Overview
Haplogroup A1b1b, more commonly known by its defining mutation M42 and designated BT, represents one of the single most transformative nodes in the entire history of the human Y chromosome. Unlike most haplogroups, which describe the ancestry of a regional population or a particular lineage, BT forms the foundational paternal root for virtually all living men outside the deepest African clades A and A1. More than 90% of the global male population descends from lineages emerging from BT. Thus, BT defines the ancestral stock that ultimately diversified into the massive B, C, D, E, F, and all F-derived haplogroups (G, H, I, J, K, P, Q, R, T, and others).
This exceptional evolutionary role makes BT central to reconstructing the origins and dispersal of Homo sapiens. BT does not represent a single migration, culture, or event, but a broad ancestral population complex that lived in Africa during the late Middle Pleistocene. The lineage emerged in an era of significant climatic fluctuation, when regional refugia allowed small human populations to survive while others collapsed. Within this dynamic landscape, BT men belonged to a population that would give rise to lineages that later dominated the Holocene world—agriculturalists of the Near East, pastoralists of the Sahara and Sahel, foragers of eastern and southern Africa, the first modern humans to reach East Asia, and the Paleolithic settlers of Europe and Oceania.
Although the BT node itself is deep in time and sparsely represented in present-day individuals, its descendants shaped every major phase of human prehistory: the Out-of-Africa expansion, the differentiation of Eurasian forager groups, the spread of early farming cultures, and the explosive Bronze Age paternal expansions. Understanding BT is, in effect, understanding the origin of nearly all modern paternal lineages.
Geographic distribution
As a lineage that predates the major radiations of human populations, BT cannot be meaningfully associated with a single geographic region in the modern world. Today, no living men are considered basal BT*, as all identified individuals fall into one of its numerous downstream groups. This is not surprising—BT is extremely ancient, and its early diversity was almost entirely replaced or absorbed by its own descendants. However, the geographic origin of BT is strongly supported to be within Africa, most likely in the eastern or southern regions of the continent, where both archaeological and genomic evidence indicate the presence of deeply diverged Homo sapiens populations during the Middle Stone Age.
The strongest indirect evidence comes from the distribution of BT’s earliest descendant haplogroups. B, one of the deepest lineages under BT, is overwhelmingly African and is associated with central, southern, and eastern hunter-gatherer populations. CT, the other major branch of BT, shows a pattern that fits an African origin followed by dispersal: all non-African paternal diversity derives from CT, but the earliest CT splits—including DE and CF—likely occurred within Africa before one branch extended beyond the continent. This suggests a demographic scenario where BT diversified inside Africa into multiple sublineages, only one of which ultimately contributed to the successful out-of-Africa migrants.
While no modern populations carry BT*, its legacy is visible everywhere. In a very literal sense, every region outside Africa that hosts human populations today is overwhelmingly dominated by BT-derived paternal lineages.
Ancient DNA
- No ancient individual sequenced to date has been assigned to basal BT*, which is expected for a lineage this ancient; instead, ancient males fall into downstream clades such as B, C, D, E, CF-derived lineages, and many others.
- Early Holocene and Late Pleistocene African genomes demonstrate considerable BT-descendant diversity, with several B and E lineages present among hunter-gatherer populations.
- The earliest CT-derived ancient samples outside Africa, including Ust'-Ishim (~45,000 BP), belong to downstream lineages ultimately tracing to BT through CT → CF.
- Recent ancient samples from East Africa, dated to 8,000–20,000 BP, include rare B2 lineages, reinforcing that BT’s diversification occurred well before the emergence of structured Holocene societies.
- Comparative mutation rate analyses show that BT predates the most robust phase of human genetic structuring and corresponds to a long, stable period of population continuity in Africa.
Phylogeny & subclades
BT forms one of the most critical branching points in the modern Y-chromosome tree. It divides into two primary descendant lineages: B and CT. Haplogroup B is almost entirely African, with the highest frequencies among central African rainforest foragers, Khoisan-speaking populations, and some east African groups. Its phylogeographic profile reflects ancient structure and continuity within Africa.
CT, the second major branch, is even more consequential. Every non-African paternal lineage descends from CT. CT divides into DE and CF; DE gives rise to the widely distributed E (dominant across Africa and the Middle East) and D (prominent among Tibetan, Japanese, and Andamanese groups), while CF spawns the expanding F-derived macro-radiation, leading to G through T—including all major Eurasian, Oceanian, and Native American haplogroups.
This topology makes BT the ancestor not of one major lineage, but of almost all major lineages. While the mutation M42 defines BT, numerous downstream SNPs define each major radiation, making BT the root of the modern Y-chromosome macro-structure. The massive asymmetry between the rarity of BT* and the overwhelming dominance of BT-derived clades is characteristic of ancient lineages where almost all diversity lies in descendant branches, not in the root node.
- B (ancestrally African; deep structure among forager populations)
- CT (ancestor of all non-African lineages)
- DE (ancestor of D and E)
- CF (ancestor of C and F)
- F (origin of G, H, I, J, K, P, R, T and related clades)
Notes & context
BT’s evolutionary legacy lies not in its present-day frequency but in the overwhelming dominance of its descendant lineages. The out-of-Africa dispersal—one of the most significant events in human prehistory—required a paternal lineage capable of surviving bottlenecks, environmental instability, and demographic challenges. BT provided this foundation. A combination of ecological opportunity, genetic drift, cultural developments, and demographic growth in descendant populations led to the replacement of earlier Y lineages across much of Africa and Eurasia.
Modern research increasingly supports the idea that human evolutionary history within Africa prior to 70,000 BP was more complex than once assumed. Multiple semi-isolated groups existed, exchanging genes intermittently over vast timescales. BT likely belonged to one such population that eventually became central to later expansions.
As whole-genome sequencing becomes more widespread in Africa—currently the least-sampled continent in paleogenetics—undiscovered deep-rooting BT* or near-BT lineages may eventually emerge. These would have transformative implications for the reconstruction of the full Y-chromosomal phylogeny and for understanding human origins.
References & external links