среда, 27 февраля 2019 г.

The Steppe Maykop enigma

Who were the Steppe Maykop people exactly? Their ancestry must surely rank as one of the biggest surprises served up by ancient DNA to date.
I always thought that they’d turn out roughly like a mixture between populations associated with the Kura-Araxes and Yamnaya cultures (mostly because their territory was located sort of in between them). Nope, that wasn’t even close. This is where they cluster compared to Kura-Araxes and Yamnaya samples in my Principal Component Analysis (PCA) of world-wide genetic variation: the Global25.



To explore the ancestry of the Steppe Maykop people in more detail I ran a couple of unsupervised Global25/nMonte tests, using basically every ancient population in the (scaled) Global25 datasheet that seemed chronologically sensible and even remotely relevant. I narrowed things down to these two mixture models.



Steppe_Maykop
Geoksiur_Eneolithic,11.2
Piedmont_Eneolithic,44.4
West_Siberia_N,44.4
distance%=1.5161
Steppe_Maykop
Piedmont_Eneolithic,46.6
Sarazm_Eneolithic,10.4
West_Siberia_N,43
distance%=1.6408



But, you might say, Global25/nMonte isn’t a published analytical method and it doesn’t run on formal statistics, the meat and potatoes of ancient DNA papers. OK then, let’s try the same models with the qpAdm software, which is a published method and does run on formal statistics, using exactly the same samples.



Steppe_Maykop
Geoksiur_Eneolithic 0.100±0.032
Piedmont_Eneolithic 0.433±0.053
West_Siberia_N 0.467±0.028
chisq 19.155
tail prob 0.159096
Full output
Steppe_Maykop
Piedmont_Eneolithic 0.429±0.051
Sarazm_Eneolithic 0.119±0.033
West_Siberia_N 0.452±0.026
chisq 18.090
tail prob 0.202699
Full output



Basically identical. Importantly, my models must reflect reality at some level, otherwise it’s extremely unlikely that I’d be able to produce a pair of essentially identical results using two such vastly different statistical methods. So the pertinent question is what do these results actually mean?
I didn’t get a chance to put together a map for this blog post. I’ll try and do that tomorrow, because when looking at the locations of the potential mixture sources in my models, it seems unlikely to me that we’re dealing here with a highly complex three-way mixture process, including populations from such far flung locations as western Siberia and southern Central Asia. Rather, I suspect that Steppe Maykop was the result of a two-way mixture between Piedmont_Eneolithic (the population that lived before it on the steppe north of the Caucasus) and someone just a little bit more easterly. I’m guessing that the latter was the (as yet unsampled) population associated with the Kelteminar archeological culture.
Like I say, I’ll add to this blog entry tomorrow. Meantime, feel free to let me know in the comments below if there are models that more accurately capture the ancestry of the Steppe Maykop people, and I might incorporate them into my effort. See also…
On Maykop ancestry in Yamnaya
Big deal of 2018: Yamnaya not related to Maykop
Late PIE ground zero now obvious; location of PIE homeland still uncertain, but…

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