A fisherman from Cornwall and an accountant from Houston may have trouble understanding each other, but the English tongue itself is in no danger of disappearing.

The same cannot be said for many of the world's languages.

"Human languages are vanishing as we speak," said K. David Harrison, a linguist at Swarthmore College in Pennsylvania. The rate of loss, he added, "makes the extinction of species look trivial by comparison."

Roughly 40 percent of the world's estimated 6,800 languages may disappear within the next century, Yale University linguist Stephen Anderson said in Seattle at a February meeting of the American Association for the Advancement of Science.

Chinese dialects, English, Spanish, Hindi, Urdu and Arabic will probably maintain their status as some of the languages with the most speakers. So why should it matter if a few obscure languages, spoken by a few people in remote corners of the world, vanish forever?

"What is lost when a language is lost is a world," Anderson said.

Native languages, he said, often convey cultural and sociological information that cannot be articulated in the same way in a different language. A tribe that loses its language may also lose a storehouse of knowledge about its members' history and their environment.

Two newly described languages in the Turkic family, documented in Siberia by Harrison, illuminate the challenges facing those who speak an obscure tongue.

Last July, Harrison traveled to Siberia to meet with the Chulym people, who live in six isolated villages using traditional means of hunting, gathering and fishing. Only 35 people in the community of 426 -- and no one younger than 52 -- could speak the Middle Chulym language fluently, Harrison found. The rest mainly spoke Russian.

Middle Chulym is a "moribund" language, destined to disappear soon after its last speaker dies, Harrison reported at the Seattle meeting. "Passive" speakers may continue to use some of the words for a while, but they, too, will die, he said.

Villagers consider Middle Chulym to be a low-prestige language, so there is no compelling reason for them to preserve it, Harrison said. Attempts to transliterate it into the Cyrillic alphabet, used in Russian, have not worked for the same reason.

"People don't feel good about speaking these languages," he said.

When Middle Chulym goes, the villagers will have lost a rich oral history as well as specific environmental knowledge, such as what plants to harvest, what animals to hunt and how to read local weather, Harrison said. He is working on a new transliteration to create a children's storybook in Middle Chulym of which the villagers can be proud.

In similar straits is another Siberian language called Tofa, spoken by just 40 people in a community of 600. Tofa speakers have a detailed language for their reindeer herds, with specific ways of describing animals on the basis of sex, age, fertility, ridability and color.

That kind of information does not translate in the same fashion into Russian, Harrison said.

Sometimes languages vanish by choice.

In the Caucasus, for instance, the relatively complex Ubykh language disappeared partly because of a tradition that a couple, upon marrying, should adopt the phonetically simpler language of the pair, Anderson said. Thus, every time an Ubykh-speaking man married a woman who spoke a simpler language, he stopped speaking Ubykh.

Several linguistic, conservation and religious groups have devoted themselves to documenting as many of the world's vanishing languages as possible. But working against them is the fact that nobody knows exactly how many languages exist.

The Ethnologue database, sponsored by Dallas-based SIL International, catalogs 6,809 languages worldwide. That number represents a "best guess" but is essentially meaningless, linguists said at the Seattle meeting.

For starters, there is the oft-blurred distinction between what constitutes a dialect and what constitutes a language.

Languages are sometimes defined by political power rather than by linguistic categories, said Laurence Horn of Yale. Cantonese and Mandarin are considered dialects of Chinese when they are really distinct languages. On the other hand, Norwegian, Swedish and Danish are more like dialects than separate languages, he said.

In the end, perhaps it is best to regard all the world's languages as manifestations of a single language, said David Lightfoot of Georgetown University.

And that, he said, could be called simply Human.