Environmental factors have helped shape California's diversity of Indian ethnic-and-language groups in the last 12,000 years, researchers say.

Populations of peoples have followed the greenery, they said, with successive waves of migrating tribes settling first on the lush Pacific coast and moving on to progressively drier, less-vegetated habitats.

"Trying to explain why linguistic diversity is high in some places and low in others has been a big issue in anthropology," lead study author Brian Codding of the University of Utah said.

"For a number of years, people have shown a correlation between ecological diversity and linguistic diversity," he explained. "What we did in this study that was different was to look at it over time -- to actually see the process through which different populations came to live side-by-side as neighbors or replaced one population with another. We're showing how the diversity actually developed over time."

The researches said they set out to determine if the suitability of ancient California habitats correlated with when and where wave after wave of Native American immigrants settled during the past 12,000 years, resulting in prehistoric California's extreme diversity of ethnicities and languages.

The comparative lushness of California regions, mostly unchanged for millennia, correlated to the order in which nine major prehistoric ethnolinguistic groups migrated to California and colonized the state, they found.

From about 12,000 to 8,000 years ago, the researchers found, the earliest settlers colonized the most suitable, lush habitats along the Pacific coast, particularly estuaries and river mouths, while from 8,000 to 4,000 years ago, "migrants settled in more marginal habitats" in California's Central Valley and the Sierra Nevada.

"We can use these general results to try to understand how ethnic diversity builds over time in different areas," Codding said. "It suggests that where we see a lot of ecological diversity, migration patterns probably should result in the buildup of linguistic diversity."

The finding "may aid in the explanation of prehistoric hunter-gatherer migrations across the globe, including the initial spread of people out of Africa into Europe, Asia and across to Australia-New Guinea," the researchers said in reporting their study in the journal Proceedings of the National Academy of Sciences.