Reader Matt points me to two new papers on the linguistic phylogenetics of the Sino-Tibetan language families, Dated language phylogenies shed light on the ancestry of Sino-Tibetan and Phylogenetic evidence for Sino-Tibetan origin in northern China in the Late Neolithic. You should read Matt’s whole comment, but one thing he mentions is that by ~3,000 years ago, individuals who were genetically similar to modern Burmese were already present in the territory of modern Burma. Burmese are quite distinct from Cambodians or Vietnamese because there is a distinct “northern” element, which perhaps resembles Tibetans.*
Matt observes that this means the expansion of agriculture into Southeast Asia occurred through a few pulses in rapid succession, rather than gradually over time, as seems likely the case in Europe and South Asia (“Early European Farmers” to Corded Ware, or Iranian agriculturalists to Central Asian agro-pastoralists). Austro-Asiatic speaking groups pushed out from the highlands of southern China 4-4,500 years ago. Meanwhile, people from further north seem to have pushed into the uplands of the western portion of upland Southeast Asia 500 to 1,000 years after this. Further east, Austronesians were sweeping along the coast and expanding into the maritime fringe.
But I am not intending to talk about Southeast Asia. Rather, I want to focus on China. Or perhaps more precisely the region and cultures that became China. Both the above papers suggest that the diversification of the Sino-Tibetan languages occurred around ~7,000 years ago. And, that they began expanding from the zone of inland China, the upper Yellow River basin, from the area occupied by the Yangshao culture. This would explain some peculiar genetic facts. First, the northern affinities of Tibeto-Burman groups in northeast India and in Burma itself (which might otherwise require later Tai migrations) mentioned above. But second, about ten years ago when the early work on EPAS1 and high altitude adaptation was done on Tibetans, their genetic relatedness to Han Chinese was surprisingly close! In fact, some estimate of divergence put it as recently as 3,000 years before the present (I think this was an underestimate, but it gets at the qualitative result).