Genetic Chaos

Monday, May 10, 2004

The Place of the Indian mtDNA Variants in the Global Network of Maternal Lineages and the Peopling of the Old World

Both archaeology and genetics suggest that modern humans originated 100,000 to 200,000 years ago in Africa (Cann et al. 1987; Stringer 1990). Their first skeletal remains outside Africa are about 100,000 years old but have been found so far only in the immediate vicinity - from the caves in Near East (McDermott et al. 1993; Stringer 1992; Stringer et al. 1989; Aitken and Valladas 1992). There is no substantial evidence supporting further spatial dispersal of modern humans earlier than around 50,000 years ago. By that time they seem to have reached Papua New Guinea and Australia and soon after that they are found also in Europe. What happened during this 50,000 year long gap and where did the initial radiation of the Eurasian population take place remains largely an open question. Western Asia and India stand geographically on the road early modern humans had almost inevitably pass to reach eastern Asia, New Guinea and Australia. Did some of the migrating waves of humans settle there instead of going in corpore further eastwards? Was it the place where the initial radiation of Eurasian mitochondrial DNA lineages took place? There is a lack of skeletal evidence of modern humans from East Asia older than the Upper Cave Zhoukoudian crania (Foley 1998) that are dated to around 30,000 years before present (BP). The earliest skeletal evidence from South Asia comes from Sri Lanka, where the Fa Hien Lena finds put forward 34,000 year old carbon datings (Deraniyagala 1998). These archeological dates imply the approximate time line for the dispersal of modern humans in Asia. Driven by fast mutation rate and lack of recombination, distinctive clusters of mtDNA lineages have emerged during the last tens of thousands of years. Low overall population density during Palaeolithic and vast geographic distances favoured the isolation of human populations and thus played an important role in secluding the differences arisen in DNA lineages. The present day mtDNA variability is highly continent-specific (Chen et al. 1995; Torroni et al. 1996; Wallace 1995). Therefore, already at the level of present day knowledge about the worldwide variation of mtDNA genome, one can reliably distinguish between mtDNAs of eastern Asian, European or sub-Saharan African origin (Fig. 1). Nevertheless, the same knowledge base shows that all mtDNA variants outside Africa, studied so far, derive from a single Pan-African mtDNA cluster L3a (Watson et al. 1997).

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