This week it was announced that a new genetic study published in the journal Nature shows that the genomes of over 27,000 Icelanders display an admixture of modern human and Neanderthal DNA, along with a noticeable trace of Denisovan ancestry that was not expected at all. These findings suggest that past introgression took place between the direct ancestors of Icelanders, most obviously archaic Norwegians, and either Neanderthals carrying Denisovan DNA or Neanderthals and Denisovans separately. Similar results have not been noted in modern-day Scandinavians, either due to insufficient study of their genomes, or the fact that any residual Denisovan DNA has been diluted through interbreeding with other European groups with no traceable Denisovan ancestry.
So Denisovan DNA among modern-day Icelanders either came from Neanderthals, through their own introgression with Denisovans, or, alternately, the ancestors of the Icelanders had themselves encountered Denisovans somewhere in the ancient world, this all taking place many thousands of years before the establishment of the first Viking colony on Iceland during the ninth century AD.
The discovery of Iceland by the Vikings in 872. (Public Domain)