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The high level of biological diversity found in today's New World tropical forests began at least 52 million years ago, according to a study by US and Argentinean researchers.

The findings, published in this week's Science, contradict the typically held belief that tropical plant biodiversity is relatively recent, arising during the past two million years.

"Tropical South America is the most biodiverse region today," says one of the researchers, Peter Wilf of Pennsylvania State University, the United States. "There has been little evidence but much debate about the history of [this] exceptional plant diversity."

The researchers collected more than 1,500 fossils and identified more than 100 plant species in from a volcanic-lake deposit preserved in Patagonia. They dated the fossils to the warmest period of the past 65 million years, when tropical climates and plants stretched into the middle latitudes.

"With good dating information, an adjusted sample size and a large number of species, we can say that South American plant diversity began very far in the past and continues to this day," says Wilf.

According to a related article in Science by Sandra Knapp and James Mallet, the findings support Charles Darwin's belief that speciation occurs not only when populations are geographically isolated, but also when they spread over large areas and experience "isolation by distance".

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Link to Science research paper by Wilf et al

Link to Science article "Refusing Refugia?" by Sandra Knapp and James Mallet

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