ratchet_fan
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I think the SE Asian origin of K2b is far from certain, it's mainly based on the haplogroups of modern populations. My hypothesis is that the origin was further west in India or Iran. There's no direct evidence for this, but my reasoning is that haplogroup F must have diversified fairly quickly between about 55 and 50 kya, leading to GHIJK, then HIJK, then IJK, then IJ and K. Before 47 kya K had diversified further to L, K2a, K2b and subclades of K2b (such as K2b1 in Sahul).
What's interesting about haplogroup K2 is that by 40 kya it had become very widespread from Europe to Sahul. The other haplogroups that descended from F, such as G, H, IJ and L seem to be less widespread at that stage. The only other Y haplogroup that was similarly widespread from Europe to Sahul at that time was C, particularly C1.
My admittedly speculative theory is that C1 and K2 (including K2a and K2b) co-migrated widely in HG bands that were not pure but mixed in their Y haplogroups. An example of this would be the initial settlement of Australia (Sahul) sometime before 47 kya. Since it involved an ocean crossing, the founding population must have been small, but it apparently had mixed haplogroups, including K2*, K2b1 and C1b2.
K2a was present early on in western Siberia (Ust'-Ishim 45 kya) and Europe (Oase 40 kya), suggesting an origin in Iran or western India, then a migration north to the Kirghiz Steppe. It's possible K2b diverged from basal K2* at a similar time and place as K2a, and that some subclades of K2b co-migrated north with K2a. Even the K2b in North China (Tianyuan) may have arrived via the steppe route rather than via SE Asia. Obviously at least one subclade of K2b did migrate to Sahul via SE Asia though.
full still underestimates so K2b might be closer to 50-55K old. I like your theory. Let's hope we get enough ancient DNA to figure out what really happened. So all these clades would have diversified prior to the east -west split of Eurasians. So they then admixed into East and West populations leading to M and P respectively in your opinion?