Today, in the English language, we call the third number three. The word derives from the Old English word thri, which is related to the German word drei. In Latin, the number was called tres, and in her daughter languages, French and Spanish, the word is trois and tres respectively. In Greek, the word is treis. In Russian it is tri. In Irish Gaelic, triúr. And in Sanskrit, the mother of most of the languages of the Indian subcontinent, the word is trayas. Are these similarities coincidence, or are all of these languages somehow related, perhaps as the descendants of a single, ancient parent tongue?
People have long speculated that their languages came from a single source; during the Renaissance, for example, scholars noted that languages seemed to group themselves in the ways they said "God": Latin Deus, Spanish Dios, vs. Swedish Gud, English God, vs. Russian and Polish Bog, etc. As early as 1767, physician James Parsons collected number-names from many European languages, as well as those of Iran and India, and found them to be quite similar and concluded that they must have all come from one source --Noah's Ark.
But Sir William Jones, Chief Justice of India, took this observation a step further. As an educated man, he was trained in the classical languages. When he came to India and began to examine Sanskrit he saw right away how similar it was to Greek and Latin, not only in vocabulary, but even in inflections and grammatical features. The three languages are so similar, both in the roots of verbs and in the forms of grammar, than could possibly have been produced by accident. So similar, indeed, that no scholar could examine all three without believing them to have sprung from some common source, which perhaps no longer existed.
Jones' speech to the Bengal Asiatic Society in 1786 contained the first clear declaration of the existence of an ancestor language for the languages of Europe, Persia and India -- an Indo-European "proto-language".
Building upon this observation, Rasmus Rask said that scholars had to do more than just use intuition to set up this hypothetical proto-language; they had to find regularities and systematic developments. For example, he noticed that certain sounds in Greek, such as the "ph" in "phrater" (brother) and "phero" (I carry) corresponded regularly to other sounds in Germanic, such as the sound "b", as in English "brother" and "bear". Jacob Grimm, famous as a collector of fairly tales, was the first to publish a systematic explanation of how Germanic must have diverged, in extremely consistent ways, from this proto-language. His explanation is now called "Grimm's Law."
The detailed reconstruction of the original Indo-European proceeded by stages, and by 1880 the main outlines of the language were established. Acceptance of the language grew during the 20th century, and the last few decades have witnessed a convergence of linguistics, archeology, prehistory and comparative religions that has resulted in a precise and comprehensive picture of the Proto Indo-European language.
The reconstruction of the proto-language, however, also established the existence
of prehistoric society, the community that created and used this language. Which
leads to the question: Who were the Indo-Europeans?