Two poachers lie in shallow graves beside the Indian Ocean after they trespassed on an endangered tribe’s island. Now even relatives of the victims’ want the killers left alone. Dan McDougall reports from Port Blair

Describing the Sentinelese tribe of India's remote Andaman islands in his travel journals, the notoriously trite 13th-century explorer Marco Polo wrote: 'They are a most violent and cruel generation who seem to eat everybody they catch.'

While their cannibalism has never been proven, little has changed here in the remotest parts of the Bay of Bengal over seven centuries and Delhi's furthest-flung outpost is still occupied by aggressive 'stone-age' tribes who hunt wild pigs and fish with arrows, believe that birds talk to spirits, and lack both the skills to make fire and a word to describe a number greater than two.

Having survived occupations of the islands by the Burmese, the British and the Japanese and most recently a tsunami which took the lives of almost 2,000 other islanders in the archipelago, the elusive Sentinelese remain one of the most enigmatic peoples on earth - but today the very existence of the tribe may be under threat, and not because of the encroachment of the rest of the world.

The remarkable story behind the murders of Indian fishermen Sunder Raj, 48, and Pandit Tiwari, 52, sounds like a chapter from a Joseph Conrad novel, but it happened here in the Andamans late last month. The two men were killed by loin-clothed Sentinelese warriors on 27 January, after their boat accidentally drifted on to the shore of North Sentinel Island, a tiny outcrop in the Indian Ocean.

Other fishermen, who witnessed the attack from the water, described how the pair, believed to be drunk on palm wine, died after they were attacked by near-naked axe-wielding tribal warriors when their craft beached on the island, a preservation area strictly out of bounds to the outside world.

An Indian coastguard helicopter, sent out to investigate, was attacked with bows and arrows by the same tribal warriors, leaving the pilot under no illusions as to the safety of landing. The fishermen's macheted bodies were exposed in their shallow graves when the down-draught from the chopper's rotor blades blew away the sand. One of the crew later remarked to police that he was surprised to see bodies. 'I thought they roasted and ate their victims,' he said.

The incident has divided opinion in the archipelago. Relatives of Sundar Raj are calling for justice and government compensation. But the local authorities, under pressure from international preservation groups and a largely sympathetic local population, are reluctant to pursue the matter. And they are backed by the father of the second victim.

'Believing in justice is one of the pillars of your society but for me it's different,' says schoolteacher RK Tiwari, slurping noisily on a hot cup of sweet chai in his home on the outskirts of Port Blair, his grandson on his lap.

From his balcony the corrugated iron slums of the provincial capital can be seen stretching down the harbour where the smell of dried fish and raw sewage keeps tourists away. 'My son Pandit got his own justice. He was breaking the law, poaching and trespassing on land that wasn't his own and he was murdered. What more is there to say?'

The 74-year-old father of seven continues: 'As far as I am concerned the Sentinelese are the victims in this, not my son. They live in constant terror of heavily armed poachers from Myanmar [Burma] and Port Blair. They were only defending themselves with bows and arrows and rocks in the only way they know how. What I do want is my son's body back so my wife and I can bury him; we don't want retribution. It is an impossible case to prosecute anyway.'

The belief that the case could never get to court is shared by the Andamans' police chief, Dharmendra Kumar: 'We have witnesses, yes, illegal poachers who won't testify because they can be imprisoned. Then there are the language barriers; nobody speaks the Sentinelese language. This is before we think about identifying the culprits and compiling forensic evidence. We would have to arrest the entire tribe.'

He adds: 'We are in an impossible situation. If we raided the island there would be casualties on both sides. If the tribesmen go inland we might be able to sneak back there and collect the bodies - that's as far as this will probably go.'

According to Kumar, a number of people - government officials, anthropologists and fishermen - have tried to get on the island before but the tribe are clearly determined to live their lives without interference. 'Even when rescuers in helicopters and boats approached the island after the 2004 tsunami to check on any casualties, they were met with arrows and spears, so how are we going to conduct an investigation?'

According to some police officers in Port Blair, there remains a great deal of empathy with the victims' families, especially over the fact that the fishermen's bodies may now never be recovered from their crude beach graves.

'It's too dangerous,' one told me. 'If we go in with guns we face international condemnation, if we go in unarmed we will be killed by poison darts and arrows smeared with blood. What can we do? It's the question we're all asking. The law should be upheld. Murder cannot be accepted, they have broken the most fundamental of our laws.'

Changal Das, the wife of Sundar Raj, has now demanded a police investigation into the murders and claims she may take her case to the Indian government.

She told The Observer that she would expect a prosecution. 'My husband has been murdered and nobody is left to care for me and my family. The government and police have washed their hands of this matter: nobody seems to want to offend the tribe but two men have been killed. We want the bodies to be retrieved and the police to arrest the murderers. Whether my husband was poaching or not, he didn't deserve to be killed with an axe.'

The Andamans, the term probably taken from the Sanskrit word nagnamanaba, meaning 'naked man', form, along with the Nicobar Islands, part of an archipelago of 572 islands that lies 1,000km off India's east coast in the Bay of Bengal and stretches in a north-south arc for more than 800km. Although closer to Burma, the islands are administered by Delhi and form the subcontinent's most remote state.

Anthropologists separate the indigenous tribes living on the archipelago into two groups. It's thought that those living on the Nicobar islands - the Shompen and Nicobaris - are of Asian descent, while the four surviving Andaman tribes - the Great Andamanese, Onge, Jarawa and the Sentinelese - all originated in Africa, a fact that makes their survival all the more remarkable.

The most reclusive of all are the Sentinelese, who have violently rebuffed all approaches from the outside world. According to a recent study of the tribes carried out by a team of biologists at the Centre for Cellular and Molecular Biology in the southern Indian city of Hyderabad, the indigenous islanders, often described by anthropologists as 'pygmies', may actually represent the first Asians - an early wave of 'out of Africa', who reached the Far East more than 40,000 years ago and have since evolved separately from most of the other native people of Asia, the South Seas and Australia.

The scientific team's findings, based on DNA samples, fit into an ongoing debate about how and when the hominids who evolved in Africa to become Homo sapiens moved out into the Middle East, Asia and the rest of the world. One relatively new idea is that beaches exposed by low sea level provided a useful pathway, and the oceans supplied reliable food, allowing these humans to migrate easily.

The 'Stone Age' moniker, so regularly applied to the islanders, refers to the fact that the Sentinelese have lived in isolation for 60,000 years: genetically, therefore, there is a direct line between them and their pre-Neolithic ancestors. Unlike real Stone Age tribes, though, they probably use metal salvaged from shipwrecks, although their hostility to outside incursions means nobody has properly studied the question.

The first documented contact with the islands was made more than 1,000 years ago by Chinese and Arabian travellers, who were met with a hail of arrows when they tried to land. They described the Andamanese as three feet tall with human bodies and bird beaks. During the 18th and 19th centuries, Christian missionaries also encountered fierce resistance, and the islands and their people seemed destined to remain an enigma.

It was the British that made the first significant 'breakthrough' with the Andamans' indigenous tribes, even putting them 'on display' at Calcutta Zoo after they established a penal colony on South Andaman in 1858 and attempted to civilise and educate the natives at special 'homes' in which they were dressed in Western clothes and then taught to read and write.

This proved disastrous for many of the tribes - with no resistance to common diseases, they quickly succumbed to epidemics of pneumonia, measles and influenza. At the time of first contact with the British there were an estimated 5,000 Great Andamanese; today only 41 remain.

However, the British Raj was not solely responsible for this decline. During the Second World War, the islands were occupied by the Japanese, who killed hundreds of indigenous people suspected of collaborating with the allied forces. After India gained independence in 1947, the expansionist policies of its first prime minister, Jawaharlal Nehru, continued the devastation of the Andamans' native tribes. As part of India's own plan for colonisation, thousands of settlers were sent to the Andamans from the mainland.

Over the past 50 years, the islands' population has increased more than tenfold to 30,000 and - thanks to disease, de-forestation and Nehru's vision of a modern India - the indigenous groups now comprise less than 1 per cent of that total.

According to campaigning groups such as Survival International, there is a horrible pattern from the day when first contact is made between a 'lost' tribe and the rest of humanity. Curiosity often gives way to fear. A nomadic lifestyle then develops into furtive flight to a more remote part of the forest.

There is then the prospect of enforced relocation, along with a loss of culture, language and other skills that have been honed by generations of ancestors and passed on by word of mouth. Finally, there comes the indignity of government hand-outs, disease, alcoholism and the acceptance that you are no longer part of a tribe with its own mother tongue and unique way of life.

Miriam Ross, of the London-based Survival International, says: 'These murders were tragic but they happened because the laws of the area were not upheld. In the Andamans the law strictly states that people should be kept away from Sentinel Island as any contact with the outside world is potentially deadly for the Sentinelese. For a start they are incredibly susceptible to common diseases to which they have no immunity.

'We need to look at the wider picture here. Some of the neighbouring tribes to the Sentinelese have been assimilated into our world and paid a terrible price. Fifty years ago the Great Andamanese tribe were 5,000 in number; today, as the modern world has encroached on them, there are now only 41, most of them alcoholics. These tribals are not carrying out murder with impunity as some people might think, they were simply defending their lives.'

When the tsunami struck in 2004, the Sentinelese knew the evil spirits were up to no good. Minutes before the waves struck, tribal leaders scattered pig and turtle skulls around their settlement and hurled stones toward the ocean before gathering their baskets, bows and arrows and amulets of ancestral bones for protection.

They all survived - for how long, though, it is hard to say.

How hostility kept them alone and alive

It is the sheer, intractable hostility of the Sentinelese that makes them so appealing to science. Had they been amenable to the advances of the West, they would have been assimilated long ago, and their unique identity drowned in an inundation of foreign genes. Today, their isolation - biological and geographical - makes them invaluable to studying our own species.

Scientists now accept Homo sapiens evolved in sub-Saharan Africa 200,000 years ago and quickly began an expansion that took them to the Middle East 100,000 years ago, though it is not clear what factors - language, superior memory, better social cohesiveness or longevity - gave the critical advantage over other hominids.

Equally uncertain is the route our forebears took. When the 'out of Africa' theory, pioneered by Britain's Chris Stringer and others, was established a decade ago, it was assumed they took the direct, overland path, reaching south-east Asia at least 60,000 years ago. From there, they crossed the sea to Australia. Europe was not settled until 40,000 years ago.

But this idea has problems. Going by land would have meant some formidable geographical obstacles. And, having reached south-east Asia, how did our ancestors - who, it was assumed, lacked any abilities to sail - get to Australia?

Even in the middle of the Ice Age, when sea levels were lower, there were still gaps in the chain of islands between Asia and Australia.

Some researchers now believe mankind left its homeland by sea, sailing to south-east Asia before moving on to Australia, creating outposts as they went. One of the last of these may be the territory controlled by the Sentinelese. Hence their scientific attraction.

The trouble, of course, is that the very factor that helped the islanders retain independence, their propensity to kill intruders, makes it rather tricky to investigate them.

Robin McKie