Indo-European is originally a linguistic term, referring to the Indo-European language family. By extension, it became a collective name for cultures and religions associated with these languages. Hypothetically, these cultures arose from the expansion of an ancient people, the Proto-Indo-Europeans, possibly originating from somewhere around the Black Sea region from the 5th millennium BC onward.
See main article Indo-European languages.
The Indo-European language family is attested in twelve branches, some of them extinct, with a historical distribution over most of Europe, Anatolia, Iran, India and parts of Central Asia (East Turkistan). During the age of colonialism, Indo-European languages spread from Europe to all continents, and today there are over three billion speakers of Indo-European languages, distributed over all the world.
The languages are traditionally separated into a Satem group in the east (Balto-Slavic, Indo-Iranian, Armenian) and a Centum group in the west (Greek, Italic, Celtic, Germanic), according to their different treatment of PIE velar sounds. The two groups are considered paraphyletic, i.e. there are no separate proto-languages for each group and their common characteristics are likely due to prolongued contact because of their geographical proximity. Also, there is evidence that the Anatolian, Tocharian and Albanian branches belong to neither of the two groups.
See main article Indo-European studies.
The existence of the Proto-Indo-Europeans has been inferred by comparative linguistics. The discovery of the genetic relationship of the various Indo-European languages goes back to William Jones, a British judge in India, who in 1782 observed the strong affinity of Sanskrit, Greek and Latin.
The language group was briefly referred to as "Indo-Germanic", until it became apparent that the group included most of the other languages of Europe, as well. "Indo-European", the term now current in English, was coined in 1813 by the British scholar Sir Thomas Young. Franz Bopp performed extensive comparative work.
At first, the related languages were simply compared, with no attempt at reconstruction. August Schleicher was the first scholar to compose a tentative text in the extinct "common source" Jones had predicted. The reconstructed Proto-Indo-European language (PIE) represents, by definition, the common language of the Proto-Indo-Europeans.
In the 20th century, great progress was made due to the discovery of more language material belonging to the Indo-European family, and by advances in comparative linguistics, by scholars such as Ferdinand de Saussure.
See main article Proto-Indo-European.
The scholars of the 19th century that originally tackled the question of the original homeland of the Indo-Europeans (also called Urheimat after the German term), were essentially confined to linguistic evidence. A rough localization was attempted by reconstructing the names of plants and animals as well as the culture and technology. The scholarly opinions became basically divided between a European hypothesis, positing migration from Europe to Asia, and an Asian hypothesis, holding that the migration took place in the opposite direction.
However, from its early days, the controversy was tainted by romantic, nationalistic notions of heroic invaders at best and by imperialist and racist agendas at worst. The question is still the source of much contention.
In the twentieth century, Marija Gimbutas (1956) created a modern variation on the traditional invasion theory (the Kurgan hypothesis) which regards the Indo-Europeans as a nomadic tribe in what is today Ukraine, expanding in several waves during the 3rd millennium BC. Colin Renfrew (1987) is the main propagator for another theory according to which the Indo-Europeans were farmers in Asia Minor who migrated peacefully into southeast Europe from around 7000 BC where they assimilated the Preindoeuropeans.
The rise of Archaeogenetic evidence which uses genetic analysis to trace migration patterns added new elements to the puzzle. Luigi Luca Cavalli-Sforza recently used genetic evidence to in some ways combine Gimbutas' and Renfrew's theory.
PIE religion was centered on sacrificial rituals where animals were slaughtered to establish good relations with the gods. The chief god of the Indo-European pantheon, probably mirroring the position of the king in human society, was the sky-god *Dyeus.
These kings were ragarded as being able to "reach out and defend their people". This is an important fact, as the word reach is the only Germanic word to maintain a very well-known Indo-European root, meaning king, that can be seen in Latin rex, Irish rí, and Sanskrit raj.