PROVIDENCE, R.I. [Brown University] Around 30 to 40 million years ago, grasses on Earth underwent an epic evolutionary upheaval. An assemblage capitalized on falling levels of atmospheric carbon dioxide by engineering an internal mechanism to concentrate the dwindling CO2 supply that, like a fuel-injection system in a car, could more efficiently convert sunlight and nutrients into energy.
The rise of C4 grasses is not disputed. They dominate in hot, tropical climes and now make up to 20 percent of our planet's vegetational covering. Scientists have pinned the rise of C4 plants primarily to dwindling concentrations of CO2. But C4 grasses have been closely linked with warmer temperatures. Indeed, on a map, C4 grasses are found along tropical gradients, while C3 grasses occupy the northern, or colder, end of the temperature gradient. Considering knowledge of their past and their current distribution, what was left to question?
Everything, apparently, according to Erika Edwards, an evolutionary biologist at Brown University. In a paper published online in the Proceedings of the National Academy of Sciences, Edwards and Stephen Smith, a postdoctoral researcher at the National Evolutionary Synthesis Center in North Carolina, have found that rainfall not temperature was the primary trigger for C4 grasses' evolutionary beginnings. Moreover, the pair say C4 grasses were already in tropical forests before moving out of the shade of the taller trees and into drier, sunnier environments.
"We've kind of changed the story a bit," said Edwards, assistant professor of biology.
The paper is important, Smith said, because it "demonstrates the importance of precipitation in the evolution of grasses and particularly in the evolution of C4 grasses specifically, their movement into drier, not necessarily warmer climates."
To arrive at their findings, the biologists compiled a database of roughly 1.1 million specimens of grasses
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