The deepest branch of the human paternal tree
Haplogroup A sits at or near the root of the human Y-chromosome tree — every other paternal lineage descends from branches that split off after it. It is found almost entirely in Africa, at its highest frequencies among certain Khoisan-speaking groups of southern Africa and among some populations of the Nilotic and Ethiopian regions.
Haplogroup A is overwhelmingly African. It reaches notable frequencies among Khoisan speakers in southern Africa, among some Nilo-Saharan populations in Sudan and South Sudan, and in parts of Ethiopia. Elsewhere it is rare. Because it is so basal, the label covers a set of very deep and quite distinct sublineages rather than a single tight cluster, and two men both recorded as 'A' may sit on branches that diverged tens of thousands of years ago.
Haplogroup A represents the deepest split in the surviving Y-chromosome tree, and its most basal branches are among the oldest paternal lineages that have been sampled in living men. Age estimates for the very deepest divergences run into many tens of thousands of years and vary considerably between studies. The picture has been complicated, rather than settled, by the discovery of extremely deep and rare lineages that push the estimated age of the common paternal ancestor further back than earlier work assumed.
A is conventionally divided into deeply separated branches, often written A00, A0, A1a and A1b, which split very early. A00 in particular is a strikingly deep and rare lineage identified in a small number of individuals of West African descent, and its discovery notably revised estimates of how old the paternal tree is. These branches are not close relatives in any recent sense; the shared label reflects position on the tree, not a recent shared ancestor.
An A result points to deep African paternal ancestry, but the top-level label is far too broad to be genealogically informative on its own — the branches beneath it are enormously divergent. Downstream SNP testing is essential to establish which specific sublineage you carry, and only then does comparison with others on the map become meaningful.
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