New research suggests rice was thrice domesticated. On three separate occasions, farmers in different parts of the world began cultivating wild rice strains chosen for their desired traits.

The three domestication events, researchers say, explain the world's three main types of rice -- Indica, the long-grain, non-sticky rice of lowland Southeast Asia; Japonica, short-grain sticky rice most famous for its presence on sushi rolls; and Aus, a drought-tolerant variety cultivated in Bangladesh and India.

Until now, scientists thought rice was only domesticated once or twice. Most agreed that Japonica had been singularly domesticated some 10,000 years ago. But researchers disagreed on whether Indica was a hybridization of Japonica or had been separately domesticated.

Researchers at the University of Manchester analyzed the genes of 446 samples of wild rice varieties from across Asia. They compared the wild rice genes to those of domesticated varieties, paying specific attention to "domestication sweeps," the portions of domesticated genomes that differ most from wild rice genomes.

These sweeps reveal the characteristics that farmers long ago sought when selecting and cultivating wild strains -- attributes like a tendency to grow vertically and allow more dense planting, or resistance to drought and pests.

In their analysis, researchers found these advantageous characteristics where present in wild rice varieties across South Asia, supporting the idea that rice farmers domesticated varieties separately in different parts of Asia.

"Our conclusions are in accord with archaeological evidence that suggests widespread origins of rice cultivation," Manchester researcher Terry Brown said in a press release. "We therefore anticipate that our results will stimulate a more productive collaboration between genetic and archaeological studies of rice domestication."

The domestication of rice in Asia was an integral part of the development of civilization in that part of the world. A stable food source allowed people to gather and organize in larger numbers. Brown and his colleagues say their work will help researchers better understand early human history in Asia.