Family History - Northern Europe

Scandinavia (Denmark, Sweden, Norway): Inhabited by Germanic peoples (the Norse and their ancestors) from before the time period we are looking at. The northern inland regions of Norway and Sweden has been home to the Samek (formerly called the Lapps), a people speaking a non-Indo-European language, throughout the same time. So if your ancestors were Danes, Swedes, Norwegians, or Lapps, they would have been up in Scandinavia for much of this time. Indeed, this region has been a net donor of genes. But.....

In 1 CE, the Danes lived in southern Sweden and occupied what is now Denmark during the later Roman and early Medieval periods. Southernmost Sweden was considered Danish until well past 1490. Thus, if your ancestors lived in Denmark in 1000 CE, their ancestors probably lived in both Denmark and southern Sweden in 1 CE.

Finland: Various Finnish peoples (Karelians, Varsinais-Suomi, Tavastians) lived here throughout the interval we are looking at. Some Norse (Scandinavian) settlements on the coastlines since the 1100s. In the 18th to and 19th centuries, Finland was occupied and ruled by Sweden and Russia in turn. If your ancestors lived in Finland in 1800 or 1900, their ancestors could have lived in Sweden or Russia in 1490 or 1800. Family folklore and surnames should help you sort this out. In the cases of Karelia and Tavastia there have been Russian colonists since the 12th century.