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Indo-European
Geographic distribution	Worldwide
Linguistic classification	One of the world's primary language families
Proto-language	Proto-Indo-Hittite?   • Proto-Indo-European
Subdivisions	Albanoid?   • Albanian   • Illyrian †   • Messapic † Anatolian † Armenian Balto-Slavic Celtic Germanic Graeco-Phrygian?   • Hellenic   • Phrygian † Indo-Iranian Italic Tocharian † Unclassified or poorly attested:   • Dacian †   • Elymian †   • Liburnian †   • Lusitanian †   • Paeonian †   • Philistine? †   • Thracian †
Language codes
ISO 639-2 / 5	ine
Glottolog	indo1319
Present-day distribution of Indo-European languages in Eurasia:   Albanoid (Albanian)   Armenian   Baltic (East)   Slavic  Celtic (Brittonic and Goidelic)   Germanic (North and West)   Hellenic (Greek)   Iranian   Indo-Aryan   Nuristani   Italic (Romance)   Non-Indo-European languages Dotted/striped areas indicate where multilingualism is common (more visible upon full enlargement of the map).
Notes	† indicates this branch of the language family is extinct

Part of a series on
Indo-European topics
Languages List of Indo-European languages Extant Albanoid Albanian Armenian Balto-Slavic Baltic Slavic Celtic Germanic Hellenic Greek Indo-Iranian Indo-Aryan Iranian Nuristani Italic Romance Extinct Anatolian Tocharian Paleo-Balkan Dacian Illyrian Liburnian Messapic Mysian Paeonian Phrygian Thracian Reconstructed Proto-Indo-European language Phonology: Sound laws, Accent, Ablaut Hypothetical Balkanic Daco-Thracian Graeco-Albanian Graeco-Armenian Graeco-Aryan Graeco-Phrygian Indo-Hittite Italo-Celtic Thraco-Illyrian Grammar Vocabulary Root Verbs Nouns Pronouns Numerals Particles Other Proto-Albanian Proto-Anatolian Proto-Armenian Proto-Germanic (Proto-Norse) Proto-Italo-Celtic (Proto-Celtic · Proto-Italic) Proto-Greek Proto-Balto-Slavic (Proto-Slavic · Proto-Baltic) Proto-Indo-Iranian (Proto-Iranian)
Philology Hittite inscriptions Hieroglyphic Luwian Linear B Rigveda Avesta Homer Behistun Greek epigraphy Phrygian epigraphy Messapic epigraphy Latin epigraphy Gaulish epigraphy Runic epigraphy Ogham Gothic Bible Bible translations into Armenian Tocharian script Old Irish glosses Albanian Kanun
Origins Homeland Proto-Indo-Europeans Society Religion Mainstream Kurgan hypothesis Indo-European migrations Eurasian nomads Alternative and fringe Anatolian hypothesis Armenian hypothesis Beech argument Indigenous Aryanism Baltic homeland Paleolithic continuity theory
Archaeology Chalcolithic (Copper Age) Pontic Steppe Domestication of the horse Kurgan Kurgan stelae Kurgan culture Steppe cultures Bug–Dniester Sredny Stog Dnieper–Donets Samara Khvalynsk Yamnaya Mikhaylovka culture Novotitarovskaya culture Caucasus Maykop East Asia Afanasievo Eastern Europe Usatovo Cernavodă Cucuteni Northern Europe Corded ware Baden Middle Dnieper Bronze Age Pontic Steppe Chariot Yamnaya Catacomb Multi-cordoned ware Poltavka Srubnaya Northern/Eastern Steppe Abashevo culture Andronovo Sintashta Europe Globular Amphora Corded ware Bell Beaker Únětice Trzciniec Nordic Bronze Age Terramare Tumulus Urnfield Lusatian South Asia BMAC Yaz Gandhara grave Iron Age Steppe Chernoles Europe Thraco-Cimmerian Hallstatt Jastorf Caucasus Colchian India Painted Grey Ware Northern Black Polished Ware
Peoples and societies Bronze Age Anatolian peoples (Hittites) Armenians Mycenaean Greeks Indo-Iranians Iron Age Indo-Aryans Indo-Aryans Iranians Iranians East Asia Wusun Yuezhi Europe Celts Gauls Celtiberians Insular Celts Cimmerians Hellenic peoples Italic peoples Germanic peoples Paleo-Balkan/Anatolia Thracians Dacians Illyrians Paeonians Phrygians Scythians Middle Ages East Asia Tocharians Europe Albanians Balts Slavs Norsemen/Medieval Scandinavians Middle Ages Indo-Aryan Medieval India Iranian Greater Iran
Religion and mythology Reconstructed Proto-Indo-European mythology Proto-Indo-Iranian paganism Historical Vedic religion Ancient Iranian religion Historical Hittite Indo-Aryan Vedic Hinduism Buddhism Jainism Sikhism Iranian Persian Zoroastrianism Kurdish Yazidism Yarsanism Scythian Ossetian Others Armenian European Paleo-Balkan (Albanian · Illyrian · Thracian · Dacian) Greek Roman Celtic Irish Scottish Breton Welsh Cornish Germanic Anglo-Saxon Continental Norse Baltic Latvian Lithuanian Slavic Practices Fire rituals Horse sacrifice Sati Winter solstice/Yule
Indo-European studies Scholars Marija Gimbutas J. P. Mallory Institutes Copenhagen Studies in Indo-European Publications Encyclopedia of Indo-European Culture The Horse, the Wheel, and Language Journal of Indo-European Studies Indogermanisches etymologisches Wörterbuch Indo-European Etymological Dictionary
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The Indo-European languages are a language family native to the northern Indian subcontinent, the overwhelming majority of Europe, and the Iranian plateau. Some European languages of this family—English, French, Portuguese, Russian, Dutch, and Spanish—have expanded through colonialism in the modern period and are now spoken across several continents. The Indo-European family is divided into several branches or sub-families, of which there are eight groups with languages still alive today: Albanian, Armenian, Balto-Slavic, Celtic, Germanic, Hellenic, Indo-Iranian, and Italic; another nine subdivisions are now extinct.

Today, the individual Indo-European languages with the most native speakers are English, Spanish, Portuguese, Russian, Hindustani, Bengali, French, and German; many others are small and in danger of extinction.

In total, 46% of the world's population (3.2 billion people) speaks an Indo-European language as a first language—by far the highest of any language family. There are about 445 living Indo-European languages, according to an estimate by Ethnologue, with over two-thirds (313) of them belonging to the Indo-Iranian branch.

All Indo-European languages are descended from a single prehistoric language, linguistically reconstructed as Proto-Indo-European, spoken sometime during the Neolithic or early Bronze Age. The geographical location where it was spoken, the Proto-Indo-European homeland, has been the object of many competing hypotheses; the academic consensus supports the Kurgan hypothesis, which posits the homeland to be the Pontic–Caspian steppe in what is now Ukraine and southern Russia, associated with the Yamnaya culture and other related archaeological cultures during the 4th millennium BC to early 3rd millennium BC. By the time the first written records appeared, Indo-European had already evolved into numerous languages spoken across much of Europe, South Asia, and part of Western Asia. Written evidence of Indo-European appeared during the Bronze Age in the form of Mycenaean Greek and the Anatolian languages of Hittite and Luwian. The oldest records are isolated Hittite words and names—interspersed in texts that are otherwise in the unrelated Akkadian language, a Semitic language—found in texts of the Assyrian colony of Kültepe in eastern Anatolia dating to the 20th century BC. Although no older written records of the original Proto-Indo-European population remain, some aspects of their culture and their religion can be reconstructed from later evidence in the daughter cultures. The Indo-European family is significant to the field of historical linguistics as it possesses the second-longest recorded history of any known family, after the Afroasiatic Egyptian language and Semitic languages. The analysis of the family relationships between the Indo-European languages, and the reconstruction of their common source, was central to the development of the methodology of historical linguistics as an academic discipline in the 19th century.

The Indo-European language family is not considered by the current academic consensus in the field of linguistics to have any genetic relationships with other language families, although several disputed hypotheses propose such relations.

History of Indo-European linguistics

During the 16th century, European visitors to the Indian subcontinent began to notice similarities among Indo-Aryan, Iranian, and European languages. In 1583, English Jesuit missionary and Konkani scholar Thomas Stephens wrote a letter from Goa to his brother (not published until the 20th century) in which he noted similarities between Indian languages and Greek and Latin.

Another account was made by Filippo Sassetti, a merchant born in Florence in 1540, who travelled to the Indian subcontinent. Writing in 1585, he noted some word similarities between Sanskrit and Italian (these included devaḥ/dio "God", sarpaḥ/serpe "serpent", sapta/sette "seven", aṣṭa/otto "eight", and nava/nove "nine"). However, neither Stephens' nor Sassetti's observations led to further scholarly inquiry.

In 1647, Dutch linguist and scholar Marcus Zuerius van Boxhorn noted the similarity among certain Asian and European languages and theorized that they were derived from a primitive common language that he called Scythian. He included in his hypothesis Dutch, Albanian, Greek, Latin, Persian, and German, later adding Slavic, Celtic, and Baltic languages. However, Van Boxhorn's suggestions did not become widely known and did not stimulate further research.

Ottoman Turkish traveler Evliya Çelebi visited Vienna in 1665–1666 as part of a diplomatic mission and noted a few similarities between words in German and in Persian. Gaston Coeurdoux and others made observations of the same type. Coeurdoux made a thorough comparison of Sanskrit, Latin, and Greek conjugations in the late 1760s to suggest a relationship among them. Meanwhile, Mikhail Lomonosov compared different language groups, including Slavic, Baltic ("Kurlandic"), Iranian ("Medic"), Finnish, Chinese, "Hottentot" (Khoekhoe), and others, noting that related languages (including Latin, Greek, German, and Russian) must have separated in antiquity from common ancestors.

The hypothesis reappeared in 1786 when Sir William Jones first lectured on the striking similarities among three of the oldest languages known in his time: Latin, Greek, and Sanskrit, to which he tentatively added Gothic, Celtic, and Persian, though his classification contained some inaccuracies and omissions. In one of the most famous quotations in linguistics, Jones made the following prescient statement in a lecture to the Asiatic Society of Bengal in 1786, conjecturing the existence of an earlier ancestor language, which he called "a common source" but did not name:

The Sanscrit [sic] 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.

— Sir William Jones, Third Anniversary Discourse delivered 2 February 1786, ELIOHS

Thomas Young first used the term Indo-European in 1813, deriving it from the geographical extremes of the language family: from Western Europe to North India. A synonym is Indo-Germanic (Idg. or IdG.), specifying the family's southeasternmost and northwesternmost branches. This first appeared in French (indo-germanique) in 1810 in the work of Conrad Malte-Brun; in most languages this term is now dated or less common than Indo-European, although in German indogermanisch remains the standard scientific term. A number of other synonymous terms have also been used.

Franz Bopp wrote in 1816 On the conjugational system of the Sanskrit language compared with that of Greek, Latin, Persian and Germanic and between 1833 and 1852 he wrote Comparative Grammar. This marks the beginning of Indo-European studies as an academic discipline. The classical phase of Indo-European comparative linguistics leads from this work to August Schleicher's 1861 Compendium and up to Karl Brugmann's Grundriss, published in the 1880s. Brugmann's neogrammarian reevaluation of the field and Ferdinand de Saussure's development of the laryngeal theory may be considered the beginning of "modern" Indo-European studies. 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 of ablaut in the wake of Kuryłowicz's 1956 Apophony in Indo-European, who in 1927 pointed out the existence of the Hittite consonant ḫ. Kuryłowicz's discovery supported Ferdinand de Saussure's 1879 proposal of the existence of coefficients sonantiques, elements de Saussure reconstructed to account for vowel length alternations in Indo-European languages. This led to the so-called laryngeal theory, a major step forward in Indo-European linguistics and a confirmation of de Saussure's theory.

Classification

The various subgroups of the Indo-European language family include ten major branches, listed below in alphabetical order:

• Albanian, attested from the 13th century AD; Proto-Albanian evolved from an ancient Paleo-Balkan language, traditionally thought to be Illyrian, or otherwise a totally unattested Balkan Indo-European language that was closely related to Illyrian and Messapic.
• Anatolian, extinct by Late Antiquity, spoken in Anatolia, attested in isolated terms in Luwian/Hittite mentioned in Semitic Old Assyrian texts from the 20th and 19th centuries BC, Hittite texts from about 1650 BC.
• Armenian, attested from the early 5th century AD. It evolved from the Proto-Armenian language which, according to the Armenian hypothesis, developed in situ from the Proto-Indo-European language of the 3rd millennium BC.
• Balto-Slavic, believed by most Indo-Europeanists to form a phylogenetic unit, while a minority ascribes similarities to prolonged language-contact.
  • Slavic (from Proto-Slavic), attested from the 9th century AD (possibly earlier), earliest texts in Old Church Slavonic. Slavic languages include Bulgarian, Russian, Polish, Czech, Slovak, Silesian, Kashubian, Macedonian, Serbo-Croatian (Bosnian, Croatian, Montenegrin, Serbian), Sorbian, Slovenian, Ukrainian, Belarusian, and Rusyn.
  • Baltic, attested from the 14th century AD; although attested relatively recently, they retain many archaic features attributed to Proto-Indo-European (PIE). Living examples are Lithuanian and Latvian.
• Celtic (from Proto-Celtic), attested since the 6th century BC; Lepontic inscriptions date as early as the 6th century BC; Celtiberian from the 2nd century BC; Primitive Irish Ogham inscriptions from the 4th or 5th century AD, earliest inscriptions in Old Welsh from the 7th century AD. Modern Celtic languages include Welsh, Cornish, Breton, Scottish Gaelic, Irish and Manx.
• Germanic (from Proto-Germanic), earliest attestations in runic inscriptions from around the 2nd century AD, earliest coherent texts in Gothic, 4th century AD. Old English manuscript tradition from about the 8th century AD. Includes English, Frisian, German, Dutch, Scots, Danish, Swedish, Norwegian, Afrikaans, Yiddish, Low German, Icelandic, Elfdalian, and Faroese.
• Hellenic (from Proto-Greek, see also History of Greek); fragmentary records in Mycenaean Greek from between 1450 and 1350 BC have been found. Homeric texts date to the 8th century BC.
• Indo-Iranian, attested c. 1400 BC, descended from Proto-Indo-Iranian (dated to the late 3rd millennium BC).
  • Indo-Aryan, attested from around 1400 BC in Hittite texts from Anatolia, showing traces of Indo-Aryan words. Epigraphically from the 3rd century BC in the form of Prakrit (Edicts of Ashoka). The Rigveda is assumed to preserve intact records via oral tradition dating from about the mid-second millennium BC in the form of Vedic Sanskrit. Includes a wide range of modern languages from Northern India, Eastern Pakistan and Bangladesh, including Hindustani (Hindi, Urdu), Bengali, Odia, Assamese, Punjabi, Kashmiri, Gujarati, Marathi, Sindhi and Nepali, as well as Sinhala of Sri Lanka and Dhivehi of the Maldives and Minicoy.
  • Iranian or Iranic, attested from roughly 1000 BC in the form of Avestan. Epigraphically from 520 BC in the form of Old Persian (Behistun inscription). Includes Persian, Pashto, Kurdish, Balochi, Luri, and Ossetian.
  • Nuristani (includes Kamkata-vari, Vasi-vari, Askunu, Waigali, Tregami, and Zemiaki).
• Italic (from Proto-Italic), attested from the 7th century BC. Includes the ancient Osco-Umbrian languages, Faliscan, as well as Latin and its descendants, the Romance languages, such as Italian and French.
• Tocharian, with proposed links to the Afanasevo culture of Southern Siberia. Extant in two dialects (Turfanian and Kuchean, or Tocharian A and B), attested from roughly the 6th to the 9th century AD. Marginalized by the Old Turkic Uyghur Khaganate and probably extinct by the 10th century.

In addition to the classical ten branches listed above, several extinct and little-known languages and language-groups have existed or are proposed to have existed:

• Ancient Belgian: hypothetical language associated with the proposed Nordwestblock cultural area. Speculated to be connected to Italic or Venetic, and to have certain phonological features in common with Lusitanian.
• Cimmerian: possibly Iranic, Thracian, or Celtic
• Dacian: possibly very close to Thracian
• Elymian: Poorly-attested language spoken by the Elymians, one of the three indigenous (i.e. pre-Greek and pre-Punic) tribes of Sicily. Indo-European affiliation widely accepted, possibly related to Italic or Anatolian.
• Illyrian: possibly related to Albanian, Messapian, or both
• Liburnian: evidence too scant and uncertain to determine anything with certainty
• Ligurian: possibly close to or part of Celtic.
• Lusitanian: possibly related to (or part of) Celtic, Ligurian, or Italic
• Ancient Macedonian: proposed relationship to Greek.
• Messapic: not conclusively deciphered, often considered to be related to Albanian as the available fragmentary linguistic evidence shows common characteristic innovations and a number of significant lexical correspondences between the two languages
• Paionian: extinct language once spoken north of Macedon
• Phrygian: language of the ancient Phrygians. Very likely, but not certainly, a sister group to Hellenic.
• Sicel: an ancient language spoken by the Sicels (Greek Sikeloi, Latin Siculi), one of the three indigenous (i.e. pre-Greek and pre-Punic) tribes of Sicily. Proposed relationship to Latin or proto-Illyrian (Pre-Indo-European) at an earlier stage.
• Sorothaptic: proposed, pre-Celtic, Iberian language
• Thracian: possibly including Dacian
• Venetic: shares several similarities with Latin and the Italic languages, but also has some affinities with other IE languages, especially Germanic and Celtic.

Membership of languages in the Indo-European language family is determined by genealogical relationships, meaning that all members are presumed descendants of a common ancestor, Proto-Indo-European. Membership in the various branches, groups, and subgroups of Indo-European is also genealogical, but here the defining factors are shared innovations among various languages, suggesting a common ancestor that split off from other Indo-European groups. For example, what makes the Germanic languages a branch of Indo-European is that much of their structure and phonology can be stated in rules that apply to all of them. Many of their common features are presumed innovations that took place in Proto-Germanic, the source of all the Germanic languages.

In the 21st century, several attempts have been made to model the phylogeny of Indo-European languages using Bayesian methodologies similar to those applied to problems in biological phylogeny. Although there are differences in absolute timing between the various analyses, there is much commonality between them, including the result that the first known language groups to diverge were the Anatolian and Tocharian language families, in that order.

Tree versus wave model

The "tree model" is considered an appropriate representation of the genealogical history of a language family if communities do not remain in contact after their languages have started to diverge. In this case, subgroups defined by shared innovations form a nested pattern. The tree model is not appropriate in cases where languages remain in contact as they diversify; in such cases subgroups may overlap, and the "wave model" is a more accurate representation. Most approaches to Indo-European subgrouping to date have assumed that the tree model is by-and-large valid for Indo-European; however, there is also a long tradition of wave-model approaches.

In addition to genealogical changes, many of the early changes in Indo-European languages can be attributed to language contact. It has been asserted, for example, that many of the more striking features shared by Italic languages (Latin, Oscan, Umbrian, etc.) might well be areal features. More certainly, very similar-looking alterations in the systems of long vowels in the West Germanic languages greatly postdate any possible notion of a proto-language innovation (and cannot readily be regarded as "areal", either, because English and continental West Germanic were not a linguistic area). In a similar vein, there are many similar innovations in Germanic and Balto-Slavic that are far more likely areal features than traceable to a common proto-language, such as the uniform development of a high vowel (*u in the case of Germanic, *i/u in the case of Baltic and Slavic) before the PIE syllabic resonants *ṛ, *ḷ, *ṃ, *ṇ, unique to these two groups among IE languages, which is in agreement with the wave model. The Balkan sprachbund even features areal convergence among members of very different branches.

An extension to the Ringe-Warnow model of language evolution suggests that early IE had featured limited contact between distinct lineages, with only the Germanic subfamily exhibiting a less treelike behaviour as it acquired some characteristics from neighbours early in its evolution. The internal diversification of especially West Germanic is cited to have been radically non-treelike.

Proposed subgroupings

Specialists have postulated the existence of higher-order subgroups such as Italo-Celtic, Graeco-Armenian, Graeco-Aryan or Graeco-Armeno-Aryan, and Balto-Slavo-Germanic. However, unlike the ten traditional branches, these are all controversial to a greater or lesser degree.

The Italo-Celtic subgroup was at one point uncontroversial, considered by Antoine Meillet to be even better established than Balto-Slavic. The main lines of evidence included the genitive suffix -ī; the superlative suffix -m̥mo; the change of /p/ to /kʷ/ before another /kʷ/ in the same word (as in penkʷe > *kʷenkʷe > Latin quīnque, Old Irish cóic); and the subjunctive morpheme -ā-. This evidence was prominently challenged by Calvert Watkins, while Michael Weiss has argued for the subgroup.

Evidence for a relationship between Greek and Armenian includes the regular change of the second laryngeal to a at the beginnings of words, as well as terms for "woman" and "sheep". Greek and Indo-Iranian share innovations mainly in verbal morphology and patterns of nominal derivation. Relations have also been proposed between Phrygian and Greek, and between Thracian and Armenian. Some fundamental shared features, like the aorist (a verb form denoting action without reference to duration or completion) having the perfect active particle -s fixed to the stem, link this group closer to Anatolian languages and Tocharian. Shared features with Balto-Slavic languages, on the other hand (especially present and preterit formations), might be due to later contacts.

The Indo-Hittite hypothesis proposes that the Indo-European language family consists of two main branches: one represented by the Anatolian languages and another branch encompassing all other Indo-European languages. Features that separate Anatolian from all other branches of Indo-European (such as the gender or the verb system) have been interpreted alternately as archaic debris or as innovations due to prolonged isolation. Points proffered in favour of the Indo-Hittite hypothesis are the (non-universal) Indo-European agricultural terminology in Anatolia and the preservation of laryngeals. However, in general this hypothesis is considered to attribute too much weight to the Anatolian evidence. According to another view, the Anatolian subgroup left the Indo-European parent language comparatively late, approximately at the same time as Indo-Iranian and later than the Greek or Armenian divisions. A third view, especially prevalent in the so-called French school of Indo-European studies, holds that extant similarities in non-satem languages in general—including Anatolian—might be due to their peripheral location in the Indo-European language-area and to early separation, rather than indicating a special ancestral relationship. Hans J. Holm, based on lexical calculations, arrives at a picture roughly replicating the general scholarly opinion and refuting the Indo-Hittite hypothesis.

Satem and centum languages

The division of the Indo-European languages into satem and centum groups was put forward by Peter von Bradke in 1890, although Karl Brugmann did propose a similar type of division in 1886. In the satem languages, which include the Balto-Slavic and Indo-Iranian branches, as well as (in most respects) Albanian and Armenian, the reconstructed Proto-Indo-European palatovelars remained distinct and were fricativized, while the labiovelars merged with the 'plain velars'. In the centum languages, the palatovelars merged with the plain velars, while the labiovelars remained distinct. The results of these alternative developments are exemplified by the words for "hundred" in Avestan (satem) and Latin (centum)—the initial palatovelar developed into a fricative in the former, but became an ordinary velar in the latter.

Rather than being a genealogical separation, the centum–satem division is commonly seen as resulting from innovative changes that spread across PIE dialect-branches over a particular geographical area; the centum–satem isogloss intersects a number of other isoglosses that mark distinctions between features in the early IE branches. It may be that the centum branches in fact reflect the original state of affairs in PIE, and only the satem branches shared a set of innovations, which affected all but the peripheral areas of the PIE dialect continuum. Kortlandt proposes that the ancestors of Balts and Slavs took part in satemization before being drawn later into the western Indo-European sphere.

Proposed external relations

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From the very beginning of Indo-European studies, there have been attempts to link the Indo-European languages genealogically to other languages and language families. However, these theories remain highly controversial, and most specialists in Indo-European linguistics are skeptical or agnostic about such proposals.

Proposals linking the Indo-European languages with a single language family include:

• Indo-Uralic, joining Indo-European with Uralic
• Pontic, postulated by John Colarusso, which joins Indo-European with Northwest Caucasian

Other proposed families include:

• Nostratic, comprising all or some of the Eurasiatic languages and the Kartvelian, Dravidian (or wider, Elamo-Dravidian) and Afroasiatic language families
• Eurasiatic, a theory championed by Joseph Greenberg, comprising the Uralic, Altaic and various 'Paleosiberian' families (Ainu, Yukaghir, Nivkh, Chukotko-Kamchatkan, Eskimo–Aleut) and possibly others

Nostratic and Eurasiatic, in turn, have been included in even wider groupings, such as Borean, a language family separately proposed by Harold C. Fleming and Sergei Starostin that encompasses almost all of the world's natural languages with the exception of those native to sub-Saharan Africa, New Guinea, Australia, and the Andaman Islands.

Evolution

Proto-Indo-European

The proposed Proto-Indo-European language (PIE) is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans. From the 1960s, knowledge of Anatolian became certain enough to establish its relationship to PIE. Using the method of internal reconstruction, an earlier stage, called Pre-Proto-Indo-European, has been proposed.

PIE is an inflected language, in which the grammatical relationships between words were signaled through inflectional morphemes (usually endings). The roots of PIE are basic morphemes carrying a lexical meaning. By addition of suffixes, they form stems, and by addition of endings, these form grammatically inflected words (nouns or verbs). The reconstructed Indo-European verb system is complex and, like the noun, exhibits a system of ablaut.

Diversification

The diversification of the parent language into the attested branches of daughter languages is historically unattested. The timeline of the evolution of the various daughter languages, on the other hand, is mostly undisputed, quite regardless of the question of Indo-European origins.

Using a mathematical analysis borrowed from evolutionary biology, Donald Ringe and Tandy Warnow propose the following evolutionary tree of Indo-European branches:

• Pre-Anatolian (before 3500 BC)
• Pre-Tocharian
• Pre-Italic and Pre-Celtic (before 2500 BC)
• Pre-Armenian and Pre-Greek (after 2500 BC)
• Proto-Indo-Iranian (2000 BC)
• Pre-Germanic and Pre-Balto-Slavic; proto-Germanic c. 500 BC

David Anthony proposes the following sequence:

• Pre-Anatolian (4200 BC)
• Pre-Tocharian (3700 BC)
• Pre-Germanic (3300 BC)
• Pre-Italic and Pre-Celtic (3000 BC)
• Pre-Armenian (2800 BC)
• Pre-Balto-Slavic (2800 BC)
• Pre-Greek (2500 BC)
• Proto-Indo-Iranian (2200 BC); split between Iranian and Old Indic 1800 BC

From 1500 BC the following sequence may be given:

• 1500–1000 BC: The Nordic Bronze Age of Scandinavia develops pre-Proto-Germanic, and the (pre-) Proto-Celtic Urnfield and Hallstatt cultures emerge in Central Europe, introducing the Iron Age. Migration of the Proto-Italic speakers into the Italian peninsula (Bagnolo stele). Migration of Aryans to India followed by the redaction of the Rigveda; rise of the Vedic civilization and beginning of Iron Age in the Punjab. The Mycenaean civilization gives way to the Greek Dark Ages. Hittite goes extinct. Iranian speakers start migrating southwards to Greater Iran. Balto-Slavic splits into ancestors of modern Baltic and Slavic.
• 1000–500 BC: The Celtic languages spread over Central and Western Europe, including Britain. Baltic languages are spoken in a huge area from present-day Poland to Moscow. Pre-Proto-Germanic gives rise to Proto-Germanic in southern Scandinavia. Homer and the beginning of Classical Antiquity. The Vedic civilization gives way to the Mahajanapadas as the Indo-Aryan tongue reaches eastwards, giving rise to the Greater Magadha cultural sphere, where Mahavira preaches Jainism and Siddhartha Gautama preaches Buddhism. Zoroaster composes the Gathas, rise of the Achaemenid Empire, replacing the Elamites and Babylonia. Separation of Proto-Italic into Osco-Umbrian, Latin-Faliscan, and possibly Venetic and Siculian. A variety of Paleo-Balkan languages besides Greek are spoken in Southern Europe, including Thracian, Dacian and Illyrian, and in Anatolia (Phrygian). Development of Prakrits across the northern Indian subcontinent, as well as migration of Indo-Aryan speakers to Sri Lanka and the Maldives.
• 500 BC – 1 BC/AD: Classical Antiquity: spread of Greek and Latin throughout the Mediterranean and, during the Hellenistic period (Indo-Greeks), to Central Asia and the Hindukush. The Magadhan power and influence rises in ancient India, especially with the conquests of the Nandan and Mauryan empires. Germanic speakers start migrating southwards to occupy formerly Celtic territories. Scythian cultures extend from Eastern Europe (Pontic Scythians) to Northwest China (Ordos culture).
• 1 BC – AD 500: Late Antiquity, Gupta period; attestation of Armenian. Proto-Slavic. The Roman Empire and then the Germanic migrations marginalize the Celtic languages to the British Isles. Sogdian, an eastern Iranian language, becomes the lingua franca of the Silk Road in Central Asia leading to China, due to the proliferation of Sogdian merchants there. Greek settlements and Byzantine rule make the last Anatolian languages extinct. Turkic languages start replacing Scythian languages.
• 500–1000: Early Middle Ages. The Viking Age forms an Old Norse koine spanning Scandinavia, the British Isles and Iceland. Phrygian becomes extinct. The Islamic conquests and the Turkic expansion result in the Arabization and Turkification of significant areas where Indo-European languages were spoken, but Persian still develops under Islamic rule and extends into Afghanistan and Tajikistan. Due to further Turkic migrations, Tocharian becomes fully extinct while Scythian languages are overwhelmingly replaced. Slavic languages spread over wide areas in central, eastern and southeastern Europe, largely replacing Romance in the Balkans (with the exception of Romanian) and whatever was left of the Paleo-Balkan languages with the exception of Albanian. Pannonian Basin is taken by the Magyars from the western Slavs.
• 1000–1500: Late Middle Ages: Attestation of Albanian and Baltic. Modern dialects of Indo-European languages start emerging.
• 1500–2000: Early Modern period to present: Colonialism results in the spread of Indo-European languages to every habitable continent, most notably Romance (North, Central and South America, North and Sub-Saharan Africa, West Asia), West Germanic (English in North America, Sub-Saharan Africa, East Asia and Australia; to a lesser extent Dutch and German), and Russian to Central Asia and North Asia.

Important languages for reconstruction

In reconstructing the history of the Indo-European languages and the form of the Proto-Indo-European language, some languages have been of particular importance. These generally include the ancient Indo-European languages that are both well-attested and documented at an early date, although some languages from later periods are important if they are particularly linguistically conservative (most notably, Lithuanian). Early poetry is of special significance because of the rigid poetic meter normally employed, which makes it possible to reconstruct a number of features (e.g. vowel length) that were either unwritten or corrupted in the process of transmission down to the earliest extant written manuscripts.

Most noticeable of all:

• Vedic Sanskrit (c. 1500–500 BC). This language is unique in that its source documents were all composed orally, and were passed down through oral tradition (shakha schools) for c. 2,000 years before ever being written down. The oldest documents are all in poetic form; oldest and most important of all is the Rigveda (c. 1500 BC).
• Ancient Greek (c. 750–400 BC). Mycenaean Greek (c. 1450 BC) is the oldest recorded form, but its value is lessened by the limited material, restricted subject matter, and highly ambiguous writing system. More important is Ancient Greek, documented extensively beginning with the two Homeric poems (the Iliad and the Odyssey, c. 750 BC).
• Hittite (c. 1700–1200 BC). This is the earliest-recorded of all Indo-European languages, and highly divergent from the others due to the early separation of the Anatolian languages from the remainder. It possesses some highly archaic features found only fragmentarily, if at all, in other languages. At the same time, however, it appears to have undergone many early phonological and grammatical changes which, combined with the ambiguities of its writing system, hinder its usefulness somewhat.

Other primary sources:

• Latin, attested in a huge amount of poetic and prose material in the Classical period (c. 200 BC – AD 100) and limited older material from as early as c. 600 BC.
• Gothic (the most archaic well-documented Germanic language, AD c. 350), along with the combined witness of the other old Germanic languages: most importantly, Old English (c. 800–1000), Old High German (c. 750–1000) and Old Norse (c. 1100–1300 AD, with limited earlier sources dating to AD c. 200).
• Old Avestan (c. 1700–1200 BC) and Younger Avestan (c. 900 BC). Documentation is sparse, but nonetheless quite important due to its highly archaic nature.
• Modern Lithuanian, with limited records in Old Lithuanian (c. 1500–1700).
• Old Church Slavonic (c. 900–1000).

Other secondary sources, of lesser value due to poor attestation:

• Luwian, Lycian, Lydian and other Anatolian languages (c. 1400–400 BC).
• Oscan, Umbrian and other Old Italic languages (c. 600–200 BC).
• Old Persian (c. 500 BC).
• Old Prussian (c. 1350–1600); even more archaic than Lithuanian.

Other secondary sources, of lesser value due to extensive phonological changes and relatively limited attestation:

• Old Irish (ADc. 700–850).
• Tocharian (AD c. 500–800 ), underwent large phonetic shifts and mergers in the proto-language, and has an almost entirely reworked declension system.
• Classical Armenian (AD c. 400–1000).
• Albanian (c. 1450–current time).

Sound changes

As the Proto-Indo-European (PIE) language broke up, its sound system diverged as well, changing according to various sound laws evidenced in the daughter languages.

PIE is normally reconstructed with a complex system of 15 stop consonants, including an unusual three-way phonation (voicing) distinction between voiceless, voiced and "voiced aspirated" (i.e. breathy voiced) stops, and a three-way distinction among velar consonants (k-type sounds) between "palatal" ḱ ǵ ǵh, "plain velar" k g gh and labiovelar kʷ gʷ gʷh. (The correctness of the terms palatal and plain velar is disputed; see Proto-Indo-European phonology.) All daughter languages have reduced the number of distinctions among these sounds, often in divergent ways.

As an example, in English, one of the Germanic languages, the following are some of the major changes that happened:

1. As in other centum languages, the "plain velar" and "palatal" stops merged, reducing the number of stops from 15 to 12.
2. As in the other Germanic languages, the Germanic sound shift changed the realization of all stop consonants, with each consonant shifting to a different one:

   bʰ → b → p → f

   dʰ → d → t → θ

   gʰ → g → k → x (Later initial x →h)

   gʷʰ → gʷ → kʷ → xʷ (Later initial xʷ →hʷ)

   Each original consonant shifted one position to the right. For example, original dʰ became d, while original d became t and original t became θ (written th in English). This is the original source of the English sounds written f, th, h and wh. Examples, comparing English with Latin, where the sounds largely remain unshifted:

   For PIE p: piscis vs. fish; pēs, pēdis vs. foot; pluvium "rain" vs. flow; pater vs. father

   For PIE t: trēs vs. three; māter vs. mother

   For PIE d: decem vs. ten; pēdis vs. foot; quid vs. what

   For PIE k: centum vs. hund(red); capere "to take" vs. have

   For PIE kʷ: quid vs. what; quandō vs. when

3. Various further changes affected consonants in the middle or end of a word:
   • The voiced stops resulting from the sound shift were softened to voiced fricatives (or perhaps the sound shift directly generated fricatives in these positions).
   • Verner's law also turned some of the voiceless fricatives resulting from the sound shift into voiced fricatives or stops. This is why the t in Latin centum ends up as d in hund(red) rather than the expected th.
   • Most remaining h sounds disappeared, while remaining f and th became voiced. For example, Latin decem ends up as ten with no h in the middle (but note taíhun "ten" in Gothic, an archaic Germanic language). Similarly, the words seven and have have a voiced v (compare Latin septem, capere), while father and mother have a voiced th, although not spelled differently (compare Latin pater, māter).