Were Croats and Serbs one people that was 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 near past one people, of which some adopted Catholic religion (now called Croats) and some Ortodox (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 dividion.
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 ansestry, how come they speak (nearly) the same language? I guess their unrelated ansestors spoke different (Slavic) languages. Have Croats adopted Serbian language? Or vice versa?
I don't mean 19th century agreements of unification of the Serbo-Croatian language, I mean why did they speak so similar languages in the first place, long before 19th century?