What other theory do you need, they are in all probability humans. Sounds like a time to refer to Occam's razor. The simplest explanation is usually the correct one.
It has been shown that animals cannot cross bread except with creates of extremely similar DNA, and even there direct offspring cannot produce any kind of child. Yet there are many non Scandinavians who have children with Scandinavians, what does this tell us. That they are human. Also no oone has presented any evidence of this outrageous claim. I_Stab_People said "There are theories that seem to prove" but he did not say what these were and a theory is a theory not in any way evidence. But there is lots of evidence against such a theory. As i said before about the crossbreeding and the improbability of aliens visiting earth. And it should be noted that most scientists think that once you leave this universe you simply do not exist, rather then move into a new one. Also many surveys have been taken about the DNA structure of particular ethnic groups across the world, which has gone through Scandinavia and no none human DNA was found.
Something that has the same DNA as a human is a human. And I very much doubt that a completly independent evolution with a completely different environment could create an alien identical to being human.
It has been said that absence of evidence is not evidence of absence, which is true. Where there should be evidence of the Scandinavians being aliens there is no, so we can safely conclude that are human.