Quote Originally Posted by sparkey View Post

Haplogroup I seems to be the most anciently European haplogroup that is still common. As a result, "When did Haplogroup I become IE?" is similar to answering the question, "When did IE get to Europe, and how long did it take to become dominant in Europe?". It's not an easy question to answer, although I think the most popular answer to "When did IE get to Europe?" is that it came first with Corded Ware culture. As for when it became dominant, it seems to have been a clear majority by the Classical Age, and the Classical Age helped solidify it.

An interesting twist is that there seems to be some I that may have never been IE... specifically the I2-M26 in Basques. (That doesn't mean, of course, that the pre-IE culture of Haplogroup I folks was Basque. We're not sure what it was.)

It's probably because I1 bottlenecked and spread from North Germany, close to Scandinavia, around the same time that the population of these regions expanded.
Mesolithic I seems to have been become more marginal after arrival of neolithic G2a, J, F, E1b1b and T.
With arrival of IE , some I-tribes seem to florish again, while neolthical tribes become marginal.
Why would I have been the natural allies of IE?