Y-DNA haplogroup R1a is arguably the best candidate for a genetic signal of the Proto-Indo-Europeans, who are thought to have expanded across Eurasia during the Copper Age. It's been characterized as such in several academic papers during the past 15 years, and the theory now looks more plausible than ever thanks to recent discoveries about its structure and phylogeography (for example, see here and here). Moreover, it's been found in numerous ancient remains supposedly belonging to early Indo-Europeans. R1a's brother clade, R1b, has also been proposed as a marker of the Proto-Indo-European dispersals, mostly by hobby genetic genealogists on various online forums. Among other things, they argue that it shows a high correlation with the so called Centum Indo-European languages, and probably expanded rapidly across Europe at exactly the right time (ie. during and/or after the Copper Age). However, there are also several problems with this theory, such as the fact that R1b is the dominant Y-DNA haplogroup in one of Europe's few non-Indo-European speaking groups, the Basques. Moreover, there is very little R1b in South Asia - where the early Indo-Europeans are thought to have arrived en masse from Central Asia during the early Iron Age - and much of that can be explained by historic Turkic incursions anyway. Eventually, probably in the not too distant future, next-generation sequencing of a wide range of C14 dated Eurasian remains will solve the mystery of who the Proto-Indo-Europeans were genetically. However, as already mentioned, we've now seen a reasonable number of ancient Eurasian remains tested for Y-DNA, so perhaps it might be useful to review how R1a and R1b stack up as potential Proto-Indo-European markers based on these results? In the lists below, I describe all the samples from archaeological cultures usually suspected or known to be of Indo-European origin as simply "Indo-European", while I tag those that derive from archeological cultures generally not regarded as Indo-European as "non-Indo-European".
Ancient R1a recovered to date - Corded Ware, Copper Age, Germany, Indo-European - Tocharian?, Bronze Age, Tarim Basin, Indo-European - Andronovo, Bronze Age, South Siberia, Indo-European - Urnfield, Bronze Age, Germany, Indo-European - Tagar Scythian, Iron Age, South Siberia, Indo-European - Pazyrk Scythian, Iron Age, Alati Republic, Indo-European - Xiongnu, Iron Age, Mongolia, non-Indo-European - Tachtyk Scythian, Iron Age, South Siberia, Indo-European - Slavic or Germanic, Middle Ages, Germany, Indo-European - Spanish, Modern (17th–18th centuries), Canary Islands, Indo-European Ancient R1b recovered to date - Bell Beaker, Copper Age, Germany, non-Indo-European - Urnfield, Bronze Age, Germany, Indo-European - Guanches, Iron Age and/or Middle Ages, Canary Islands, non-Indo-European - Basque, Middle Ages, Spain, non-Indo-European - Merovingian, Middle Ages, Germany, Indo-European - Czech, Middle Ages, Czech Republic, Indo-European - Spanish, Modern (17th–18th centuries), Canary Islands, Indo-EuropeanObviously, the scope of the sampling could be a lot better, but I think that already it's possible to tease out some very interesting patterns from these results. For instance, the R1a list is overwhelmingly "Indo-European". Only one sample qualifies as non-Indo-European in this scheme, and that's the Xiongnu individual from Mongolia. However, in the paper where this result was reported, Kijeong Kim et al. 2010, the R1a result was actually explained as a genetic signal of West Eurasian and indeed Indo-European influence in the Xiongnu population. In comparison, the R1b list came out significantly "non-Indo-European", starting with the Copper Age Bell Beaker sample. The oldest supposedly Indo-European R1b reported so far is the one from the Urnfield burial site. However, it's important to note that R1a was found in two remains from that site. Note also the presence of R1b in the non-Indo-European indigenous Canary Island sample, and the appearance of R1a on the islands only after they were Indo-Europeanized by the Spanish. Based on these ancient DNA results, I'd say it's easy to argue that R1a was an important Proto-Indo-European marker. It's much more difficult to argue the same for R1b. Indeed, taking all evidence into account, the most plausible scenario at the moment is that R1b became an important feature of the early Indo-European gene pool during the Bronze Age, after complex interactions between Corded Ware and Bell Beaker cultures in Central Europe. References... New Vistas on the Distant Past: Ancient Western Eurasian DNA Kijeong Kim et al., A western Eurasian male is found in 2000-year-old elite Xiongnu cemetery in Northeast Mongolia, American Journal of Physical Anthropology, Published Online: 20 Jan 2010, doi: 10.1002/ajpa.21242 Rosa Fregel, Demographic history of Canary Islands male gene-pool: replacement of native lineages by European, BMC Evolutionary Biology, 2009, 9:181, doi:10.1186/1471-2148-9-181 See also... Lots of ancient Y-DNA from China Ancient Siberians carrying R1a1 had light eyes Ancient Siberians carrying R1a1 had light eyes - take 2 First R1b from Neolithic Europe...and it ain't from the steppe
The story of R1b: it's complicated