A number of paleontological observations have been consistently quoted as supporting the multiregional model and have allowed the debate on the origin of modern humans to persist. However, it is worth summarizing the results of some recent studies that bear on this issue.
Australian Populations and Homo erectus in Southeast Asia
The most often cited evidence for continuity outside Africa is that between modern Australian populations and the later specimens of Homo erectus, the Ngangdong sample from Java (Fig. 1). Australians are supposed to share with the late-surviving H. erectus a number of cranial traits (Larnach and Macintosh 1974
; Wolpoff et al. 1984). Upon superficial visual comparisons, Ngangdong and some recent Australian specimens may appear similar, but it is misleading to interpret this as part of a unique evolutionary relationship. First, the earliest known Australian fossils, those from Mungo, are quite gracile and do not show these traits (Brown 1987). The later ones, like Kow Swamp and Keilor, are very diverse, implying that some became more robust with time (Lahr 1996). Second, new dates indicate that Ngangdong may be as young as 50,000 years; if this is the case, then the supposed source population persisted unchanged long after the origins of the Australians. Third, the few characters that Australian populations do share with with Southeast Asian H. erectus are also found in the early modern human fossils of Eastern Africa (Omo Kibish) and the Levant (Skhul, Qafzeh) (Lahr 1996), and metrically, the most "primitive" of the Australian fossils (the Willandra Lakes specimen WLH50) falls closest to the African transitional or early modern specimens (Stringer 1998). These African specimens, however, also have modern proportions of the skull, unlike those found in Ngangdong that are essentially archaic and thus fall closer to Australians and represent a better ancestral model (Groves and Lahr 1994). In other words, where Australian populations look like archaic ones, it is those on the cusp of modernity in Africa that they resemble most (Fig. 1).
Regional Continuity Traits
Central to the support for multiregional evolution is the notion that certain anatomical traits show a persistence and uniqueness within regions, regardless of the time of the specimens. These are the so-called continuity traits (Wolpoff et al. 1984). Three regional groups are usually discussed: Australian, East Asian, and European. In practice, it is the first two of these that have been the primary focus of discussion and have provided the most often-cited evidence for continuity. There has, however, only been one major study of the actual frequency, distribution, and biological significance of the regional traits (Lahr 1994, 1996). In that work, Lahr took samples from the main continental populations and looked at how many of the traits showed statistically significant specific regional distributions, that is, occurred at significantly higher levels in one region of the world rather than another. The results effectively undermine the validity of the continuity traits for showing evolutionary links between archaic and modern populations in different parts of the world. For example, of the 10 traits supposedly specific to East Asia, 2 showed no regional pattern, 6 showed higher frequencies in regions other than East Asia, and only 2 could be shown to be genuine, regionally specific characteristics of present East Asian populations. Nineteen Australian traits have been suggested, but again only six proved to be regionally specific to Australia. These results do not mean that there is no regional variation in human populations, nor do they preclude the possibility that there may be continuity in some features, but they do indicate that there is no statistical basis for the claimed regional continuity of this particular set of features.
Temporal Continuity
The implication of regional continuity is that the same traits can be found consistently in populations from one time period to the next. This is seldom, if ever, the case. The situation with Southeast Asian H. erectus and the Australians has already been described. In East Asia, the earliest modern humans are those from Upper Cave Zhoukoudian, dated to ~30,000 years ago. Kaminga and Wright (1988) have shown that statistically these specimens differ from recent East Asians. As in Australia, the diversity of early moderns in eastern Asia refutes notions of temporal stability in regional patterns (Lahr 1996).
Morphometrics
Another way of testing hypotheses concerning the origins of modern humans is to look at overall morphological similarity and differences among Homo crania. Howells (1973, 1989) originally suggested that all modern human crania shared more in common with each other than any did with their local antecedents. Stringer (Stringer 1992a,b, 1995) has repeatedly also shown in multivariate analyses that "recent modern" and "early modern" human skulls are all distinct from nonmodern forms and do not show any trends within these regions that might indicate the evolution locally of modern humans from archaic ones. This would include such specimens as WLH50 from the Willandra Lakes in Australia, which despite being remarkably big and robust is also essentially very modern (Turbon et al. 1997). Pairwise comparison of the relevant populations also supports this view (Waddle 1994).
Cladistics
In paleontology, at least, there is a long-standing argument about the relative strengths of phenetics (simple assessment of overall similarities and differences, usually based on metrics) and cladistics (a more discriminating technique that, although generally less based on quantifiable traits, is more careful in weighting the evolutionary significance of each trait) as means of reconstructing evolutionary relationships. Phenetics is thought to be less powerful as it does not weight characters according to degree of ancestral sharing. However, the appropriateness of applying cladistics to within-species variation has been debated (Trinkaus 1990). Leiberman (1995) has carried out a cladistic analysis of the recent hominid cranial variation and has shown that the most parsimonious explanation is that which incorporates all modern human variation in a single clade, derived from a recent African source. Groves and Lahr (1994) have also shown that many of the continuity traits used in the multiregional model that supposedly link Australian and Javanese H. erectus populations are in fact shared ancestral characteristics of the Homo clade and therefore cannot provide useful phylogenetic information.
Neanderthal Admixture
Although there is growing acceptance among paleontologists that the western European Neanderthals show an extreme morphology and do not appear to have interbred with modern humans, it has been claimed that there is evidence for admixture in central Europe. A major element of that claim is the material from Krapina in former Yugoslavia, which lacks many of the more extreme Neanderthal features. However, recent redating of the material shows that it is in fact ~130,000 years old, which appears too old for admixture with incoming humans (Rink et al. 1995), and its lack of specialization is almost certainly a result of its position early in Neanderthal evolution. Pleistocene modern Europeans cannot be shown to have special affinities with Neanderthals (Stringer et al. 1984; Howells 1989; Van Vark 1990; Van Vark and Henke 1992).
Chronology
Dating of later Pleistocene hominids has advanced considerably over recent years, and this new dating information has been a central to the debate. The key fact to emerge is that there is a chronological fault line in the appearance of modern humans. In eastern Africa, northern Africa, southern Africa, and the adjacent Levantine region, the first modern humans are over ~100,000 years old (Stringer et al. 1989; Grun et al. 1990; Grun and Stringer 1991). In Europe, eastern Asia, and virtually everywhere else archaic hominids persist until and modern humans are not found until after these dates (Mercier et al. 1991; Hublin et al. 1996; Swisher et al. 1996). The 60,000 year or 3000 generation gap seems to suggest that African and non-African human evolution was largely decoupled until very recently.
Overall, therefore, whatever element of evidence is considered
geographical patterns, chronological distribution, or cladistic and general morphological relationshipsthere is little or no evidence for non-African admixture or continuity between archaic and modern populations. These paleontological results seem to be unequivocal and support fully the single African origin model of recent human evolution. Publications within paleoanthropology over the last decade have consistently failed to provide positive support for the widespread in situ evolution of modern humans. Although it is often stated that there is a conflict between genetics and fossils, there is little basis for this view, and new studies rather than older ones have tended to be entirely consistent with a single African origin for modern humans.
Genes and Fossils: Complementary Approaches
Given the apparent consistency between genetic and paleobiological data, it might be reasonable to ask what role the fossils now serve. If we can learn the same story from the genes, perhaps they should be seen as the primary source of information about recent human evolution. Paleontologists might soon join their specimens in extinction. In fact, though, the demise of the old debate between multiregional and single-origin models opens the opportunity for a much more complementary approach to the problems of human evolution. As more diverse sets of questions are asked, what each discipline can and cannot do is becoming clearer.
Paleoanthropology
The great strength of paleontology is that it can locate things in time and space and as such can provide the context in which evolutionary events occurred. As explaining evolution is largely a question of showing the links between changes in genotypes or phenotypes and the external conditions, the context is clearly very important. Explanations of why human evolution took the form it did must in the end depend on paleobiology. In contrast, paleontology cannot with any great ease talk about gene flow very precisely. Tiny amounts of genetic exchange are unlikely to be very visible. Only the broad patterns are observable. Thus, for example, when paleoanthropologists talk about replacement, they are talking about the general event, not whether very low-level genetic admixture may have occurred.
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