Biologist Solves Mystery Of Tropical Grasses’ Origin

February 8, 2010



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.”


“By combining all these data,” Edwards said, “we could get individual climate profiles for each grass species.”

The pair then went a step further. They whittled the list to approximately 1,230 species for which the plants’ genes had been sequenced and from there built a phylogenetic profile for the collection, the most comprehensive evolutionary tree to date for grasses. The reason for building the phylogeny, Edwards said, was to tease out the junctures at which C3 and C4 grasses diverged over time. The scientists zeroed in on 21 such “transition nodes” and examined the climatic conditions during those branching periods.

They found that in 18 of the 21 instances, precipitation, rather than temperature, had changed. “That was the clincher,” Edwards said.


“Twelve-hundred millimeters isn’t a desert,” Edwards noted. “It’s still a fairly mesic place. And so when you start looking at climate profiles, these closely related C3 and C4 lineages are straddling this transition zone between tropical forests and tropical woodlands and savanna.”



In a related finding, the scientists attempt to explain the dominance of a lineage of C3 grasses, called Pooideae, in northern, cold areas of the globe, such as the Mongolian steppes. “The global latitudinal gradients of C3 and C4 always has been explained by the physiological advantages that C4 grasses have under high temperatures,” Edwards explained. “No one has considered that the evolution of cold tolerance might have been equally important in setting up that latitudinal gradient. Climatically speaking, the cool-climate Pooideae are really the grasses that are doing something very different.”


The researchers received no outside funding for the research.

Image Caption: Erika Edwards has uncovered the origin of a grouping of grasses that now account for up to one-fifth of the vegetational covering on Earth. Rainfall, not temperature, was the primary trigger around 30 to 40 million years ago.

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