There’s a new paper (by Iosif Lazaridis and others) out on the genetics of the world’s first farmers in the Middle East. There are a number of interesting points.
They find that early farming populations are about half descended from the somewhat mysterious “Basal Eurasians”, who apparently split off from other Eurasians before they separated from each other. Those Basal Eurasians appear to have little or no Neanderthal admixture, but they are no closer to sub-Saharan Africans than other Eurasians. Which they means that they probably left sub-Saharan Africa a long time ago – but where then did they go? Somewhere without any Neanderthals? North Africa might work: we have no evidence that there were ever any Neanderthals there. But if the Basal Eurasians didn’t mix with Neanderthals, they likely mixed with someone else. Someone lived in North Africa before modern humans: we might see a trace of that in populations with Basal Eurasian ancestry. With luck and some ancient DNA from North Africa (or possibly South Arabia?), we might find the answer.
Genetic differentiation was much stronger back in those days. Fst between Natufians and the hunter-gatherers in the Zagros mountains ( western Iran) was comparable to that between Germans and Chinese today. You can bet that their languages were highly differentiated as well.
The first farmers seem to be descended from the hunter-gatherers that immediately preceded them. All the groups that picked up farming expanded outward: early Anatolian farmers into Europe (LBk and Cardial cultures), Levantine farmers into Africa (so Hamito-Semitic must have originated in the Middle East). it looks as if ur-Georgians mixed with eastern hunter-gatherers that were closely related to ANE (75%) to form the proto-Indo-Europeans, which means that pre-PIE was spoken by those eastern hunter-gatherers, and the similarities between Kartvelian languages like Georgian and Indo-European may boil down to pillow talk and lullabies.
By the Bronze age Natufians and Zagros mountaineers and Anatolian farmers were mixing a lot – but before agriculture, such mixing must have been very rare for a long time, in order to generate that big Fst. There was probably more trade with the advent of agriculture ( more mixing) , and later, technical developments like ships and wheeled vehicles probably favored mixing. Farmers can have specialists, who may make use of exotic materials ( like tin or lapis lazuli) than are imported over long trade routes. Eventually there were empires, some of which seem to have shuffled ethnic groups around the chess board simply because they could. Probably the horrible Ice Age climate played a role in keeping populations isolated before the Holocene.
But back before the Holocene, it seems that, more often than not hunter-gatherers either didn’t mix or exterminated each other. It looks as if there were two waves of replacement ( with little admixture) in Europe after modern humans replaced Neanderthals and before Anatolian farmers largely replaced the last population of European hunter-gatherers!
We see a couple of cases in which new populations are found almost entirely with males from one population and females from another: early Indo-Europeans and Amerindians.