One of the first things I wrote at length about historical population genetics, in late 2002, happened to be a rumination on the Y chromosomal phylogeography of Finnic peoples. At the time there was debate as to the provenance of the N1c Y chromosomal haplotype (this is the haplotype of the Rurikids by the way). Just as R1b is ubiquitous in Western Europe, and R1a in Eastern Europe (and to some extent in Indo-Iranian lands), N1c has an extensive distribution in the northern zone of Eurasia.
The question at the time was whether N1c was from Europe and in particular the Finnic peoples, or, whether it was from Siberia.
Today we have many of the questions resolved. At this point, we know that the Finns, Sami, and Estonians, all exhibit evidence of gene flow from a Siberian-like population. This is clear on any genome-wide analyses. Though this is very much a minority component, even among the Sami, because it is genetically very different from the Northern European background, it is clear on any analysis.
Ancient DNA has also established the likelihood that this Siberian-like element is relatively new to the Baltic region. In a recent paper, The genetic prehistory of the Baltic Sea region:
We suggest that the Siberian and East Asian related ancestry in Estonia, and Y-haplogroup N in north-eastern Europe, where it is widespread today, arrived there after the Bronze Age, ca. 500 calBCE, as we detect neither in our Bronze Age samples from Lithuania and Latvia.
This is not the only ancient DNA paper that shows this. Of course, sampling is imperfect, and perhaps they’ve missed pockets of ancient Finnic peoples. But the most thorough analysis of Mesolithic hunter-gatherers in Scandinavian does not pick them up either, Population genomics of Mesolithic Scandinavia: Investigating early postglacial migration routes and high-latitude adaptation. Populations, such as the Comb Ceramic Culture, which have been identified as possible ancestors of the modern Finnic culture and ethnicity, lack the distinctive Siberian-like component.
At the SMBE 2017, I saw a poster which had results that were sampled from Finland proper, and distinctive ancestry of Siberian-like peoples was present in an individual who lived after 500 AD. This means that in all likelihood the circumpolar Siberian population which introduced this new element into the East Baltic arrived in the period between 500 BC and 500 AD.
Someone with more knowledge of paleoclimatology and archaeology needs to comment at this point. Something happened in this period, and it probably left a big ethno-linguistic impact. But I don’t know enough detail to say much (the Wikipedia entries are out of date or don’t illuminate).
I will add when I run Treemix Finns get the Siberian gene flow you’d expect. But the Lithuanians get something from the Finns. Since the Lithuanians have appreciable levels of N1c, that is not entirely surprising to me (the basal flow from the Yakut/European region to Belorussians may be more CHG/ANE).
Additionally, I will note that on a f-3 test Lithuanians have nearly as high a z-score (absolute) as Swedes (i.e., Finn; Swede/Lithuanian, Yakut), indicating that the predominant Northern European ancestry isn’t necessarily Scandinavian, as much as something between Lithuanian-like and Swedish-like (on Admixture tests the Finns do seem to have less EEF than Swedes, and Lithuanians probably the least of all among non-Finn peoples).
Addendum: I should note here that the genetics is getting clearer, but I have no great insight into the ethno-linguistic aspect. Perhaps the Siberian-like people did not introduce Finnic languages into the Baltic. Perhaps that was someone else. But I doubt it. That being said, though the Siberian-like component adds great distinctiveness to the Finns, it is important to add that by and large Finns are actually generic (if highly drifted) Northern Europeans.