Razib Khan
Fourteen years ago, a UC Berkeley team studying altitude adaptation in Tibetans developed a method to scan the genome of various modern populations, looking for outlier gene frequencies compared to related populations. First, they compared Tibetans, Han Chinese and Northern Europeans, looking for genomic regions where Tibetans were the exception, the outlier. This approach identified EPAS1, a gene implicated in high-altitude adaptation, as a likely target of positive selection in Tibetans years before it was discovered that this gene introgressed from Denisovans.
This sort of technique obviously requires a good grasp of the genome’s broader phylogenetic patterns. You need some sense of the history of the populations to infer the peregrinations of specific genes. Next, calculating the timescale of divergences between Tibetans and Han Chinese, the authors estimated the two populations split 2,750 years ago. The problem with this estimate is that it places the proto-Tibetan stream’s separation from the proto-Han one at 750 BC, at least a millennium after we know the Han already existed as a people, and centuries into their long written history. Of course, this estimate, as estimates generally do, has multiple simplifying assumptions baked in that might be incorrect. For example, Han Chinese and Tibetans were both modeled as homogeneous groups at the tips of a bifurcating tree, rather than mixed populations. If Tibetans were a mix of two populations, each with distinct historic relatedness to the Han Chinese, then the average divergence estimate may have been misleading.
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