southern Arabia (linked by land bridges) and from there into the Indus Valley and then into Southeast Asia. This route is partially supported by the presence of Oldowan tools in northern Pakistan dated to around 1.9 Ma (Dennell et al., 1994; Larick & Ciochon, 1996).
The route leading to eastern Europe reflects a more diverse paleohabitat (Gabunia et al., 2000b). Faunal and botanical remains from the Dmanisi region are suggestive of a moderately dry climate with a fairly extensive open landscape. The northern expansion of early Homo from East Africa required an adaptation to middle latitudes, where upland habitats where marked by a number of mosaic habitat types. It may be that the populations that gave rise to the Dmanisi hominins moved through the Levantine corridor, into Georgia, and then into eastern Asia from a northern direction. The Dmanisi locality itself appears to have been part of a lake margin, adjacent to a forest-steppe formation, which would have contained rich resources, not only in plants and animals, but also raw material for tool manufacture, as indicated by nearby river gravels (see Gabunia et al., 2000b).
All this indicates that the dispersal from Africa to Asia was a complex one, with numerous routes being used. It also indicates that within Eurasia at least, there was more than one major migratory event. While there was a cultural and biological exchange between Eurasia and Africa, this appears not to have been the case between these regions and Southeast Asia, which seems to have become isolated after its initial colonization.
The early H. erectus specimens from Indonesia are unlikely to represent the ancestral population of this species, but rather reflect a recent arrival from Eurasia (that is, H. erectus is not endemic to eastern Asia) (Figures 6.6-6.8). The presence of an H. erectus-like specimen from Ceprano in present-day
Italy, dated to around 900,000 years ago, indicates a more complex process (see Ascenzi et al, 2000; Clarke, 2000; Schwartz & Tattersall, 2002). It indicates to us that the likely place of origin for this species was somewhere in Eurasia, with a later Eurasian H. erectus population moving into western Europe around 1 Ma, before becoming extinct. Indeed, the problematic Olduvai hominins OH 9 from Upper Bed II (dated to 1.2 Ma) and OH 12 and
other fragmentary specimens from Beds III/IV (dated to around 700,000 years ago), which have been considered by some to represent "classic" specimens of H. erectus (Clarke, 2000; see also partly Maier & Nkini, 1984; Rightmire, 1985, 1990; Ascenzi et al., 2000), may represent a southern expansion of this species back into Africa from Eurasia. Indeed, Clarke (2000) suggests that the Ceprano and OH 9 specimens are morphologically identical; and both specimens are morphologically closer to the earlier samples of H. erectus than to the later Asian endemic H. erectus deme from Ngandong, which are defined by increased neuro-orbital disjunction
(see later). While the Ceprano specimen has recently been allocated to its own species, Homo cepranensis (Mallegni et al, 2003), we disagree. We maintain that it represents a European deme of H. erectus, which was likely associated with dispersals "Into Africa." As demonstrated by Vrba (1985, 1999), faunal migration and territorial expansion during the Plio-/Pleistocene did not only mean an "Out of Africa" exodus, but also an "Into Africa" dispersal: There were several large mammal and rodent migrations back into Africa from Eurasia during this time, and there is no reason why such migrants did not include hominins (see Cameron, in press a). We should thus not be surprised to find later additional nonendemic African specimens of H. erectus in East Africa at least.
Further fossil evidence for a possible "Into Africa" migration by Eurasian demes of H. erectus have recently been recovered from localities in Eritrea and Ethiopia, both dating to around 1 Ma. These are the H. erectus-like fossils from Danakil and Bouri (Abbate et al., 1998; Asfaw et al., 2002). The cranium from Danakil exhibits features said to be distinctive of H. erectus, including the greatest cranial breadth across the supramastoids' crests, massive supraorbital torus, and an opisthocranion that coincides with the inion (Abbate et al., 1998). A similar H. erectus-like morphological pattern is present in the cranium from Daka (Bouri, Middle Awash), which is believed to be contemporary with the Danakil skull. The Daka hominins are said to be aligned morphologically with OH 9 and to share many derived characters with Asian demes of H. erectus (Asfaw et al., 2002).
With the arrival of H. erectus in Asia, we can see a pattern of evolutionary stasis. Asian hominins are marked by few speciation events, compared with the numerous ones that occur in Africa and Eurasia. The only specia-tion event so far recognized in eastern Asia is the evolution of a more derived species in China, H. pekinensis from the famous Zhoukoudian Cave close to Beijing and from a few other sites in China (Figure 6.9). Specimens of H. pekinensis are currently dated to around 400,000 years ago, with some specimens possibly dating to as far back as 800,000 years ago (Shen et al., 2002; see also Goldberg et al., 2001). This later Chinese species is likely to have originated from an earlier H. erectus deme, either from Eurasia or perhaps from a migrant population from Indonesia. While the African and European species H. heidelbergensis is also present in China, as represented by the Dali specimen dated to 200,000 years ago and the Jinniushan specimen dated to around 250,000 years ago (Wu & Poirier, 1995) (Figure 6.10), these appear to have been part of a short-term
migratory event from the west and not an Asian speciation event as such. There is also some evidence for Acheulean-like tools in northern China, though they are few and far between (see Foley & Lahr, 1997). This late migration of European hominins into Asia may explain the Narmada cranium (from Central India), which has clear European affinities and is associated with Acheulean tools (see M.A. de Lumley & Sonakia, 1985; Cameron et al., in press) (Figure 6.11); or Narmada may represent another, later, more Neanderthal-like incursion.
Within H. erectus we can also see an ongoing trend of evolution from the earliest specimen from Sangiran (1.5 Ma) to the later specimens from Ngandong (40,000 years ago?) (Figure 6.12), a survival in a little-changed form in a way unique from all other hominins. Homo erectus, over its long history, underwent increased neuro-orbital disjunction, while African and Eurasian Homo developed the opposite condition, increased neuro-orbital convergence. These two morphological trends can be linked to differential patterns of brain morphology, which will affect not only facial hafting to the braincase, but also degrees of postorbital constriction and differential patterns of anterior cranial base angulation (see Weidenreich, 1943; partly D.E. Lieberman, 1995, 2000; Cameron, in press a, b).
While H. erectus and H. pekinensis both show signs of increased encephalization, their overall pattern of brain development is different from that observed in later African and Eurasian Homo in that their frontal lobes are located in a more inferoposterior position so that the frontal is low, with increased supraorbital development and postorbital constriction. Overall the brain case is pushed back from the face (neuro-orbital disjunction). In later western Homo the trend is for the forehead to become higher, as the frontal lobes become situated directly above the orbits, which results not only in a reduced supraorbital, but also a high cranium and reduced postorbital constriction. These features are correlated with increased cranial base flexion, associated with the forward and superior migration of the frontal lobes (neuro-orbital convergence). The impact of increased cranial base flexion, or extension, has significant impacts on facial morphology, especially degrees of prognathism (D.E. Lieberman, 1995, 2000; Cameron, in press a, b).
Also associated with this are a number of other features including a significant reduction in mastoid size. This indicates a differential pattern of head and neck musculature, as the mastoid is the major attachment site for the sternocleidomastoideus muscle, a major neck muscle, which controls head rotation amongst other things. Homo erectus and H. pekinensis are also defined by the development of an angular torus, which suggests
increased temporalis muscle size/mass. The temporalis muscle is a major muscle of mastication. There is further differentiation of the neck and masticatory musculature between H. erectus and H. pekinensis: the Chinese species has increased postorbital constriction (increased tempo-ralis development?) and the connection of the supramastoid and mastoid crests, suggesting increased development of associated muscles, at the expense of the sternocleidomastoideus, or a repositioning of this muscle. One surprising difference between these two Asian species is that while H. pekinensis has increased postorbital constriction, suggesting increased anterior cranial base extension, its supraorbital torus is less developed than in H. erectus (Cameron et al., in press).
Two major patterns of speciation are evident in the emergence of early Homo upon leaving Africa. In Eurasia there is a pattern of rapid cladogen-esis, with the rise and fall of numerous hominin species. After the initial colonization of Asia, however, the pattern is one of anagenesis, with H. erectus slowly adapting to existing conditions, and only one known endemic speciation event, with the emergence of H. pekinensis in China from an earlier H. erectus population.
Speciation in Africa can be attributed to increasing climatic and geological upheavals, which would result in the isolation of numerous human and other animal groups (Kingdon, 2003). By 1.8 Ma the formation of large glacial ice sheets had reached its maximum in the northern hemisphere, which resulted in cooler and drier climates, associated with oscillations between more forested and more open habitats. It is also at this time that we see in East Africa renewed activity in the formation of the Rift Valley system, with valley floor spreading associated with high levels of tectonic disruption and volcanic activity. The Rift Valley system would block drainage systems, forming great scarps, and with volcanic eruptions spewing out lava and ash. The previous homogeneous habitat was split into a number of mosaic and restricted habitat zones (see Foley, 1987; Potts, 1996). Within these habitat zones numerous proto-human populations (and other fauna) adapted to their differing ecological settings; in many cases, given the differing associated resources, many groups may have been isolated not so much as a result of physical barriers, but in terms of preferred environments, such as forest as opposed to savanna (Kingdon, 2003).
Over time, geographic isolation and/or isolation based on habitat preference would propel populations down many differing evolutionary pathways. In some cases, however, populations from differing regions might evolve in parallel as they adapted to the same conditions in a similar anatomical and behavioral way. In some cases, the result would be extinction of a hominin group, in other cases the absorption of one group into another; in yet other cases the result would be speciation.
The human paleontological record is making it increasingly clear that, rather than a simplistic model of just one or two hominin species being present at any one time, numerous species, some sympatric, emerged in Africa during the Plio-/Pleistocene, most often followed by the extinction of one or more of them. This should be no surprise if we view hominins as just like any other large mammal. For at this same time, we witness in the fossil record a diversity of experiments within other mammal groups, with rapid speciation events and extinctions — hominins were just joining in on the act (Vrba, 1985, 1999; Foley, 1987; Potts, 1996). The same pattern of rapid and ongoing speciation is also observed in the earlier Miocene hominids of Africa and Eurasia, for they also had climatic and habitat hurdles to jump, which resulted in considerable hominid species diversity (Cameron, in press a; see also Harrison, 2002; Begun, 2002; Kelley, 2002; S. Ward & Duren, 2002).
The territorial expansion into Eurasia would also fit this model; as we have already discussed, the migration to the Levantine corridor and southeastern Europe (Dmanisi) would meet differing mosaic habitats. It is within Eurasia that we think the origin of H. erectus occurs, with its migration into East and Southeast Asia proper. Here, there is evidence for tropical forests as well as some grasslands. But what appears to be different is that there is no major disruption in these habitat zones through time, and populations of H. erectus were not confronted with major periods of climatic or habitat disruption, unlike the conditions in Africa and Europe, which were marked by continuous periods of such disruption. The only major difference in habitat in Asia documented to date is that at Zhoukoutian, which is associated with a more open woodland habitat, which may go some way to explaining the emergence of H. pekinensis as well as the short-lived migration event of a H. heidelbergensis population into this region around 200,000 years ago.
Finally, it is interesting to speculate that the survival of H. erectus in Southeast Asia can be attributed to their isolation from other hominin species. Accepting that populations of H. erectus survived at Ngandong as recently as just 40,000 years ago (Swisher et al., 2000), we have some insight into their extinction. This last appearance datum for H. erectus correlates with the slightly earlier appearance of modern humans in this region. There is a similar correlation in Europe, where the endemic populations of H. neanderthalensis become extinct with the appearance of modern humans in Europe at around the same time, though at this point let's not get ahead of ourselves.
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