Even the experts havent any idea. They are guessing even for WW2.
That whole region has been nothing but one big constant war since the Vikings took control of it. Before that..the Muslims and even then much speculation even to the Greeks who had outposts on there.
But, if one has to ID them as Russian, then it goes to before the Rurik dynasty. Kievan Rus, which was an offspring of the Novgorod Rus. If one REALLY wants to understand..The Ukraine was largely Cossack at one time.
The word is supposed to be derived from a Nordic word meaning Swede or something like that...Viking.
DNA studies have been done and the present Russians have been found to be more aligned with the Finnish peoples than the Ukrainians who are a mixed bag. Which would makes sense since the Crimea,etc, has been the area of many ethnic groups.
Which also make sense to me since the fins are the most stubborn people I have ever known. And they can be heartless. Believe me I know.
“Before that..the Muslims”
Correction..the mongols came along after the Vikings and then it converted to Muslims.
In other words..it is one mixed bag of particularly historical nasty ethnic groups.