CHAMPAIGN, Ill. According to a popular hypothesis, grasses such as maize, sugar cane, millet and sorghum got their evolutionary start as a result of a steep drop in atmospheric carbon dioxide levels during the Oligocene epoch, more than 23 million years ago. A new study overturns that hypothesis, presenting the first geological evidence that the ancestors of these and other C4 grasses emerged millions of years earlier than previously established.
The findings are published in the journal Geology.
C4 plants are more efficient than C3 plants at taking up atmospheric carbon dioxide and converting it into the starches and sugars vital to plant growth. (C3 and C4 refer to the number of carbon atoms in the first molecular product of photosynthesis.) Having evolved relatively recently, C4 plants make up 3 percent of all living species of flowering plants. But they account for about 25 percent of global plant productivity on land. They dominate grasslands in tropical, subtropical and warm temperate areas. They also are a vital food source and an important feedstock for the production of biofuels.
"C4 plants are very successful, they're economically very important, but we actually don't know when they originated in the geological history," said University of Illinois plant biology professor Feng Sheng Hu, who led the new analysis. "To me, it's one of the most profound geological and ecological questions as a paleoecologist I can tackle."
A previous study dated the oldest C4 plant remnant found, a tiny fragment called a phytolith, to about 19 million years ago. Other studies analyzed the ratios of carbon isotopes in bulk soil samples to determine the ratio of C3 to C4 plant remains at different soil horizons, which correspond to different geological time periods. (C3 a
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University of Illinois at Urbana-Champaign