Haplogroup A00

Haplogroup A
Possible time of origin	roughly 270,000 ybp
Possible place of origin	Africa
Ancestor	Human Y-MRCA
Descendants	A00, A0, A1, A2, A3, BT
Highest frequencies	macro-haplogroup subclades found among various populations in Northwest-Central Africa

Haplogroup A is a human Y-chromosome DNA haplogroup. Unlike other Y-DNA clades, it is not defined by a specific mutation. It is the foundational haplogroup to all known patrilineal lineages carried by modern humans, and thus is the Y-chromosomal Adam.

Formerly also known as "clade I", bearers of extant sub-clades of haplogroup A are entirely found in Africa (or among descendants of recently extracted African populations), in contrast with the descendant haplogroup BT (clade II-X) bearers of which participated in the Out of Africa migration of anatomically modern humans.

The most basal subclades of haplogroup A are, by age of divergence, "A00", "A0", "A1" (also "A1a-T") and "A2-T". Haplogroup BT, ancestral to all non-African haplogroups, is a subclade of A2-T.

There are terminological difficulties, but as "haplogroup A" has come to mean "the foundational haplogroup" (viz. of contemporary human population), haplogroup A is not defined by any mutation but refers to any haplogroup which is not descended from haplogroup BT, i.e. defined by the absence of the defining mutation of that group (M91). By this definition, haplogroup A includes all mutations that took place between the Y-MRCA (estimated at some 250 kya) and the mutation defining haplogroup BT (estimated at some 80–70 kya), including any extant subclades that may yet to be discovered.

Bearers of haplogroup A (i.e. absence of the defining mutation of haplogroup BT) have been found in Southern Africa's hunter-gatherers, especially among the San people. In addition, the most basal mitochondrial DNA lineages are also largely restricted to the San. However, the A lineages of Southern Africa are sub-clades of A lineages found in other parts of Africa, suggesting that A sub-haplogroups arrived in Southern Africa from elsewhere. The two most basal lineages of Haplogroup A, A0 and A1 (prior to the announcement of the discovery of haplogroup A00 in 2013), have been detected in West Africa, Northwest Africa and Central Africa. Cruciani et al. (2011) suggest that these lineages may have emerged somewhere in between Central and Northwest Africa. Scozzari et al. (2012) also supported "the hypothesis of an origin in the north-western quadrant of the African continent for the A1b [ i.e. A0 ] haplogroup".

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