Overview
Haplogroup A00 represents the deepest known divergence within the human Y-chromosome phylogeny and is widely considered the most basal lineage carried by living humans. First identified in a rare Y-chromosome sample from an African American individual with Cameroonian paternal ancestry, A00 demonstrated a separation time far older than any previously documented haplogroup. Genetic studies estimate its formation in the late Middle Pleistocene, potentially predating the emergence of anatomically modern Homo sapiens. Its extreme antiquity dramatically reshaped interpretations of early human paternal structure by illuminating the presence of deeply divergent lineages that persisted in isolation for tens of thousands of years. While extremely rare today, A00 provides an invaluable window into the earliest stages of human patrilineal history, representing a lineage that branched off before all other surviving Y-chromosome lineages including those grouped under A0, A1, BT and CT.
Geographic distribution
Present-day A00 lineages are highly restricted geographically. Nearly all known carriers trace paternal ancestry to the western regions of Cameroon, particularly among Mbo-speaking groups and several small, isolated communities within southwestern Cameroon. Sporadic occurrences in the African diaspora (for example, among African Americans) reflect historical population dispersal rather than a broader modern distribution. No confirmed A00 lineages have been discovered outside western Central Africa in indigenous populations. This extreme geographic confinement suggests long-term regional isolation, possibly involving small, demographically constrained groups that maintained deep Y-lineage diversity across vast spans of time.
Ancient DNA
- As of current publications, A00 has not yet been identified directly in ancient DNA remains. However, its extreme time depth suggests that ancestral A00 lineages were present among Middle Pleistocene African populations predating the major demographic expansions that shaped later Homo sapiens population structure.
- Indirect evidence from population-genetic modeling indicates that A00 may trace back to deeply structured populations within Africa, some of which contributed minimally to later widespread Y-chromosome expansions. This is consistent with hypotheses proposing long-lasting population subdivision among early Homo sapiens groups or late-surviving archaic populations in Africa.
- Although ancient A00 specimens remain undiscovered, the lineage's basal position is frequently referenced in studies of early human divergence, population structure and potential introgression from archaic African lineages.
Phylogeny & subclades
Within the Y-chromosome tree, A00 stands entirely alone at the base. It is the earliest known split in the modern Y-lineage and does not descend from A0, A1 or BT; instead, all other haplogroups form a sister branch relative to A00. The internal phylogeny of A00 itself contains several sub-branches defined by rare private SNPs, though limited sampling prevents a detailed reconstruction comparable to more numerous haplogroups. A00 is currently divided into A00* (basal) and several downstream microlineages identified in Mbo individuals, each representing exclusive clusters defined by unique mutations. These branches likely accumulated over exceptionally long timespans, reflecting the deep and isolated history of this haplogroup.
- A00* (basal, extremely rare)
- A00a (a downstream branch defined by internal SNP clusters)
- A00b (additional microlineages identified through high-resolution sequencing)
Notes & context
A00 fundamentally altered the understanding of the Y-chromosome phylogeny. Prior to its discovery, the deepest splits were thought to lie within haplogroup A broadly, but the identification of A00 demonstrated the presence of a lineage that diverged substantially earlier than all others. The estimated divergence time—sometimes exceeding 250,000 years—has led researchers to reconsider the complexity of early human male structures, including the possibility of deeply subdivided ancestral populations within Africa. Because A00 is so rare and geographically localized, obtaining high-coverage sequences has been challenging, but ongoing research continues to refine its placement, internal branching and divergence times. A00's existence highlights the need to re-evaluate simplistic models of early human demography, suggesting a mosaic of populations with limited gene flow persisting for vast periods.
References & external links