Indo-European topics
The Indo-European languages comprise a family of several hundred related languages and dialects

Classification
Many scholars classify the Indo-European sub-branches into a Satem group and a Centum group. This terminology comes from the different treatment of the three original velar rows. Satem languages lost the distinction between labiovelar and pure velar sounds, and at the same time assibilated the palatal velars. The centum languages, on the other hand, lost the distinction between palatal velars and pure velars. Geographically, the "eastern" languages belong in the Satem group: Indo-Iranian and Balto-Slavic (but not including Tocharian and Anatolian); and the "western" languages represent the Centum group: Germanic, Italic, and Celtic. The Satem-Centum isogloss runs right between the Greek (Centum) and Armenian (Satem) languages (which a number of scholars regard as closely related), with Greek exhibiting some marginal Satem features. Some scholars think that some languages classify neither as Satem nor as Centum (Anatolian, Tocharian, and possibly Albanian). Note that the grouping does not imply a claim of monophyly: we do not need to postulate the existence of a "proto-Centum" or of a "proto-Satem". Areal contact among already distinct post-PIE languages (say, during the 3rd millennium BC) may have spread the sound changes involved. In any case, present-day specialists are rather less galvanized by the division than 19th cent. scholars were, partly because of the recognition that it is, after all, just one isogloss among the multitudes that criss-cross Indo-European linguistic geography. (Together with the recognition that the Centum Languages are no subgroup: as mentioned above, subgroups are defined by shared innovations, which the Satem languages definitely have, but the only thing that the "Centum Languages" have in common is staying put.)

Satem and Centum languages
Some linguists propose that Indo-European languages form part of a hypothetical Nostratic language superfamily, and attempt to relate Indo-European to other language families, such as South Caucasian languages, Altaic languages, Uralic languages, Dravidian languages, and Afro-Asiatic languages. This theory remains controversial, like the similar Eurasiatic theory of Joseph Greenberg, and the Proto-Pontic postulation of John Colarusso. There are no possible theoretical objections to the existence of such superfamilies; the difficulty comes in finding concrete evidence that transcends chance resemblance and wishful thinking. The main problem for all of them is that in historical linguistics the noise-to-signal ratio steadily worsens over time, and at great enough time-depths it becomes open to reasonable doubt that it can even be possible to tell what is signal and what is noise.

History of the idea of Indo-European

Historical evolution

Main article: Indo-European sound laws Sound changes
The earliest attestations of Indo-European languages date to the early 2nd millennium BC. At that time, the languages were already diversified and widely distributed, so that "loss of contact" between the individual dialects is accepted to have taken place before 2500 BC.. Competing scenarios for the early history of Indo-European are thus largely compatible for times after 2500 BC, even if they are incommensurable for the 4th millennium BC and earlier. The following timeline inserts the scenario suggested by the mainstream Kurgan hypothesis for the mid 5th to mid 3rd millennia (see below for competing hypotheses).

4500–4000: Early PIE. Sredny Stog, Dnieper-Donets and Samara cultures, domestication of the horse. (The early presence of the horse at Sredny Stog has been discredited as decisive—genetic evidence does not supply a single origin for the domesticated horse.)
4000–3500: The Yamna culture (prototypical kurgan-building) emerges in the steppe, and the Maykop culture in the northern Caucasus. Indo-Hittite models postulate the separation of Proto-Anatolian before this time.
3500–3000: Middle PIE. The Yamna culture reaches its peak: it represents the classical reconstructed Proto-Indo-European society, with stone idols, early two-wheeled proto-chariots, predominantly practising animal husbandry, but also with permanent settlements and hillforts, subsisting on agriculture and fishing, along rivers. Contact of the Yamna culture with late Neolithic Europe cultures results in the "kurganized" Globular Amphora and Baden cultures. The Maykop culture shows the earliest evidence of the early Bronze Age, and bronze weapons and artifacts enter Yamna territory. Probable early Satemization.
3000–2500: Late PIE. The Yamna culture extends over the entire Pontic steppe. The Corded Ware culture extends from the Rhine to the Volga, corresponding to the latest phase of Indo-European unity, the vast "kurganized" area disintegrating into various independent languages and cultures, but still in loose contact and thus enabling the spread of technology and early loans between the groups (except for the Anatolian and Tocharian branches, already isolated from these processes). The Centum-Satem division has probably run its course, but the phonetic trends of Satemization remain active.
2500–2000: The breakup into the proto-languages of the attested dialects has done its work. Speakers of Proto-Greek live in the Balkans, speakers of Proto-Indo-Iranian north of the Caspian in the Sintashta-Petrovka culture. The Bronze Age reaches Central Europe with the Beaker culture, whose people probably use various Centum dialects. Proto-Balto-Slavic speakers (or alternatively, Proto-Slavic and Proto-Baltic communities in close contact) emerge in north-eastern Europe. The Tarim mummies possibly correspond to proto-Tocharians.
2000–1500: Invention of the chariot, which leads to the split and rapid spread of Iranian and Indo-Aryan from the Andronovo culture and the Bactria-Margiana Archaeological Complex over much of Central Asia, Northern India, Iran and Eastern Anatolia. Proto-Anatolian splits into Hittite and Luwian. The pre-Proto-Celtic Unetice culture has an active metal industry (Nebra skydisk).
1500–1000: The Nordic Bronze Age develops (pre-)Proto-Germanic, and the (pre-)Proto-Celtic Urnfield and Hallstatt cultures emerge in Central Europe, introducing the Iron Age. Proto-Italic migration into the Italian peninsula. Redaction of the Rigveda and rise of the Vedic civilization in the Punjab. Flourishing and decline of the Hittite Empire. The Mycenaean civilization gives way to the Greek Dark Ages.
1000 BC–500 BC: The Celtic languages spread over Central and Western Europe. Northern Europe enters the Pre-Roman Iron Age, the formative phase of Proto-Germanic. Homer initiates Greek literature and early Classical Antiquity. The Vedic civilization gives way to the Mahajanapadas. Zoroaster composes the Gathas; rise of the Achaemenid Empire, replacing the Elamites and Babylonia. The Scythians supplant the Cimmerians (Srubna culture) in the Pontic steppe. Armenians succeed the Urartu culture. Separation of Proto-Italic into Osco-Umbrian and Latin-Faliscan, and foundation of Rome. Genesis of the Greek and Old Italic alphabets. A variety of Paleo-Balkan languages have speakers in Southern Europe. The Anatolian languages suffer extinction. Indo-European expansion

Main article: Proto-Indo-European language Proto-Indo-European
Scholars have dubbed the common ancestral (reconstructed) language Proto-Indo-European (PIE). They disagree as to the original geographic location (the so-called "Urheimat" or "original homeland") from where it originated. Mainstream opinion locates PIE in the Pontic-Caspian steppe in the Chalcolithic (from ca. 4000 BC; see Kurgan hypothesis). The main competitor of this is the Anatolian hypothesis advanced by Colin Renfrew, dating PIE to several millennia earlier, associating the spread of Indo-European languages with the Neolithic spread of farming (see Indo-Hittite). A rapid divergence of the Romance, Celtic and Balto-Slavic languages around 6,500 years ago
It should be noted that theories of the origin of Indo-European languages are not based on purely linguistic concepts. These theories are highly dependent on extra-linguistic factors, particularly interpretations of archaeological findings and the unattested meaning of words dating back as much as 3500 years or more before writing. The reference above to "mainstream" opinion concerning origins in the Pontic-Caspian steppes relies on such extra-linguistic conclusions. Since there is no direct way of knowing what language was spoken by a particular archaeological culture or how the meaning of words changed over thousands of years, theories about the location of the origin of Indo-European languages remain largely conjectural.

Location hypotheses

Main article: Kurgan hypothesis Kurgan hypothesis

Main article: Anatolian hypothesis Anatolian hypothesis
The Armenian hypothesis of Tamaz Gamq'relidze and Vyacheslav V. Ivanov in 1984 placed the Indo-European homeland on Lake Urmia This event occurred in still clearly Neolithic times and happened rather too early to fit with Kurgan archaeology. One can still imagine it as an event in the remote past of the Sredny Stog culture, with the people living on the land now beneath the Sea of Azov as possible pre-Proto-Indo-Europeans.
A recent version of the hypothesis of European origin of PIE is the "Paleolithic Continuity Theory" proposed by Italian theorists, which derives Indo-European languages from the Proto-Indo-European Paleolithic cultures, arguing for linguistic continuity from genetic continuity.

Other hypotheses

Bibliography

See also