Haplogroup O2a (Y-DNA)

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In genetics, Haplogroup O2a (M95) is a human Y-chromosome DNA haplogroup. Haplogroup O2a is a descendent branch of Haplogroup O2. Its closest extant phylogenetic relatives are the Haplogroup O2* Y-chromosomes found at a low frequency throughout most of East Eurasia and the Haplogroup O2b Y-chromosomes found at low frequencies among the indigenous populations of Inner Mongolia and Manchuria and at much higher frequencies in Korea and Japan.

Haplogroup O2a is distributed widely in Asia, from southern India to the Altai Mountains and Central Asia in the west, and from Indonesia to northern China and Japan in the east. It is found only at marginally low frequencies of approximately 1% at the periphery of its distribution in southern India, Central Asia, northern China, and Japan, but many populations within the vast intervening territory in South Asia, Southeast Asia, and southern China display a greatly elevated frequency of Haplogroup O2a Y-chromosomes. Patrilines within Haplogroup O2a predominate among the Austro-Asiatic populations of South and Southeast Asia, such as the Khmer of Cambodia and the Khasi of Meghalaya in northeastern India. Some researchers have reported that slightly over half of all men in a composite sample of Austro-Asiatic speakers belonged to Haplogroup O2a. Haplogroup O3 (M122), which attains its peak frequency among the Sino-Tibetan and Miao-Yao peoples of China and Southeast Asia, and Haplogroup O1a (M119), which predominates among Taiwanese aborigines and many populations of the Philippines, also generally occur among speakers of Austro-Asiatic languages, but at much lower frequencies than Haplogroup O2a.

Haplogroup O2a has also been found to occur at a rather high frequency among the Daic-speaking peoples of Thailand and neighboring areas, which may reflect assimilation of the older Austro-Asiatic Mon-Khmer populations that have left ample evidence of their presence in the region prior to the immigration of Daic speakers.

Outside of the region in which Austro-Asiatic languages are currently spoken or have a historically attested presence, Haplogroup O2a reaches its highest frequencies among the populations of the islands of Java, Bali, and Borneo in central Indonesia. The reason for its substantial presence in these islands, all of which are Austronesian-speaking, is yet to be elucidated.