PRE- Indo-Europeans

Not Proto-Indo-Europeans, whom everyone usually talks about.  Immediately before that. 

I wrote a longer post which I barely understood myself about six months ago. It is more detailed, but also less coherent. I am giving the information to you in shortened form mostly for myself, to lock the overview in. The rest of you are just along for the ride, in the hope that you may be interested in the simplified version yourself.

There was a group on the Lower Volga that was mostly Caucasian Hunter Gatherer. Some of them moved south through the Kuban steppe and the Caucasus into Anatolia near Trebizond on the NE coast of Turkey on the Black Sea. Those became the Hittites and other now long-extinct groups. Another group moved west toward the Dniestr and mixed with Ukrainian Neolithic Farmers and a half-dozen other groups. Everyone sat and fermented for a while, still mixing with nearby others, and then suddenly burst out in all directions (called a star phylogeny) conquering their neighbors. Some of them keep the women as wives or slaves while killing all the males, others kill everyone. 

They thus start to differentiate, but the men speak the same language, have the same customs, and maintain trade connections with each other. The similarities fade, and trading narrows to nearer groups.

One of the downstream star points are the Corded Ware, which give way to the Bell Beakers, groups that replicate the previous expansion of fermenting for a few centuries, then exploding into a star phylogeny of its own. Most but not all European ancestry comes from this explosion.