"Rice doesn't like saltwater," he said, noting that sea levels were rising at the time due to climate warming.
"We think [saltwater levels] must have been managed. Otherwise you would see a gradual rise in the brackish water influence," he said.
The water may have been held back by small earth dikes known as bunds, Zong said.
The team also detected increased levels of animal and human dung on the rice fields.
"Whether the dung was deliberately used as fertilizer, or whether it was just washed naturally into the paddy fields, it's very difficult to be certain," Zong said.
Rice fragments found in the swamp belonged to wild strains, the team found.
The discovery of unusually large rice pollen grains, however, may signal the beginnings of domesticated varieties, Zong said.
The team's findings are published in the latest issue of the journal Nature.
Other recent studies have dated the first fully domesticated rice in China to about 6,000 years ago.
Dorian Fuller of University College London, author of one such study, said the "evolution of rice as a domesticated crop was a long, drawn-out process which may have taken millennia."
The inhabitants of Kuahuqiao would have been "forager-cultivators," Fuller said.
"Rice cultivation isn't the only thing they do, and it's possibly not the main thing they do," he added.
"People who were using a wide range of other resources, including acorns and water chestnuts, started to manipulate marshland environments where rice was wild," Fuller added.
Who Farmed It First?
The new study provides the earliest known evidence of rice paddies, Fuller said, though other, less solid evidence points to rice farming elsewhere in China around the same period.
Wild rice grains from Stone Age sites along the middle Yangtze River have been dated to 6000 B.C., he noted.
"People were using rice earlier than this," he added.
(See related photo: "4,000-Year-Old Noodles Found in China" [October 12, 2005].)
He explained that rice farming likely evolved independently in different parts of Asia, such as along the Ganges River in India.
"It's very clear now from the genetics of modern rice that it has multiple origins from the wild gene pool right across southern China and northern and eastern India," Fuller said.
Gary Crawford, an anthropologist at the University of Toronto at Mississauga, said that the new study is "an important contribution to understanding agricultural origins in the rice regions of East Asia."
The study, he said, provides "a fascinating interpretation that rice cultivation was taking place in slightly brackish coastal wetlands that were regularly flooded."
The study team says the move toward rice farming by the Kuahuqiao people was likely spurred by the onset of warmer, wetter conditions ideally suited to growing the cereal plant.
The changing climate acted as a "critical environmental prompt to cultural change, permitting rice cultivation at this latitude," the team said.
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