Every kind of Asian rice traces back to one domestication in China 9,000 years ago
More than 40,000 cultivars of Asian rice exist today, from sticky japonica to long-grain indica. Genetic studies point to a single starting point: wild rice tamed in China's Yangtze basin about 9,000 years ago. Africa, meanwhile, domesticated a completely separate species of its own, some 3,000 years ago.
The key clue is a gene for nonshattering, the trait that keeps ripe grains on the stalk instead of scattering, which is the hallmark of a domesticated cereal. Vaughan and colleagues found in 2008 that this allele, along with five other genetic markers, is identical in indica and japonica, implying one origin. A 2011 study agreed that all Asian rice sprang from the wild species Oryza rufipogon. More recent genomic work suggests japonica came first, and that indica emerged when japonica reached India around 4,500 years ago and crossed with a local wild or semi-wild rice.
Two regions of the Yangtze probably pioneered rice and wetland farming. The lower river, linked to cultures such as Hemudu and Liangzhu, is associated with the ancestors of Austronesian speakers, who built stilt houses, carved jade and built boats. The middle river, home to cultures such as Pengtoushan and Shijiahe, is tied to early Hmong–Mien speakers. The two traded with each other and with neighbours, spreading rice across southern China. Northern millet farmers adopted it as a secondary crop by around 4000 to 3800 BC, though the colder climate kept millet dominant there.
Between 3500 and 2500 BC populations in the rice heartlands boomed, with intensive paddy fields and settlements large enough that some archaeologists call them true states. Then, in the following five centuries, Liangzhu vanished and Shijiahe shrank, probably as the Longshan culture pushed south; walls and moats suggest widespread conflict. A global cooling event known as the 4.2 k event also appears to have driven tropical japonica south and encouraged a temperate japonica able to grow farther north.
Africa's own rice, Oryza glaberrima, is less widespread but still grown in West Africa, and it crossed to the Americas through the transatlantic slave trade, though exactly how is unclear. Asian rice reached the Americas through the Columbian exchange. North America's four wild rice species belong to a different genus, Zizania, while sharing a botanical tribe with the domesticated kinds.
Source: History of rice cultivation
