The Maternal Ancestry of Great Nicobar: What Mitochondrial Genomes and Deep-Time Isolation Reveal about the Peopling of the Bay of Bengal

The maritime geography of the eastern Indian Ocean is recorded not just in underwater bathymetric contours but also within the genomes of its most isolated inhabitants. Located in the southernmost reaches of the Andaman and Nicobar archipelago, Great Nicobar Island serves as a home to two distinct communities: the coastal, agricultural Nicobarese and the interior, semi-nomadic Shompen hunter-gatherers. For decades, the origin of these groups remained a subject of intense academic dispute, with early anthropologists relying on surface morphological comparisons to assert mixed origins. Today, high-resolution genetic sequencing has bypassed these highly speculative typologies, offering a clearer picture written in mitochondrial DNA (mtDNA) (genetic material inherited solely through the maternal line).

The emerging genomic evidence reveals a deep-time maternal genetic bottleneck (a drastic reduction in population size that limits genetic variation) dating back to the Last Glacial Maximum (LGM) (the period during the last Ice Age when global ice sheets reached their greatest extent). This deep-time bottleneck, followed by long-term ecological isolation, has heavily shaped the modern genetic profile of the region.

The Stratigraphy of the Genome

In classical field archaeology, stratigraphy (the analysis of soil and cultural layers over time) allows researchers to trace relative timelines from the lowest, oldest deposits to the surface. Population genetics operates on a similar principle. Mutations accumulate in human DNA at a relatively steady rate, allowing scientists to reconstruct ancestral chronologies and pinpoint when specific maternal lineages branched off from a common ancestor.

For the Shompen, early attempts to categorize their ancestry suggested they were an isolated offshoot of the Negrito populations of the Andaman Islands. However, molecular studies have firmly dismantled this theory. The maternal ancestry of the Shompen is characterized by two distinct mitochondrial clades (groups of organisms sharing a common ancestor) within the broader R lineage: B5a and R12 (subsequently classified as R22).

The B5a lineage is particularly revealing. It exhibits a coalescence age of roughly 17,000 years, a timeline that aligns closely with the end of the Last Glacial Maximum. While B5a is widely distributed across littoral (coastal) and insular Southeast Asia, the specific sub-branches found among the Shompen point to a profound founder effect (the loss of genetic variation when a new population is established by a very small number of individuals). The genetic diversity within this maternal lineage is exceptionally low, indicating that the modern Shompen descend from a highly restricted maternal pool that survived severe climatic and geographic constraints.

Similarly, the R22 maternal lineage is shared between the Shompen, the coastal Nicobarese, and ancient Southeast Asian populations from mainland Indochina and Island Southeast Asia. In a study published in the Journal of Human Genetics, researchers analyzed mitochondrial and Y-chromosomal markers from Shompen individuals. While their paternal lineages strongly link them to Austroasiatic speakers of mainland Asia, their maternal mtDNA reveals an ancient, deep-seated connection to the Indonesian archipelago. This dual signature indicates that the Shompen are descendants of Mesolithic (middle Stone Age) hunter-gatherers of Southeast Asian origin, deriving from at least two ancestral source populations.

Redefining the Timeline of Settlement

The timing of human arrivals in the Nicobar archipelago has long been debated. Traditional linguistic reconstructions—often cited in linguistic journals—held that the ancestors of the Nicobarese settled the islands during the early Holocene (the geological epoch starting approximately 11,700 years ago). These models suggested that early Austroasiatic speakers migrated southward as sea levels stabilized after the glacial melt.

However, high-resolution genomic data has forced a significant revision of this chronological framework. In a milestone study published in the European Journal of Human Genetics, a research team led by Dr. Kumarasamy Thangaraj of the CSIR-Centre for Cellular and Molecular Biology (CSIR-CCMB) and Prof. Gyaneshwer Chaubey of Banaras Hindu University (BHU) analyzed DNA markers from 1,559 individuals across South and Southeast Asia.

The study demonstrated that while the Nicobarese share a powerful Austroasiatic ancestral component with populations across Southeast Asia, their actual settlement of the Nicobar Islands occurred much later than previously assumed. The molecular clock indicates a settlement date of approximately 5,000 years ago, contrasting sharply with the 11,700-year linguistic estimate.

Among mainland populations, the genomic analysis highlighted a close maternal and autosomal connection between the Nicobarese and the Htin Mal—an Austroasiatic-speaking group residing in the mountainous borderlands of Thailand and Laos. Despite this ancient shared ancestry, the Htin Mal have experienced a pronounced genetic drift (random fluctuations in gene frequencies over time) due to their long-term geographic and cultural isolation, making them a valuable genetic proxy for understanding the ancestral Austroasiatic homeland.

Refuting Simple Migration Models

The genetic landscapes of both the Shompen and the Nicobarese show that simple, linear migration models are insufficient to explain the history of the eastern Indian Ocean. During the Last Glacial Maximum, the dramatic drop in global sea levels exposed the Sunda Shelf, connecting modern-day Sumatra, Java, Borneo, and mainland Southeast Asia into a contiguous landmass. This massive land-bridge facilitated human movement across the region.

As the ice sheets melted and sea levels rose, these land bridges flooded, stranding populations in isolated pockets. This ecological isolation triggered a severe maternal genetic bottleneck, leaving the ancestors of the Shompen with extremely restricted mitochondrial diversity. The preservation of these ancient, unaltered genetic markers—such as the B5a and R22 lineages—is a direct consequence of this geographical containment. The Nicobarese and Shompens have remained largely unmixed with outside populations for millennia.

While popular headlines frequently seek to brand indigenous island groups as "living fossils" or "unchanged since the Stone Age," the genetic data tells a far more nuanced and dynamic story of adaptation, geographic restriction, and biological survival in the face of changing post-glacial environments.