So this is in the news, Police: American adventurer John Allen Chau killed by isolated Sentinelese tribe on Indian island. There is some talk about whether the guy was a Christian missionary or not, but that’s not really too relevant. Whether he believes in evolution or not (he was a graduate of a very conservative Christian college), he definitely won a Darwin award before he expired.
North Sentinel is totally isolated, and the people who live there, the Sentinelese, are out of contact with the rest of the world. They are hostile to the outside world. And this is probably why the Sentinelese are still around, as the outside world does not have a good track record with hunter-gatherers. The Andamanese as a whole had a reputation for being very hostile to outsiders, as traders knew not to stop too long for water.
First, they are not that genetically unique. A recent paper on the genetics of Southeast Asia using ancient samples makes their affinities clear. The Onge, an Andamanese tribe, are positioned close to the two ancient samples from Laos and Malaysia. They emerge out of the same milieu as Paleolithic Southeast Asians (whose Hoabinhian culture persisted deep into the Holocene).
The Andamanese themselves are probably from mainland Southeast Asia. The gap between the islands and the mainland was smaller ~20,000 years ago when the sea levels were lower. They could have come up from the south or the north.
Second, they are not the most “ancient” people. That doesn’t make any sense. We are all people who are equally ancient. We all descend by and large outside of Africa from a migratory wave that expanded ~60,000 years ago. Andamanese, Chinese, and Europeans. What is “ancient” about them is that they are hunter-gatherers who have continued to practice that mode of production down to the present. But that’s a matter of culture and not genetics.
Third, in alignment with the above two points, they are not uniquely and distinctly isolated from all other human populations. They are not descendants of an early wave out of Africa preserved on these islands. They are not distinct from all other non-Africans. Rather, they seem to be closer to the peoples of Oceania, Papuans, and Australian Aboriginals, than Northeast Asians. And closer to Northeast Asians than they are to West Eurasians. The latest evidence is that the Andamanese were part of a broader diversification of lineages ~40-50,000 years ago to the east of India that gave rise to the peoples of the western Pacific Rim. Within this broader set of groups, some form a distinct clade that is not with Northeast Asians (often these are like “Australasian”).
Finally, the census size for the Sentinelese is in the range of 100 individuals. This seems on the edge of viability over the long term.