A few months ago someone asked me (via email) which populations I would love to get typed (genetically that is). There is one population which did not come to mind at the time: the Sumerians. Why? Because these are arguably the first historic nation. The first self-conscious ethnic group which operated by the rules which we define as the fundamentals of literate civilization. Strangely, they are an ethno-linguistic isolate. My own assumption until lately has been that this is not too surprising, in that prior to the rise of expansive civilizations (Sargon of Akkad) there was much more linguistic and ethnic diversity than we currently see around us. Or, was evident even in the early Iron Age. In other words, the ancient Fertile Crescent may have resembled the highlands of Papua, with Hurrians, Akkadians, Gutians, Elamites, Sumerians, etc., all speaking mutually unintelligible dialects which diverged very far back in the mists of antiquity.
I am no longer quite so sure about this model. That is largely due to the possibility that there was a great deal of demographic change between the Mesolithic and the Bronze Age, with successive waves of layering and replacement. My rough model is that a few groups of farmers may have expanded to swallow up thousands of hunter-gatherer groups. These homogeneous farmer societies eventually would diversify, because they were not united by the institutional forces which cemented later imperial regimes, in particular, literate elites which had a sense of consciousness which extended deep into the past because of written records. Therefore, the diversification would presumably have been similar to what we see with Romance languages, or Indo-Aryan, branching out from an common root language which replaced many competitors rapidly. Without writing and large scale polities the divergence would be more rapid, and there would be many more tips on the phylogenetic tree.