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Phylogenetic models have been reconstructed on Indo-European populations through comparative studies using both genotyping based on haplotypes, specifically haplogroup R1a,  as well as the analysis of their linguistic lexicon.
The Graeco-Armeno-Aryan group supposedly branched off from the parent Indo-European stem by the mid-3rd millennium BC. Proto-Armenian, would have been between Proto-Greek and Proto-Indo-Iranian, which would be consistent with the fact that Armenian shares some features only with Indo-Iranian (the satem change) but others only with Greek (s > h).
In the context of the Kurgan hypothesis, Graeco-Aryan is also known as "Late Proto-Indo-European" or "Late Indo-European" to suggest that Graeco-Aryan forms a dialect group, which corresponds to the latest stage of linguistic unity in the Indo-European homeland in the early part of the 3rd millennium BC. By 2500 BC, Proto-Greek and Proto-Indo-Iranian had separated, moving westward and eastward from the Pontic Steppe, respectively.
If Graeco-Aryan is a valid group, Grassmann's law may have a common origin in Greek and Sanskrit. However, that Grassmann's law in Greek postdates certain sound changes that happened only in Greek and not Sanskrit, which suggests that it could not have been inherited directly from a common Graeco-Aryan stage. Rather, it is more likely that an areal feature spread across a then-contiguous Graeco-Aryan–speaking area. That would have occurred after early stages of Proto-Greek and Proto-Indo-Iranian had developed into separate dialects but before they ceased to be in geographic contact.
Evidence for the existence of a Graeco-Aryan subclade was given by Wolfram Euler's 1979 examination on shared features in Greek and Sanskrit nominal flection. Graeco-Aryan is invoked in particular in studies of comparative mythology such as Martin Litchfield West (1999) and Calvert Watkins (2001).
A hypothesis has placed Greek in a Graeco-Armenian subclade of Indo-European, though some researchers have integrated both attempts by including also Armenian in a putative Graeco-Armeno-Aryan language family, further divided between Proto-Greek (possibly united with Phrygian) and thus arriving at a Armeno-Aryan subclade, the putative ancestor of Armenian and Indo-Iranian.