Something must have caused languages, even in the past and under traditional conditions, to diverge and to remain separate, in the face of all that contact. Here's how it happens. Any of us over the age of 40 has observed that languages change even over the course of just a few decades, with some words dropping out of use, new words being coined, and pronunciation shifting. For instance, whenever I revisit Germany , where I lived in 1961, young Germans notice that they have to explain to me some new German words (e.g, the new word Handi for cell phones, which didn't exist in 1961), and that I still use some old-fashioned German words that have been going out of use since 1961(e.g, jener/jene for "that/those"). But young Germans and I can still mostly understand each other well. Similarly, you American readers under the age of 40may not recognize some formerly popular English words like "ballyhoo," but incompensation you daily use the verb "to Google" and the participle "Googling" which didn't exist in my childhood.