OGRABRAJ, India — The police on South Andaman Island know what to do when members of the isolated Jarawa tribe venture into the villages that surround them, hoping to snatch rice and other prized goods, like cookies, bananas or, for some reason, red garments.

The policy is to send the Jarawas back into the 300 square miles of forest that has been set aside for the tribe, where they are expected to survive by hunting and gathering, as they have for millenniums. Inspector Rizwan Hassan, whose precinct includes a “buffer zone” beside the tribe’s reserve, is under clear orders: to interfere as little as possible in the traditional life of the tribe, which India prizes as the last remnant of a Paleolithic-era civilization.

This did not prepare him for the criminal complaint that was registered at his station in November. A 5-month-old baby was dead, and witnesses came forward willingly, leaving the police, for the first time in history, confronting the prospect of arresting a Jarawa on suspicion of murder.

The Jarawas, who number about 400 and whom one geneticist described as “arguably the most enigmatic people on our planet,” are believed to have migrated from Africa around 50,000 years ago. They are very dark-skinned, small in stature and until 1998 lived in complete cultural isolation, shooting outsiders with steel-tipped arrows if they came too near.