title_body: Is there evidence that any natural languages were actually constructed? Is there any evidence that a ‘natural’ language that exists today was created by somebody a thousand (or two) years ago. And that it evolved till today?
upvoted_answer: Possibility? Of course, there is always a possibility, but rather remote one, if you mean a full featured well-known modern language.
  
  The best candidates are "taboo languages", where the lexical items are replaced because of taboo/religious pressure. Perhaps the best known is Dyirbal, where a special version of the language is to be used in front of "tabooed" persons.
  
  Though it probably does not go millennia into the past, and it's questionable if it should be considered a language, since it is just a vocabulary encoding.