In 2015 a paper was published which tried to model the major admixture events of the last 3,000 years in West Eurasians. Table S5 provides a list of all the computed events and guesses of their magnitude/timing: cell.com/current-biolog… I want to talk specifically about the Greeks
The Greek cluster is modeled as 65/35 mix of two populations most like today's Cypriot Greeks/Lithuanians and the estimated date of the mixture is 630 AD, which would conform with the arrival of Slavic tribes onto the mainland.
Here, non-Slavic Lithuanians serve as a better representation of Proto-Slavs than say, Poles because of their greater isolation (Balts are the most European Europeans) These numbers explain a bit: for example, why Greeks are less Middle Eastern than South Italians.
The Slavic migration to Greece is well known, but very little talked about. I read Norwich's great little history of Byzantium and I don't think they got one full page of attention, despite contributing a huge chunk of the ancestry of today's Greeks. People don't care about Slavs
To put this in perspective, David at Eurogenes modeled Mycenenan Greeks as being as about 20% steppe herder and 80% Minoan. If these numbers are right, that would mean that Slavs had a greater genetic impact on today's Greeks than the actual Greeks themselves did.
The principal source of ancestry belongs to the mysterious Pre-Greek population which donated the consonant clusters -nth and -ss as in Corinth or Thalassa and which may have been Pelasgian (who may themselves have been Lemnians).
