OK, after posting "Cats on a Slide" (the post below) I feel I have to post something with more intellectual content.
Richard Hershberger suggested reading Don Ringe's this blog post on "Language Log". This is long for a blog post, but a fascinating summary of some theories of how languages developed.
I just finished Cunliffe's "Europe between the Oceans", which is a "history" of Europe from 9000 BC to 1000 AD, which covers this same geography and time period from an archaeological perspective, and it was interesting to contrast some of the points made.
Ringe notes that "no single language can occupy, for more than a few centuries, an area too large for all its native speakers to communicate with each other regularly." Once a language spreads too far out, the language tends to diverge and split into dialects or perhaps a new language because it develops in different directions as new experiences are encountered.
In contrast, Cunliffe notes that artifacts and technologies tend to spread via trade and perhaps migration -- so bronze and later iron spread across Europe and artifacts made in Greece might easily end up in the Baltic region. A technology can spread over a wider area than a language and remain relatively the same (although the artistic style of the artifacts might change).