Were Croats and Serbs one people that were later (politically) divided, or two unrelated peoples that came in the same area and (linguistically) merged?
Before, I thought that Serbs and Croats were, in the near past, one people of which some adopted Catholic religion (now called Croats) and some Orthodox (now called Serbs). I still think that this is the case of Serbs and Bosnians: Bosnians are Serbs converted into Islam under Turkish rule, so no wonder they speak (almost) the same language. Hindi and Urdu is another example of such division.
The more I read, the more I feel Serbs and Croats are unrelated Slavic tribes which came to Balkans separately and never had shared history or culture. Not one people divided, but rather two unrelated peoples culturally and linguistically merged, as they happened to settle next to each other. Is this true?
And if they don't share (recent) common ancestry, how come they speak (nearly) the same language? I guess their unrelated ancestors spoke different (Slavic) languages. Have Croats adopted the Serbian language? Or vice versa?
I don't mean 19th century agreements of unification of the Serb-Croatian language, I mean why did they speak such similar languages in the first place long before the 19th century?