By David Cyranoski
Researchers mapping a massive array of genomes across Asia say they have found evidence that humans covered the continent in a single migratory wave, and share a common ancestry.
The findings were released by the Human Genome Organisation (HUGO) Pan-Asian SNP Consortium which looks at single-nucleotide polymorphisms (SNPs), or variations at individual bases that make up the genetic code. The results challenge the view that Asia was populated by at least two waves of migration.
"In Asia, we are all related," says Edison Liu, a lead author from the Genome Institute of Singapore. "It brings us closer together."
It is thought that a wave of humans emerged from Africa some 60,000-75,000 years ago and travelled along the southern coast of India, into southeast Asia and down to Oceania. But scientists struggled to explain some of the variation seen in Asia today -- such as the obvious physical differences between Malaysian and Filipino Negrito populations and other Asians. Some researchers have postulated that a second wave, or series of waves, from a northern route largely repopulated the area, leaving the Negrito and others as relicts of the earlier migration.
The new study, a five-year examination of variation at some 55,000 SNPs in 1928 individuals, found that Negrito populations had a high level of genetic overlap with other southeast Asia populations, suggesting a common ancestry. East Asians, the analysis suggests, share a large degree of common genetic background with southeast Asians but very little with central Asians, seeming to preclude a peopling of east Asia through a northern route via the Eurasian Steppes. And genetic variation within local populations decreased from southeast to northeast Asia. The two observations suggest that diverse peoples living in southeast Asia migrated northwards.
"It's an impressive collection of samples, a huge amount of work and analysis, and it will contribute greatly to the field," says Mark Stoneking, an evolutionary geneticist at Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who was not involved in the study.
Merely organizing the work was a massive task. Researchers in 11 countries and regions took samples from 73 populations, requiring countries often at political or economic loggerheads to share ideas, technology and genomes. For countries lacking the technological capabilities to do the genetic analysis but loath to ship genetic samples to another country, Liu established a system by which researchers could bring the samples to host countries and do the studies themselves, in collaboration with their hosts. "The chain of custody was never broken," he says. "It was extraordinarily collegial."
The result is not a complete shock. While this study provides the most detailed analysis of genetic diversity among Asians to date, a 2005 study on mitochondrial DNA came to a similar conclusion2. Martin Richards, at the University of Leeds, UK, is a specialist in genetic variation in southeast Asia who led that study. "By and large, [the new study] is not surprising for fans of mitochondrial DNA, I think, but naturally it is very heart-warming," he says.
The new study also supports mitochondrial DNA evidence that challenges the customary "out of Taiwan" model, in which migration from mainland China through Taiwan led to the settlement of southeast Asia and the Pacific islands. Instead it seems Taiwan may have been largely settled from islands in southeast Asia.
But the results are not conclusive, as the authors admit. Stoneking says he was "very surprised that the Negrito populations were not more genetically distinct", and would like to see other supposed relict populations, such as those in New Guinea and Australia, studied in the same kind of detail. He argues that it is not possible to tell whether extensive genetic intermingling with surrounding populations might have obscured evidence for two waves of migration. He says he has evidence to support the two-wave theory in work yet to be published that looks specifically at mitochondrial DNA and Y-chromosomes of Negrito populations.
Liu says he is discussing plans for a second phase study with much higher resolution -- based on 600,000-1 million SNPs. Possible extensions for the new project will be a look at copy number variation (duplications in sections of DNA), a resequencing of mitochondrial DNA and a focus on specific genetic components such as differences between enzymes that metabolize drugs, and human leukocyte antigen variations. It will be especially tantalizing, says Liu, to see if drug-metabolism genes show the same north-south variation in east Asia. "There would be implications for drug response and clinical trials," he says - although he adds that it will not be possible to link specific health information to genotypes across the continent.