In this story:

• Speakers formed a single population
• Similar words for birch bark
• Evidence of several migrations

WASHINGTON (Reuters) -- A few words in one of the planet's most obscure languages support the theory that Native Americans left Asia in several separate migrations, a linguist said in an article to be released Monday.

Merritt Ruhlen of Stanford University has found compelling similarities between Ket, a language spoken by just 500 people in remote Siberia, and Na-Dene, a family of Native American languages.

Writing in the Proceedings of the National Academy of Sciences, he gives examples of 36 words that are similar in the two language families, including the words for birch bark, children and rabbit.

Ket is a member of the Yeniseian family of languages. All the other languages in the family became extinct in the 19th century. Na-Dene (pronounced nah-den-EY) has four branches, including Tlingit and Eyak, spoken in western Canada and Alaska, as well as Navajo and Apache.

"It would seem that Na-Dene and Yeniseian must have once formed a single population in Eurasia," Ruhlen wrote.

Comparing words is a basic tool in linguistics and can help show how languages and the populations that speak them are related. For instance, English is an Indo-European language, one of a family of languages that ranges from Sanskrit, the ancient Indian language, to German and French.

Related words are often easy to spot -- for instance the German word "mutter" is similar to its English counterpart mother, while the Russian word "brat" looks very much like brother and is similar to the Latin root for words like "fraternal."

Ruhlen stumbled onto the link between Yeniseian and Na-Dene languages while doing other comparisons. He found striking similarities.

"I like (the word for) birch bark quite a bit," he said in a telephone interview. "It's so specific. It seems to me that it would be extremely improbable that two families would invent the same word for birch bark."

In Ket the word is pronounced something like "ch'ee" -- a sound hard to transliterate into English. In several existing Na-Dene languages it is pronounced similarly.

The words for breast also correlate. In Ket the word is "tuhguh" and in the Na-Dene Koyukon language it is "t'uga.'"

Ruhlen found enough other similarities to convince him of the link. "I just picked out 36 for this article that looked like the best and most obvious and strongest," he said.

More evidence points to Native Americans crossing a land bridge over what is now the Bering Strait from Siberia into Alaska, including genetic, archeological and other linguistic comparisons.

Ruhlen pointed out that Eskimo Aleut shows a number of similarities with European and Asian languages.

But such close similarities between more isolated and rare languages supports theories that not just one migration took place, but several.

Some of the original Yeniseian speakers would have stayed behind in Siberia, while others moved across the land bridge to help establish the populations in North America, Ruhlen said.