Ranko Matasovic presented rich typological evidence (consonant-to-vowel ratio, tonal accent, number suppletion in personal pronouns, the presence of gender and the morphological optative and, possibly, the presence of glottalized consonants and ergativity) in favor of areal contacts between Proto-Indo-European (PIE) and North Caucasian (NC) in the eastern part of the Pontic steppe. He writes (pp. 306-307):
“The adduced typological parallels between PIE and Caucasian languages make it likely that PIE was, indeed, in contact with languages of the northern Caucasus. However, these contacts could also have been of indirect nature, since there are no demonstrable loanwords from North Caucasian languages in PIE, or vice versa. If such loanwords exist, their number is certainly not high. If direct contacts did exist, we cannot determine their nature: both long-term bilingualism due to exogamy and trade networks, as well as rather rapid language shift appear equally possible.”
Matasovic (p. 288-289) mentions one of such loanwords, namely the PIE kinship term *snuso- (Gr. nuos, Arm. nu, OCS snuxa, etc. ‘daughter-in-law’), apparently borrowed into many North Caucasian languages (Chechen nus, Avar nus, Akhvakh nusa, Kabardian nesa) as well as into Megrelian (nosa). But he fails to mention the cases of 1) PIE *swesor ‘sister’ and Chechen sesag ‘wife’, Lak sus, Lezgin swas, Ubykh sasa ‘bride’ and 2) PIE *geme- ‘son-in-law’ (Skrt jamatar, Avest zamatar, Gk gambros, etc.) and Chechen zam-o, Ingush sam-e ‘best man’, Lezgin c:am, Agul zam (Nikolayev S. L., and S. A. Starostin. ? North Caucasian Etymological Dictionary. Moscow, 1994). The combination of the these three apparent loanwords, all referring to marriage and affinity, favor the long-term exogamy hypothesis between PIE speakers and the natives of the Caucasus.
Naturally, if long-term marriage exchange was indeed practiced between PIE speakers and the natives of the Caucasus, we may expect to find its genetic traces in mitochondrial, Y-DNA and/or autosomal DNA. The amateur genome blogger Dienekes Pontikos recently reported the elevated frequencies of a “Caucasus” autosomal component among modern Indo-Europeans and its absence among Basques and low frequencies among Finns. While he labeled it “West Asian” and misconstrued it as suggesting a Near Eastern origin for Indo-Europeans (in contrast to the mainstream Kurgan hypothesis), it seems to be the best available evidence for genetic exchange between Indo-European speakers and the speakers of North Caucasian languages in the Pontic steppe and North Caucasus areas. The Chechens whose language contains all three kinterm loans from PIE, show this component at 54.6%. Matasovic focuses on the Maykop culture (3700-2500 BC) as providing the best archaeological correlate to the contact zone between PIE and North Caucasian languages. The Maykop culture is centered in Adygeia, which shows 52.5% of that “Caucasus” autosomal component. We have, therefore, a very strong fit between linguistics, kinship studies, population genetics and archaeology in this case.
It remains to be seen if this fit is real or spurious. It’s noteworthy that all NC societies are strictly patrilocal and patrilineal. PIE society is also reconstructed as patrilineal and patrilocal (see The Encyclopedia of Indo-European Cultures, with the original proposal in Friedrich, Paul, “Proto-Indo-European Kinship,” Ethnology 5 (1966), pp. 1-36). This commonality would facilitate marriage exchange between the two populations in both directions, as PIE women marrying NC men would go live with their husbands without violating the rules of either society. The same works for NC women. At the same time, PIE men and NC men would rarely end up as son-in-laws in foreign households, unless they had been first taken as prisoners. We don’t know yet if PIE speakers contributed any genes to the NC speech community. Neither do we know if genetic admixture between PIE and NC speakers manifested in mtDNA and Y-DNA. If it did, then we may be able to infer from the pattern of this admixture whether this gene exchange was sex-biased and whether it took place under peaceful or violent circumstances.