Well, language certainly does evolve. I know that English that is more than 400 years old, is almost a different language. Old Chinese writings also are quite different from modern Chinese. I think linguists refer to this as linguistic drift. Some causes are political; another people enter a territory, such as, the French invasion of England in 1066. Other causes are technological; new phenomena require new language to describe. But, I blame teenagers for most of linguistic drift, because they want to be independent of their parents and make up new ways of expressing themselves with their friends. Some fashions catch on, others fade away, but over time, the language changes.