European philologists had long known that many European languages shared similarities, but in 1786 Sir William Jones demonstrated that the Celtic languages, the Germanic languages, Latin, Greek, and Sanskrit (an ancient language of India) all had such deep parallels that they had to have derived from a common predecessor. In 1833, Franz Bopp published his Vergleichende Grammatik, which took the first consistently successful steps toward the reconstruction of the "proto-language;" Bopp also added to the family the Slavic languages, Armenian, and Avestan (an ancient language of Persia - modern Iran).

The Nineteenth Century and Effects of Romantic Nationalism

Romanticism had a deep effect on nineteenth century European worldview. One outgrowth of this intellectual climate was Romantic Nationalism, the notion that each "culturally distinct" group of people was a "folk nation" and had the right to be its own political entity. One demonstration of cultural distinctiveness was a distinctive language, and this assumption of a one-to-one correlation between the folk nation and some particular language naturally led scholars considering the "Indo-European problem" to assume the existence of a single cultural group speaking a single language. Today this hypothetical ur-language is called proto-Indo-European (PIE) and the people who are presumed to have spoken it are called the proto-Indo-Europeans.