They found an entirely different vocabulary and linguistic structure.
Even the speakers of the tongue, called Koro, did not realize they had a distinct language, linguist K. David Harrison said Tuesday.
Culturally, the Koro speakers are part of the Aka community in India's Arunachal Pradesh state, and Harrison, associate professor of linguistics at Swarthmore College, said both groups merely considered Koro a dialect of the Aka language.
But researchers studying the groups found they used different words for body parts, numbers and other concepts, establishing Koro as a separate language, Harrison said.
"Koro is quite distinct from the Aka language," said Gregory Anderson, director of the Living Tongues Institute for Endangered Languages. "When we went there we were told it was a dialect of Aka, but it is a distant sister language."
People of the Aka culture live in small villages near the borders of China, Tibet and Burma (also known as Myanmar). They practice subsistence hunting, farming and gathering firewood in the forest and tend to wear ornate clothing of hand-woven cloth, favoring red garments. Their languages are not well known, though they were first noted in the 19th century.
The region where they live in the foothills of the Himalaya Mountains requires a special permit to enter. There, the researchers crossed a mountain river on a bamboo raft and climbed steep hillsides to to reach the remote villages, going door-to-door among the bamboo houses that sit on stilts.
Harrison and Anderson spoke at a news conference organized by the National Geographic Society, which supported their work.
The northeast corner of India is known as a hotspot of language diversity and researchers were documenting some of the unwritten tongues when they came across Koro in research started in 2008.
The timing of their discovery was important.
"We were finding something that was making its exit, was on its way out. And if we had waited 10 years to make the trip, we might not have come across close to the number of speakers we found," said Anderson.
Previously undocumented languages are "noticed from time to time" Harrison said, so such a discovery is not rare. But at the same time linguists estimate that a language "dies" about every two weeks with the loss of its last speakers.
Counting Koro there are 6,910 documented languages in the world, Harrison said. But he added that is really just a best estimate that can change regularly.
Many languages around the world are considered endangered, including Koro, he explained, because younger people tend to shift to the more dominant language in a region.
Unusually, Koro has been maintained within the Aka community, the researchers said, even though there is intermarriage and the groups share villages, traditions, festivals and food. In addition to the estimated 800 to 1,200 Koro speakers, the West Kameng and East Kameng districts of Arunachal Pradesh contain 4,000 to 6,000 Aka speakers.
The Koro speakers "consider themselves to be Aka tribally, though linguistically they are Koro. It's an unusual condition, such arrangement doesn't usually allow for maintenance of the minor language," Anderson said.
The threat, however, is from the spread of Hindi, a dominant language in India, and many youngsters go to boarding schools where they learn Hindi or English.
The researchers said they hope to figure out how the Koro language managed to survive within the Aka community.
They said Koro is a member of the Tibeto-Burman language family, a group of some 400 languages that includes Tibetan and Burmese. While Koro differs from Aka, it does share some things with another language, Tani, which is spoken farther to the east.
The research was started in 2008 to document two little known languages, Aka and Miji, and the third language, Koro, was discovered in that process.
"We didn't have to get far on our word list to realize it was extremely different in every possible way," Harrison said.
They said Koro's inventory of sounds was completely different, and so was the way sounds combine to form words. Words also are built differently in Koro, as are sentences.
The Aka word for "mountain" is "phu," while the Koro word is "nggo." Aka speakers call a pig a "vo" while to Koro speakers, a pig is a "lele."
"Koro could hardly sound more different from Aka," reported Harrison, author of a new book "The Last Speakers," about vanishing languages. Joining the two was linguist Ganesh Murmu of Ranchi University in India.
The researchers detail Koro in a scientific paper to be published in the journal Indian Linguistics.