The first systematic theory of the relationships between human languages began when Sir William Jones proposed in 1788 that Greek and Latin, the classical languages of Europe, and Sanskrit, the classical language of India, had all descended from a common source. The evidence for this came from both the structure of the languages -- Sanskrit grammar has similarities to Greek and to nothing else -- and the vobcabulary of the languages. Thus, "father" in English compares to "Vater" in German, "pater" in Latin, "patêr" in Greek, "pitr." in Sanskrit, "pedar" in Persian, etc. On the other hand, "father" in Arabic is "ab," which hardly seems like any of the others. This became the theory of Indo-European languages, and today the hypothetical language that would be the common source for all Indo-European languages is called Proto-Indo-European. Source