Proponents of the Indo-Hittite hypothesis claim the separation may have preceded the spread of the remaining branches by several millennia, possibly as early as 7000 BC. In this context, the proto-language before the split of Anatolian would be called Proto-Indo-Hittite, and the proto-language of the remaining branches, before the next split, presumably of Tocharian, would be called Proto-Indo-European (PIE). This is a matter of terminology, though, as the hypothesis does not dispute the ultimate genetic relation of Anatolian with Indo-European, it just means to emphasize the assumed magnitude of temporal separation.
Traditionally there has been a strong notion among Indo-European linguistics that the Anatolian branch was separated earlier than other branches. Within the Kurgan framework the split time is estimated at roughly 4000 BC.
Some fundamental shared features, like the verbal aorist category (this is a verb form denoting action without reference to duration or completion) having the perfect active particle -s fixed to the stem, link the Anatolian languages closer to the southeastern languages such as Greek and Armenian, and to Tocharian.
Features like the lack of feminine gender in the declensions of nominals, a division between an "animate" common gender and an "inanimate" neuter gender, a reduced vowel system, a tendency towards a greater simplicity of the case system, a less typical Indo-European vocabularity and other striking features have been interpreted alternately as archaic debris, as the nucleus for future developments or just as being caused by prolonged contacts in strange surroundings "en route" or after their arrival in Anatolia. In favor of the Indo-Hittite hypothesis are the very Indo-European agricultural terminology conserved in Anatolia, otherwise considered the cradle of agriculture, and the laryngeal theory that hypothesize the existence of one or more additional stop or spirant consonants in the Indo-European parent language that has only been attested in Hittite and of which only traces are left outside Anatolian. However, in general this hypothesis is considered to attribute too much weight to the Anatolian evidence and it has been demonstrated already in 1938 that the Anatolian group should be placed on the same level as other Indo-European subgroups and not as equal with Indo-European. 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 early separation, rather than indicating a special ancestral relationship.
From the point of view of phylogenetics, the Indo-Hittite hypothesis is strongly corroborated by the lack of any Indo-European innovations in Anatolian. Notably, Anatolian doesn't have the IE gender system opposing masucline : feminine; instead we have a rudimentary noun class system based on an older animate : inanimate opposition reminiscent of noun class systems in non-Bantu Niger-Congo languages.
Hence, a crucial question is, whether the Anatolian branch split off before the beginning of the Bronze Age, or even the Chalcolithic. A Bronze Age society is usually reconstructed from PIE vocabulary, but it is unclear whether this necessarily holds for inherited vocabulary in Anatolian. The Early Bronze Age starts in Anatolia at least with the beginning of the 3rd Millennium cal. BC. In the Caucasus the Bronze Age begins roughly 3300 BC. It is possible that the Proto-Anatolians were involved with the earliest development of Bronze metallurgy. In any case, while evidence that Anatolian shares common terminology of metallurgy with other branches would speak against Indo-Hittite, discarding the value of this evidence does not automatically favour the concept of Indo-Hittite, since even a 'moderate Indo-Hittite' split around 4000 BC would clearly predate the Bronze Age.
Validation of the theory would consist of identifying formal-functional structures that can be coherently reconstructed for both branches but which can only be traced to a formal-functional structure that is either (a) different from both or else (b) shows evidence of a very early, group-wide innovation. As an example of (a), it is obvious that the Indo-European perfect subsystem in the verbs is formally superimposable on the Hittite ḫi-verb subsystem, but there is no match-up functionally, such that (as has been held) the functional source must have been unlike both Hittite and Indo-European. As an example of (b), the solidly-reconstructable Indo-European deictic pronoun paradigm whose nominatives singular are *so, *sā (*seH₂), *tod has been compared to a collection of clause-marking particles in Hittite, the argument being that the coalescence of these particles into the familiar Indo-European paradigm was an innovation of that branch of Proto-Indo-Hittite.
Greater Anatolia and the Indo-Hittite Language Family: Papers Presented at a Colloquium Hosted by the University of Richmond, March 18-19, 2000. (book review)
Mar 01, 2002; Middle East & Egypt ROBERT DREWS (ed Greater Anatolia and the indo-hittite language family: papers presented at a...
Reconstructing the History of Residence Strategies in Indo-European-Speaking Societies: Neo-, Uxori-, and Virilocality
Feb 01, 2011; Abstract Linguists and archaeologists have used reconstructions of early Indo-European residence strategies to constrain...