The modern Slavic peoples carry a variety of mitochondrial DNA haplogroups and Y-chromosome DNA haplogroups. Yet two paternal haplogroups predominate: R1a1a [M17] and I2a2a [L69.2=T/S163.2]. The frequency of Haplogroup R1a ranges from 63.39% in the Sorbs, through 56.4% in Poland, 54% in Ukraine, 52% in Russia, Belarus, to 15.2% in Republic of Macedonia, 14.7% in Bulgaria and 12.1% in Herzegovina. The correlation between R1a1a [M17] and the speakers of Indo-European languages, particularly those of Eastern Europe (Russian) and Central and Southern Asia, was noticed in the late 1990s. From this Spencer Wells and colleagues, following the Kurgan hypothesis, deduced that R1a1a arose on the Pontic-Caspian steppe.