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).
Relation to the possible homeland
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.
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.
^Underhill, Peter A, et al. "Separating the Post-Glacial Coancestry of European and Asian Y Chromosomes within Haplogroup R1a." European Journal of Human Genetics, vol. 18, no. 9, 2010, pp. 1074-1074., doi:10.1038/ejhg.2010.65.
^Martin Litchfield West, Indo-European poetry and myth (2007), p. 7.
^Wolfram Euler: Indoiranisch-griechische Gemeinsamkeiten der Nominalbildung und deren indogermanische Grundlagen [= Aryan-Greek Communities in Nominal Morphology and their Indoeuropean Origins]. Innsbruck, 1979 (in German).
^Martin Litchfield West (1999), "The Invention of Homer". Classical Quarterly 49 (364).
^Renfrew, A. C., 1987, Archaeology and Language: The Puzzle of Indo-European Origins, London: Pimlico. ISBN0-7126-6612-5; T. V. Gamkrelidze and V. V. Ivanov, The Early History of Indo-European Languages, Scientific American, March 1990; Renfrew, Colin (2003). "Time Depth, Convergence Theory, and Innovation in Proto-Indo-European". Languages in Prehistoric Europe. ISBN3-8253-1449-9.
^A. Bammesberger in The Cambridge History of the English Language, 1992, ISBN978-0-521-26474-7, p. 32: the model "still remains the background of much creative work in Indo-European reconstruction" even though it is "by no means uniformly accepted by all scholars."