The results of this study can be mostly explained by one word: slaves. When the Scandinavians went viking (yes, it's a verb), they didn't just loot, r*pe and pillage, and then turn around and leave without leaving serious scars on the lands and the peoples they invaded. They, like every culture back then, took captives, brought them back home along with the rest of their booty, bought and sold them, put them to labor, and bred with them.
Does anyone really think that these tribes only travelled abroad during the limited time span denoted in the article? Resources were scarce before 2400 BC too. Limited arable land, short growing seasons and a warrior culture built on paramilitary hierarchies were always factors that encouraged Norsemen to roam elsewhere and conquer where they could. There is no reason to think they wouldn't venture south to France, Spain, Italy, and east to the Ural Mountains (and beyond) in earlier times, just like we know they did in the times of written records.
Plus, who is to say that they didn't have visitors to their lands from foreigners? We don't know how far back in time the first trade routes were made in that part of the world, for amber, salt, copper and tin (!) and any number of "luxury goods." Trade, tourism, diplomacy, invasion, it all counts. Maybe even room for a couple of Picts (also a bad-ass warrior clan culture) whose background, experiences and values were so similar to that of the Norsemen that they fit right in?
No part of Europe is genetically homogenous, save maybe some enclaves like the Basques or the Rom who purposefully remain distinct from their neighbors. There were so many tribes and sub-tribes that moved in so many directions, so many times during and after the Bronze Age, and likely before it, too. I have studied this stuff for years, and I still don't have a handle on it. Even a cursory look at the Germanic family of tribes, from which the Norse, i.e. Danes, Swedes, Norwegians and Finns (and their regional subgroups, lol) descend, is enough to make your head spin. Because there are literally hundreds of them, from Franks to Goths, from Saxon to Suebi, and all points in between. They ebbed and flowed and mingled and warred with Celts and Slavs and Italic tribes and each other. It should surprise no one that these movements and meetings show up in ancient or modern DNA.
Frankly, I think it's exciting. Scientists are using modern technology to push back the limits of what we know, into the more distant past. It's a pretty cool study, it has brought to light new information, but I don't see anyone rewriting any of the main chapters in a history book. It's not exactly Gobleki Tepe. And, sorry to disappoint the ""inclusion" junkies in the audience, but according to the article, they haven't found any DNA from non-European groups in the results. Just a reminder before someone tries to insert some link from Scandinavia to deepest, darkest Africa. Uh, oh, too late, somebody already did.
The mutation to Blond and Blue-eyed was driven from environmental factors, and is NOT a recessive trait like albinism. I am not entirely convinced of the veracity of the "out of Africa" theory either. It doesn't make chronological sense. If humans and their cultures first emerged on that continent, the subsequent cultures would be MORE advanced than in Europe and Asia, not less. It's the same with the Native American cultures, the "early guys" in the north never progressed past neolithic art and technology, but the farther south/later you go, the more advanced the building, the metal working, the arts and crafts, the religions. It's ridiculous.