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GRASS HASN'T BEEN AROUND THAT LONG

Splendor in the grass may be a relatively recent event.

Between six and eight million years ago, the world abruptly became a much grassier place. But now, as human activities increase the carbon dioxide in the atmosphere, the world may become less hospitable to the grassy plants with which humans evolved, according to a study in the journal Nature.

By examining the teeth of more than 500 extinct horses and other large, plant-eating mammals from Asia, Africa, North America and South America, University of Utah geochemist Thure Cerling and his colleagues found that between 6 million and 8 million years ago, the animals' diets changed. Their diets became dominated by plants with a type of photosynthesis called Cel,4 nl,4, one characteristic of many warm-weather grasses.

The shift occurred because temperatures became warmer and atmospheric carbon dioxide concentrations became lower, allowing the Cel,4 nl,4 plants to outcompete plants that use Cel,4 nl,4, the other major form of photosynthesis, the researchers suggest.

"It's the first time we have grasslands like we have today, modern grasslands," Cerling said. "That's when they came into being."

Cel,4 nl,4 grasses include the agricultural crops corn, millet, sorghum and sugar cane.

However, the researchers wrote, "Cel,4 nl,4 grasses will be at an increasing disadvantage as Cel,4 nl,4 levels rise."

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