A study of the rice genome suggests that the crop was domesticated only once, rather than at multiple times in different places.

Tens of thousands of varieties of rice are known, but these are represented by two distinct sub-species.

The work published in PNAS journal proposes that rice was first cultivated in China some 9,000 years ago.

Another theory proposes that the two major sub-species of rice - Oryza sativa japonica and O. sativa indica - were domesticated separately and in different parts of Asia.

This view has gained strong support from observations of large genetic differences between the two sub-species, as well as from several efforts to reconstruct the evolutionary history of the crop.

The japonica type is sticky and short-grained, while indica rice is non-sticky and long-grained.

For more, see: Rice crops 'have single origin'