Only the remaining fourth of this group emerged at higher latitudes, according to scientists, whose study appears in the October 6 issue of Science magazine.

James Valentine, biology professor at Berkeley and one of the co-authors of the study, said plants and other animal species probably originated in large part in the tropics.

Between 23.5 degrees latitude north and 23.5 degrees south of the equator, all land and waters of the tropics receive perpendicular sunlight at noon at lease once during the year.

The warmer tropics are about 10 times as biodiverse as are the arctic regions, scientists say.

The study answered in part a question that biologists have asked for more than a century: why is biodiversity on land and sea so much greater in the tropics?

"These species are spilling out of the tropics and increasing the diversity in temperate and arctic regions," he said. "We should preserve the tropics, because without them, there is no source anymore for diversity in higher latitudes," Valentine said.

"The tropics are the engine for global biodiversity," added coauthor Kaustuv Roy, associate professor of biology at UC San Diego.

"What this means is that human-caused extinctions in the tropics will eventually start to affect the biological diversity in the temperate and high latitudes. This is not going to be apparent in the next 50 years, but it will be a long-term consequence," he said.

Valentine and Roy wrote the paper with University of Chicago professor of geophysical sciences David Jablonski.

Earlier scientists assumed that life forms had evolved pretty much where they had found them. The late evolutionary biologist George Ledyard Stebbins assumed that the tropics were more diverse simply because they produced more species, and called the topics the "cradle" of biodiversity.

This study traces migrations rates, which point to higher rates of origin from the tropics of species that then spread to northern climates, which the authors say, qualifies the tropics as both cradle and museum.

"I think we've killed the idea that the tropics are either a cradle or a museum of biodiversity. They're both," Valentine said.

"We've seen this pattern in most forms of life for a century, now we know the dynamics behind it. But we still don't know the ultimate cause. Why is the origination rate higher in the tropics?"

"It's a global village, even for organisms," said Roy.

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Explore The Early Earth at TerraDaily.com
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