Hey, we take our good news where we get it.
Here we successfully sequenced 67 complete mitochondrial DNA genomes of 5200 to 300-year-old humans from the plateau. Apart from identifying two ancient plateau lineages (haplogroups D4j1b and M9a1a1c1b1a) that suggest some ancestors of Tibetans came from low-altitude areas 4750 to 2775 years ago and that some were involved in an expansion of people moving between high-altitude areas 2125 to 1100 years ago, we found limited evidence of recent matrilineal continuity on the plateau.
Congratulations to the authors. I enjoyed this paper and hope they have more in the works.
Skipping past some of the technical discussion, let’s get down to the nitty-gritty of what it all means:
The haplogroup networks and haplotype–haplogroup sharing demonstrated to us that there was partial matrilineal continuity in Tibetans from 5200 years ago.
That means modern-day Tibetans descended from a mix of peoples, some of whom have been there for over 5,000 years, and some of whom arrived recently.
Under this continuity, some people spread from low-to-high altitudes 4750 to 2775 years ago and some expanded within high-altitude areas 2125 years ago. The timing revolved around the high-altitude agriculture transformed by barley, which appeared 5.2 ka near the northeastern edge of the plateau and moved into high altitudes by 3.6 ka .
Farmers. I wonder how difficult it was to get barley to grow up there. Tibet seems like a pretty harsh environment.
However, based on the 16 haplogroups that have a frequency in Tibetans (a subset of 21 unique haplogroups), D4j1b and M9a1a1c1b1a would represent about 13% (2 out of 6) as the footprint of that event. Thus, our findings did not favour a substantial migration of lowland farmers to the high-altitude areas.
So, farmers did expand into Tibet, but not a ton of farmers (or at least, not a ton of farming women.) Probably because Tibet is a really harsh place–both for people and barley strains that aren’t adapted to living there.
An explanation for the surplus of unaccounted maternal lineages could be that there were earlier waves of populations who settled into higher altitudes and underwent isolation by distance . The earlier settlers were potentially hunters and gatherers who left behind no human fossils, perhaps connected to the blade tool assemblages or fossilized handprints and footprints dating to as far back as 40–30 ka  or 13–7 ka . Our results could support a recent diffusion of plateau populations into an otherwise stable population continuous with previous high-altitude populations. A similar point of view has been made from analysing the whole genomes of present-day Tibetans .
What would being a hunter gatherer in Tibet have been like?
I figured there was probably some ancient population that has contributed to the modern Tibetan population both because of the aforementioned environmental difficulties, and also because the Tibetans show adaptations to the area, which take time to accumulate. Among those adaptations, Tibetans have some DNA they appear to have picked up from the Denisovans, and Denisovans probably haven’t lived in Tibet in a very, very long time.
Take care and stay healthy, everyone.