A tribeswoman in the Andaman and Nicobar islands. Credit: Subhra Priyadarshini
Tenacious gene hunters armed with better techniques to read genes have challenged prevailing theories on the colonisation of the Andaman and Nicobar islands. They say Negrito tribes from Africa might have wandered in to populate these mystery-shrouded islands between 24,000 and 10,000 years ago1. Previous scientific evidence suggests that the colonisation of the islands happened about 45,000-50,000 years back, a theory the new research does not completely reject.

The new finding implies that the origin of the Andaman tribes could be far more recent than thus far described. It also suggests that the islands might have been populated in a not so distant past. However, this is a competing theory and does not overthrow the earlier theories on colonisation of the Andaman and Nicobar islands.

Trying to understand the earliest genomic footprints in the subcontinent, a joint team of Indian, Estonian and Spanish researchers studied mitochondrial DNA (mtDNA) samples from teeth preserved at the Natural History Museum of London as also from 10 Jarawa tribesmen in the Andaman and Nicobar islands. The mtDNA, passed from mother to child, preserves ancient signature of genes that help scientists figure out when and how modern humans fanned out across the globe. The rate at which new variations in mtDNA occur helps decipher the past of our species.

The team significantly found two subgroups of the mitochondrial DNA variety M31. These were the Andaman-specific M31a1 and India-specific M31a2. The M31 and M32 varieties have earlier been described as Andaman-specific by researchers at the Centre for Cellular and Molecular Biology in Hyderabad. Based on the study of the subgroups, the researchers have proposed several models of recent colonization of the Andaman and Nicobar islands. The first model, which relies on the genetic study of the Andaman-specific subgroup, suggests that recent peopling could have happened within the last 10,000 years.

But, this model faces a stumbling block. In case of recent colonization, there should be some proof of presence of these two subgroups in the other nearby islands of South-East Asia. But they didn't find any on these in neighbouring Malaysia, Thailand or the Yunnan province of China.

However they found the subgroup M31 having deep roots in mainland Indian tribes2 such as Chenchu, Lambodi, Pauri Bhuiya, Munda and Lodha. The presence of M31 subgroups (M31b and M31a2) in mainland India raises the possibility of Andaman islanders migrating back to India via Myanmar and from Myanmar to Andaman Islands. Myanmar still holds promise because there have been no significant mtDNA studies in that country till now.

"Whether M31a2 evolved in India or Myanmar, the migration of people to Andaman islands occurred tentatively around 24,000 years ago," says Vadlamudi Raghavendra Rao, who led the research. The finding will give insights into the tribes' adaptive traits which have implications in health and disease and their variation across human populations.

Rao says the main concern is missing data from Myanmar, which the team believes, is crucial in terms of South or South-East Asia as the source population that peopled the Andaman and Nicobar islands.

A tribeswoman in the Andaman and Nicobar islands © Subhra Priyadarshini

Previous research also suspects that Myanmar could be a vital transit point for colonization of the islands and back-migration to India. The continental shelf joining Andaman with Myanmar is relatively shallow and might have been crossed over a land bridge or short stretches of water during a depressed sea level period in the last ice age. “They didn't even need a land-bridge. They were well known sea-farers,” Rao says.

Genetic studies have shown that Andamanese evolved from anatomically modern humans who ventured out of Africa around 50,000 to 70,000 years ago. These humans colonized the Andaman and Nicobar islands while migrating along the tropical coast of the Indian Ocean to southeast Asia and Australia about 45,000 to 50,000 years ago. The study results offered important insights into the prehistory of the Andaman and Nicobar islands. They advance our understanding of the history of human migrations and settlement of regions from Africa to Australia between 50,000 and 70,000 years ago.