In Indo-European linguistics, the term Indo-Hittite (also Indo-Anatolian) refers to Sturtevant's 1926 hypothesis that the Anatolian languages may have split off a Pre-PIE language considerably earlier than the separation of the remaining Indo-European languages. The term may be somewhat confusing, as the prefix Indo- does not refer to the Indo-Aryan branch in particular, but is iconic for Indo-European, and the -Hittite part refers to the Anatolian language family as a whole.
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.
According to Craig Melchert, the current tendency is to suppose that Proto-Indo-European evolved, and that the "prehistoric speakers" of Anatolian became isolated "from the rest of the PIE speech community, so as not to share in some common innovations." Hittite, as well as its Anatolian cousins, split off from Proto-Indo-European at an early stage, thereby preserving archaisms that were later lost in the other Indo-European languages.
Traditionally there has been a strong notion among Indo-European linguistics that the Anatolian branch was separated earlier than other branches. Within the framework of the Kurgan hypothesis, the split is estimated to have occurred in roughly 4000 BC.
Some fundamental shared features, such as the aorist category of the verb (which denotes action without reference to duration or completion), with 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 such as 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 vocabulary, 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 typologically alien 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 hypothesizes the existence of one or more additional spirant or stop 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 as early as 1938 it was demonstrated 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.
Recent subgrouping calculations of Indo-European branches using a method that accounts for the distribution of PIE verbs (SLR-D) reject an early separation of Anatolian languages altogether and yield results that place a genealogical split of Anatolian (and Tocharian) within a more recent grouping together with Greek, Albanian and Armenian, in a single branch together with Indo-Iranian, though at distance from the genealogical splits of Balto-Slavic, Italo-Celtic and Germanic that are harboured within another branch, thus supporting proponents of an IE expansion that roughly parallels the adoption of the bronze metallurgy.
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), 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.