An interesting paper in PNAS, Rare dental trait provides morphological evidence of archaic introgression in Asian fossil record:
The recently described Denisovan hemimandible from Xiahe, China [F. Chen et al., (2019) Nature 569, 409–412], possesses an unusual dental feature: a 3-rooted lower second molar. A survey of the clinical and bioarchaeological literature demonstrates that the 3-rooted lower molar is rare (less than 3.5% occurrence) in non-Asian Homo sapiens. In contrast, its presence in Asian-derived populations can exceed 40% in China and the New World. It has long been thought that the prevalence of 3-rooted lower molars in Asia is a relatively late acquisition occurring well after the origin and dispersal of H. sapiens. However, the presence of a 3-rooted lower second molar in this 160,000-y-old fossil hominin suggests greater antiquity for the trait. Importantly, it also provides morphological evidence of a strong link between archaic and recent Asian H. sapiens populations. This link provides compelling evidence that modern Asian lineages acquired the 3-rooted lower molar via introgression from Denisovans.
You need to look at the supplemental Excel table. This morphology is not unknown among Africans and Europeans, so there is a base rate, though the authors seem to indicate this can be attributed to novel mutation. I don’t understand why introgression is more likely (from Denisovans) than selection on standing variation?
From the table it looks like frequencies in Jomon, ancient Taiwanese and early Malays are low and closer to West Eurasian numbers, which is more indicative of selection.
OTOH Bantus and Ugandans, furthest into SSA among the samples, have 0% so maybe there was both introgression and selection in Eurasians.
If it really is introgressed, it would have to be a substantially monogenic trait, but nothing is said about Mendelian inheritance or linkage?
why monogenic? it would be simpler. but admixture could be like introducing more standing variation…
Introducing more standing variation is possible, but it’s harder to imagine so strong a simultaneous selection on multiple loci in a population where the percentage of the introgressed DNA is small. Not if ALL of the loci are needed together for a trait expression