In late November, an American missionary attempted to bring Christianity to the Sentinelese people, a remote community in the Andaman and Nicobar Islands, an Indian territory in the Bay of Bengal. Circumventing Indian authorities, John Allen Chau paddled a kayak to North Sentinel, the community's island home. The Sentinelese killed him.

The Sentinelese are one of the last isolated ethnic minorities in Asia. There are also some hold-outs in West Papua, Indonesia, and in South America, where a handful of tribal communities in the Amazon basin have shunned contact with the world. All face a deeply uncertain future as the modern world collides with their ancient traditional cultures.

The danger was already clear when I visited the Andaman Islands 20 years ago, for a project supported by The British Library to document indigenous music in Asia. I had recorded the music of several ethnic minorities, including the Kalash in Pakistan, descendants of the Greek armies of Alexander the Great, and the Moken in Thailand, sea nomads who have sailed along the Andaman coast for centuries.

All these communities have something in common, beyond the uniqueness of their languages, faiths and cultures: They do not assimilate easily into the nation states that have sprung up around them, in part because of their unique identities, but also because they are often treated as illegal migrants or second-class citizens. But as modernity reaches every part of the globe, and as the natural environment is destroyed, these peoples are being assimilated whether they want it or not.

The horticulturalist cultures usually make the best of it; the Nicobarese on the Nicobar Islands, to the south of the Andamans, have adopted Christianity. But hunter-gatherer cultures are not well-equipped to hold on to their identities. Four hunter-gatherer minorities remain on the Andaman Islands -- the Andamanese, Onge, Jarawa and Sentinelese -- accounting for less than 1,000 of the archipelago's half million inhabitants.

Two decades ago, on Little Andaman, I met Babora, an Onge -- a pygmy people who are part of the Negrito family, a term for the short-statured, peppercorn-haired, dark-skinned people living in small pockets all over tropical Asia. It is assumed that the Onge walked to their current home some 50,000 years ago across a then-existing land bridge from Africa.

Babora, foreground left, with other Onge, in the Onge transit house in Hat Bay, Little Andaman (Photo by Tom Vater)

Babora was on his way to the territory's capital, Port Blair, on South Andaman Island, with two couples who were to have fertility tests because the Onge had stopped having children. He told me that the Onge had been unable for years to perform their most sacred ritual, sena garu -- the young men's coming-of-age pig hunt -- because settlers had killed all the pigs. He did not want to work in a coconut plantation, and could not see the point of the same monotonous activity day in, day out. He did not want money or clothes, or to eat rice and dhal.

I walked through Hut Bay, the main settlement on Little Andaman, with Babora. Foreigners were unusual there, and Onge were also rare. When we walked together into a chai shop, the owner handed both cups of tea to me, eyed Babora suspiciously and asked us to drink outside.

Palaji, a social worker assigned to look after the Onge, told me that they did not know what was good for them. "These people, they have nothing, they are primitive," he said. "Socio-economic forces are at work here. There is simply no space left for the Onge. India is a poor country and we need the Andaman Islands. In 20 years, the Onge will be gone."

The social worker's prediction is slowly becoming reality. The Onge live in a couple of tiny enclaves on Little Andaman which has been almost completely logged. In 2008, eight Onge men died after drinking a liquid that had washed up on the beach. The Andamanese number only around 50 and have forgotten their language.

A group of Onge in Hat Bay in Little Andaman (Photo by Tom Vater)

India's Supreme Court ordered the main road through the Jarawas' territory closed in 2002. It remains open, and the community is subjected to human safaris. As a result, some have died of measles and others are said to be HIV-positive.

The Sentinelese killed two fishermen in 2006, making it clear they did not want to be disturbed. Yet in August last year, India opened several islands that were previously off-limits to tourism, including North Sentinel. There was no consultation with the affected populations.

Some of the world's most isolated communities are being denied a say in their future. With travel restrictions no longer in place, John Allen Chau, the missionary, was just the first of many unwelcome visitors likely to follow.

Tom Vater is a Bangkok-based writer.