I don’t really understand much of this but have often been struck by how certain groups seem to be really different from close neighbors or even majorities in their own land.
For instance, the Bushmen of Southern Africa, the Finns, of course the Ainu, and some American Indian tribes don’t look anything like others.
I used to work at a Summer retreat which had a large number of Japanese students come through every Summer. The Japanese seem to be two separate groups. The fairly tall, sort of wide faced ones and the small, delicate, and often very pretty or handsome ones.
The ancient Greeks were composed of two groups. The Dorics typified by the Spartans who were tall and often fair looking and the Ionians who were smaller, and appeared more like what we call Mediterranean complexion today. Despite the differences, they spoke the same language and had the same ancient history and religion.
People keep migrating, to this very day.
Some groups travel very far as a group, or in in a wave of groups. The Finns, whom you mentioned, have a very distinct language from other Scandanavians - a whole different language group. Finnish langauge is of the Uralic group, mostly distributed in modern-day Russian (but also Hungarians and Estonians).
Central Asian people repeatedly covered long distances in large groups in relative modern times. After the fall of the Hunnic period in what is now Mongolia, Huns traveled to gates of Rome, where the Pope came out to negotiate with Attila, and married a Roman princess to him. They founded Hungaria. The Bulgars from northern Central Asia migrated to found Bulgaria. It was possible even in ancient times to travel great distances by land in a single generation
Three big waves of migratory peoples came from Asia to America over about 15,ooo years to populate the Americas, before the major European migration after Columbus.
It is entirely possible that a large group developed ocean-going technology during the Stone Age (like the Polynesians did later, but with Stone Age technology), aand lost the technology in a few generations after migrating a great distance.