What Does Genomic History Reveal About the World’s Most Isolated Tribe?

The Sentinelese, who live on North Sentinel Island, are some of the most isolated Indigenous groups in the world. Since they firmly oppose contact with outsiders and are legally protected by the Indian government, scientists do not possess any direct DNA samples of the Sentinelese. For this reason, researchers examine other Andamanese groups such as the Onge, the Jarawa, and the Great Andamanese, as well as historical remains and specimens held in museums. Although these groups give insights into the wider genetic history of the Andaman Islands, they should not be regarded as exact genetic stand-ins for the Sentinelese.

Genomic studies indicate that the Andamanese people retain some of the oldest branches of human ancestry and that their ancestors must have split off from the other eastern Eurasian groups relatively early on. This lends support to the idea that their ancestors were linked to the early human movements throughout southern Asia following the spread of Homo sapiens out of Africa. According to the Southern Dispersal hypothesis, certain of the early migrants travelled through Arabia and South Asia along or near the Indian Ocean before arriving in Southeast Asia and Australia.

Mitochondrial DNA shows that there are ancient maternal lineages. The Andamanese have particular branches of the haplogroup M, namely M31 and M32. As for their paternal history, it involves the Y-chromosome haplogroup D, a lineage which is also present today in certain areas of Tibet, among the Ainu of Japan, and in the Andaman Islands. These distributions could be the remains of a once much wider ancient population which was later altered by migrations and population expansions.

The Andamanese are not unchanged survivors from 60,000 to 70,000 years ago; rather, their genetic makeup has been altered over thousands of years due to their small population sizes, isolation, genetic drift, and bottlenecks. What is important about them is that they preserve some of the unusually old branches of the human family tree.

Autosomal DNA gives a more comprehensive view than studies of mtDNA or the Y-chromosome. The Andamanese seem to form an early-splitting branch within the eastern Eurasian populations and thus are useful for reconstructing the early population history of Asia. Their genomes are also helpful when looking at Ancient Ancestral South Indian (AASI)-related ancestry, and the Onge in particular can act as a genetic reference or stand in for modelling this deep ancestry, even though the Onge are not themselves AASI.

The fact that ancient DNA has been obtained from Hoabinhian hunter-gatherers in Southeast Asia has now provided further evidence for an ancient population group which extended throughout South and Southeast Asia. This lends support to the hypothesis that ancestors related to both the Andamanese and to the ancient Southeast Asian hunter-gatherers were at one time widely distributed before later Neolithic population expansions.

The article also talks about archaic humans; the Andamanese have Neanderthal ancestry, as do other non-African populations, but they do not display a significant amount of Denisovan ancestry comparable to that found in Oceania. A previous suggestion of a mysterious "ghost hominin" contribution to the South and Southeast Asian populations has since been challenged, and the article says that the present paleogenetic consensus does not support the idea of a unique ghost-hominin ancestry in the Andamanese.

In the past anthropologists have grouped the Andaman Islanders with African Pygmy populations because of their physical resemblance, using the problematic term "Negrito"; however modern genetics has rejected a recent common African ancestry as the explanation and now understands similar features such as short stature to be the result of convergent adaptation to specific environments.

The article also points out that isolation has brought about both evolutionary benefits and biological drawbacks. Although long-term isolation has enabled the Andamanese people to develop adaptations to their tropical surroundings, it has also left them susceptible to diseases brought in from outside. This is clearly shown by the severe decline in population that occurred among the Great Andamanese groups after they came into contact with Europeans during the colonial period.

In a nutshell, the genomic evidence does not indicate that the Sentinelese are a "frozen" or "primitive" group dating back 60,000 years; on the contrary, it shows that related Andamanese populations retain deep branches of ancient Eurasian ancestry and thus provide a valuable insight into early human population history, whereas their long isolation and distinct history mean that it is very important to protect them from outside contact.