First components of the alliance
On one side - the hunter-gatherers, roaming the forests and highlands, with seasonal interested in large rivers for seasonal fishing and transport.
On the other side - the first wave of Middle-easterners bringing the neolithic revolution in the region - farmers moving in and taking in use the fertile plains near the river beds of Danube and major tributaries.
As the burial places in the Danube gorges reveal "a picture of peaceful coexistence" side by side for many generations, meaning that they traded rather than fought, exchanging goods, genes and ideas. Farmers needed HG knowledge of nature in the region and survival skills when weather damaged crops. HG needed to supplement their diet in the winter and new blood for their small communities.
When next farmer waves came, fighting for good farming grounds was inevitable, pushing westward the descendants of previous wave. But by then those farmers closer to the HG communities would go upriver hiding and adapting to farm smaller fields between high mountains. These would become the nuclei of a population that found a way to stay in this region no matter what, using a combination of farming and HG strategies.
Later to the party
The component needed for this population to fare through millennia, until today in the region, was by that time on the move with other waves of people: pastoralists. Fast moving in search of greener pastures, pastoralist meant trouble for stable farmer communities. But for that mixture of farmers and HG living in the highlands, herding came to replace dwindling game. One particular tamed animal fitted well with their way of life: the sheep. Able to graze on high altitude lush grasses in the summer and walk even hundreds of kilometers to winter near the farms. The skill set forged by the early farmers and HG alliance made sheep herding and farming of the fields hidden between the mountains into a way of life lasting to this day in this region, the transhumance. The high mobility of sheep herds allowed for long distance trade and gene exchange so that remote communities from the mountains were connected to each other into a strong common culture throughout the region.
Wave after wave
When new masses of people were moving in, their first target were the fields near large rivers. Earlier settlers were either pushed away to the west or upriver toward the mountains. In time the older a population was in the region closer to the mountains they were.
Such a stratification can be seen by analyzing river and locality names between southern Carpathian Mountains and the Danube: most names are non Slavic and older than the arival of slavic speaking populations, but few major rivers watering the Danube plain got Slavic names to this day, like Prahova, Ialomitza, Dambovitza (citations needed).
Going back to population interactions, depending on their numbers, culture and weapon technology the incoming waves were more or less able to wage war to settle. Once settled in, trade, ideas and intermarriage were taking place with upriver neighbors. In centuries they were becoming locals and in turn their time would come to feel pressure from south and east. They faced same options of going up on Danube into middle of Europe or go closer to the mountains. But the later was possible only if they were accepted by the descendants of the early farmers - HG - pastoralist alliance. They had strongholds up in the mountains, easily defendable and a network of support from their kin throughout the region.
In millennia this network of support perpetuated a continuous blood line with additions from those incoming waves, that had the chance to stay longer in this region, with its members transmitting from a generation to the next the survival values that enabled their descendants thrive here to this day.
