CONCLUSIONS AND DISCUSSION
Validation of the utility of microsatellites in reconstructing evolutionary history of human populations has been made not only theoretically (20–23) but also empirically; the relationships based on microsatellites are generally consistent with morphological and paleontological evidence and other types of genetic markers (8–10). However, many of such studies used distantly related populations and, therefore, the utility of such markers in the study of closely related populations is yet to be explored. The current study reflects, to some extend, a lack of resolution of microsatellites in the reconstruction of closely related populations, probably because of an insufficient number of loci and a large number of populations studied but less likely because of the insufficient number of samples for each population as demonstrated by Shriver et al. (20). This is so because the variance of the genetic distance between loci is much larger than the variance due to sampling error (20) in the estimation of genetic distance. Small bootstrap values reflect insufficient amount of information available to resolve the genetic relationship among closely related populations in the presence of strong gene flow among those populations. But the employment of a much larger number of microsatellite loci in the current analysis may not guarantee a better resolution under such a scenario. Nevertheless, it is not our primary intention to reveal the detailed genetic relationship among those closely related populations, rather we are interested in exploring the major pattern of evolutionary history of the human populations currently residing in East Asia.
In both phylogenies with different loci and populations, populations from East Asia always derived from a single lineage, indicating the single origin of those populations. It does not preclude the possibility of an independent origin of modern humans in East Asia, but its contribution to the extant populations is not detectable in this analysis. It is now probably safe to conclude that modern humans originating in Africa constitute the majority of the current gene pool in East Asia. A phylogeny with very different topological structure would have been expected if an independent Asian origin of modern human had made a major contribution to the current gene pool in Asian populations. Since the methods employed in this analysis can detect only major genetic contribution from particular sources, a haplotype-based analysis will probably detect minor contribution from an independent origin of modern humans in East Asia (24, 25).
In contrast with previous studies (2–4) where distinction between southern and northern populations was clear, our current analysis showed that northern populations belong to two different groups, although statistical support was still weak. One noticeable difference in our study is the employment in the phylogeny reconstruction of the neighbor-joining method, which is supposedly more robust in the presence of genetic admixture. The use of microsatellites, a different type of genetic markers from previous studies, and the measures of genetic distance introduced further complication. However, the northern populations in cluster N2 were sampled from the southwestern part of China, except for Ewenki, where genetic admixture with the southern population was more likely to occur. This might explain why this group of northern populations clustered with southern populations.
Another noticeable feature from this analysis is that the linguistic boundaries are often transgressed across the six language families studied (Sino-Tibetan, Daic, Hmong-Mien, Austro-Asiatic, Altaic, and Austronesian). Such a phenomenon is even more pronounced among southern populations, where populations from the same geographic regions tend to cluster in the phylogeny (see Fig. ​Fig.11B). This observation is consistent with the history of Chinese populations, where population migrations were substantial.
The current analysis suggests that the southern populations in East Asia may be derived from the populations in Southeast Asia that originally migrated from Africa, possibly via mid-Asia, and the northern populations were under strong genetic influences from Altaic populations from the north. But it is unclear how Altaic populations migrated to Northeast Asia. It is possible that ancestral Altaic populations arrived there from middle Asia, or alternatively they may have originated from East Asia.
The analyses of metric and nonmetric cranial traits of modern and prehistoric Siberian and Chinese populations showed that Siberians are closer to Northern Chinese and Mongolian than European (26, 27). The same notion holds for the facial flatness (26–28). European populations did not appear in Siberia, western Mongolia, and China until the Neolithic and Bronze Age (26, 27, 29, 30). Furthermore, cranial and dental analyses have linked the Arctic peoples, Buryat and east Asians with American Indians (31–35), which arrived through Beringia (Bering land bridge) somewhere between 15,000 and 30,000 years ago (36). These observations are generally consistent with the genetic evidence based on this research and mitochondrial DNA data (37–40). Therefore, it is more likely that ancestors of Altaic-speaking populations originated from an East Asian population that was originally derived from Southeast Asia, although the current Altaic-speaking populations undeniably admixed with later arrivers from mid-Asia and Europe (see Fig. ​Fig.2,2, thin solid lines). The possibility of early northern route migration from mid-Asia to Siberia is doubtful, given the fact that the last glacier started to recede only 15,000 years ago (see Fig. ​Fig.2,2, dashed lines).
This conclusion can be tested by using simple inductive logic. If the ancestral Altaic-speaking population was of northern origin, the genetic relationship of extant populations should follow the phylogeny presented in the bottom of Fig. ​Fig.3.3. The phylogeny generated in the current study apparently supports the upper phylogeny of Fig. ​Fig.3.3. In this analysis, Altaic populations are represented by Buryat and Yakut. Southern Chinese populations are those populations from Yunnan and Taiwan that reportedly did not have any admixture with Altaic populations. Populations from Middle Asia were not available to this study.
Figure 3
Phylogenetic relationships of worldwide populations under two hypotheses; see text for discussion.
Now that we have established that populations in East Asia were subjected to genetic contributions from multiple sources: Southeast Asia, Altaic from northeast Asia, and mid-Asia or Europe. It would be interesting to estimate relative contributions from each source. Unfortunately, the current study involved only mostly minority populations. A study involving populations across the country is necessary to reveal such a picture.
