This was the Chernoles culture - arose from a contact between proto Balts? and proto-Scythians? in the Dniepr region maybe? (Dniepr Balts)
Main mistake that people often makes is to separate Balts from Slavs (ie Sarmatians). So, I will tell only few words here.
Think logically. If one people inhabit island, what is the origin of that people? Logically, same people that inhabit closest landmass is ancestral people of people on that island. That is as a rule. See, same is with coasts. If one people hold coast, it is impossible to separate that people from nearest people deeper in the inland. Logically, its one and same people. Again, its a rule.
So, that about origin of the Balts. They were Sarmats, pure and simple. Difference, if existed, was only conditional and still any difference don`t change fact about Sarmatian origin of the Balts.
But, if we insist to underline difference between Balts and rest of Sarmats, that difference could come only as result of gradual influence on Balts by foreign elements that are usually present on the coasts due to trade and influx of foreign ideas, languages and people thanks to that trade. Sure, theoretically, something also could happen (invaded, natural catastrophe, etc) with the bulk of population in the inland while people on the coasts may stay isolated from the event and that way start to differ from their ancestral population.
Its nothing new. It happened all-around Europe with Sarmatians. And not only in Europe. On the coasts, some islands, on the fringes of realm of our ancestors, people started to change, to differ from their original ancestors.
And its alright. Its evolution. What isn`t right is that those who changed, in many cases, tried/tries to annihilate those who refuse to change. And no, we don`t speak about refusing to change in the sense refusing to progress. Its not that. Those who stayed to exist as Sarmats until our times are those who, for some reason, decided to stay loyal to the ancestral culture and language. And it was and is their right.
If we Europeans refusing truth about our past and history for political reasons, what future do we have? Shall we falsify science itself to satisfy politics? Is there any future for any sentient species in the universe, if decide to ignore scientific facts? No. Not of course.
So what shall be? Truth must prevail. Sure, that truth, political consequences of that truth, suit to somebody more and to somebody less. With time, we will learn to live with it. We will invent new reasons to support and explain our different paths.