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The Proto-Indo-European language (PIE) is the unattested, reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans. The existence of such a language has been accepted by linguists for over a century, and reconstruction is far advanced and quite detailed.
Scholars estimate that PIE may have been spoken as a single language (before divergence began) around 4000 BC, though estimates by different authorities can vary by more than a millennium. The most popular hypothesis for the origin and spread of the language is the Kurgan hypothesis, which postulates an origin in the Pontic-Caspian steppe of Eastern Europe and Western Asia. In modern times the existence of the language was first postulated in the 18th century by Sir William Jones, who observed the similarities between Sanskrit, Ancient Greek, and Latin. By the early 1900s well-defined descriptions of PIE had been developed that are still accepted today (with some refinements).
As there is no direct evidence of Proto-Indo-European language, all knowledge of the language is derived by reconstruction from later languages using linguistic techniques such as the comparative method and the method of internal reconstruction. PIE is known to have had a complex system of morphology that included inflections (adding prefixes and suffixes to word roots, as is common in Romance languages), and ablaut (changing vowel sounds in word roots, as is common in Germanic languages). Nouns used a sophisticated system of declension and verbs used a similarly sophisticated system of conjugation.
Relationships to other language families, including the Uralic languages, have been proposed though all such suggestions remain controversial.
There are several competing hypotheses about when and where PIE was spoken. The Kurgan hypothesis is "the single most popular" model, postulating that the Kurgan culture of the Pontic steppe were the hypothesized speakers of the reconstructed Proto-Indo-European language. Alternative theories such as the Anatolian urheimat and Armenian hypothesis have also gained acceptance.
The satemization process that resulted in the Centum-Satem isogloss probably started as early as the 4th millennium BC and the only thing known for certain is that the proto language must have been differentiated into unconnected daughter dialects by the late 3rd millennium BC.
Mainstream linguistic estimates of the time between PIE and the earliest attested texts (ca. nineteenth century BC; see Kültepe texts) range around 1,500 to 2,500 years, with extreme proposals diverging up to another 100% on either side. Proposed models include:
Indo-European studies began with Sir William Jones making and propagating the observation that Sanskrit bore a certain resemblance to classical Greek and Latin. In The Sanscrit Language (1786) he suggested that all three languages had a common root, and that indeed they may all be further related, in turn, to Gothic and the Celtic languages, as well as to Persian.
His third annual discourse before the Asiatic Society on the history and culture of the Hindus (delivered on 2 February 1786 and published in 1788) with the famed "philologer" passage is often cited as the beginning of comparative linguistics and Indo-European studies. This is Jones' most quoted passage, establishing his tremendous find in the history of linguistics:
The Sanscrit language, whatever be its antiquity, is of a wonderful structure; more perfect than the Greek, more copious than the Latin, and more exquisitely refined than either, yet bearing to both of them a stronger affinity, both in the roots of verbs and the forms of grammar, than could possibly have been produced by accident; so strong indeed, that no philologer could examine them all three, without believing them to have sprung from some common source, which, perhaps, no longer exists; there is a similar reason, though not quite so forcible, for supposing that both the Gothic and the Celtic, though blended with a very different idiom, had the same origin with the Sanscrit; and the old Persian might be added to the same family.
This common source came to be known as Proto-Indo-European.
The classical phase of Indo-European comparative linguistics leads from Franz Bopp's Comparative Grammar (1833) to August Schleicher's 1861 Compendium and up to Karl Brugmann's Grundriss published from the 1880s. Brugmann's junggrammatische re-evaluation of the field and Ferdinand de Saussure's development of the laryngeal theory may be considered the beginning of "contemporary" Indo-European studies.
PIE as described in the early 1900s is still generally accepted today; subsequent work is largely refinement and systematization, as well as the incorporation of new information, notably the Anatolian and Tocharian branches unknown in the 19th century.
Notably, the laryngeal theory, in its early forms discussed since the 1880s, became mainstream after Jerzy Kuryłowicz's 1927 discovery of the survival of at least some of these hypothetical phonemes in Anatolian.
Julius Pokorny's landmark Indogermanisches etymologisches Wörterbuch ("Indo-European Etymological Dictionary", 1959) gave a detailed overview of the lexical knowledge accumulated up until that time, but neglected contemporary trends of morphology and phonology (including the laryngeal theory), and largely ignored Anatolian.
The generation of Indo-Europeanists active in the last third of the 20th century (such as Calvert Watkins, Jochem Schindler and Helmut Rix) developed a better understanding of morphology and, in the wake of Kuryłowicz's 1956 Apophonie, understanding of the ablaut. From the 1960s, knowledge of Anatolian became certain enough to establish its relationship to PIE; see also Indo-Hittite.