Language is constantly evolving. English, despite all attempts at standardisation, and the fact that cultures are no longer in isolation, changes every year. Pronounciation, spelling, slang, and grammar. Look at English as spoken in say, Jamaica, versus the east end of London, the Bronx or Brisbane.

Animals, too, have different dialects and languages. Birdcalls of the same species are often different in different regions, so is whalesong - pacific and atlantic groups of the same species sound different. Chimpanze and monkey calls etc also vary between tribal groups.

The fact is, language groups do have common origins and all languages can be seen to fall into the same patterns. Studying these patterns is a discipline. Extinct languages and protolanguages can be reconstructed by linguists. Look in Brittanica to see how, for instance, the Celtic groups are related, and related in turn to the Germanic and Latin goups from a common indo-european tongue that is in turn related. Language evolves in certain ways, with given rules, and this can be proven. Has been proven. Just like evolution in living creatures has been proven. That you decide you don't want to 'believe' the evidence is neither here nor there. It is utterly irrelevant.
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Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -
Albert Einstein