M31

Origins and Evolution

mtDNA haplogroup M31 is a descendant lineage within macro-haplogroup M, part of the deep maternal diversity that expanded across South and Southeast Asia during the Late Pleistocene. Based on its phylogenetic position and the distribution of closely related M subclades, M31 most likely arose between approximately ~15–25 kya, during a period of regional diversification following initial colonization of South and Southeast Asia. Its pattern — a concentrated presence on islands with scattered low-frequency occurrences on adjacent mainland coasts — is consistent with early coastal dispersal and subsequent isolation of insular populations.

Subclades

Several sublineages of M31 have been reported in the literature (commonly annotated as M31a, M31b, etc.), with M31a particularly associated with populations of the Andaman Islands in published studies. These subclades show limited internal diversity in insular populations (suggesting founder effects and long-term isolation) and greater diversity where observed on the mainland, reflecting sporadic gene flow and local differentiation.

Geographical Distribution

The geographic distribution of M31 is highly localized. It reaches its highest relative frequency in certain Andaman Island groups and appears at low to very low frequencies across parts of South Asia (coastal India, Bangladesh, Sri Lanka), Myanmar, and into mainland Southeast Asia (Thailand, Myanmar regions). Scattered occurrences in some Himalayan foothill and island rim populations likely reflect either ancient coastal dispersals or later low-level gene flow. Ancient DNA studies that include South Asian and insular remains have occasionally recovered M-related lineages that help anchor the antiquity and regional continuity of these maternal branches.

Historical and Cultural Significance

M31 is best interpreted as a marker of pre-Neolithic coastal and insular maternal populations. Its concentration in island groups such as the Andaman Islanders highlights its relevance to studies of island colonisation, founder effects, and long-term genetic continuity among small, isolated hunter-gatherer communities. Unlike mtDNA lineages associated with agricultural expansions (for example some R and N-derived lineages in parts of South Asia), M31 does not show robust association with major farming-associated cultural dispersals and instead reflects older demographic structure in the region.

Conclusion

mtDNA haplogroup M31 represents a Late Pleistocene maternal lineage that helps document early coastal and insular population histories in South and Southeast Asia. Its distribution — concentrated in insular groups with low-frequency mainland occurrences — and its phylogenetic pattern are consistent with an origin in the South/Southeast Asian coastal region followed by drift and isolation in island populations and limited mainland dispersal.