Rice Genomics and Genetics 2024, Vol.15, No.2, 83-93 http://cropscipublisher.com/index.php/rgg 84 2 The Origin and Early History of Rice 2.1 The geographical origins of wild rice The domestication of rice (particularly Oryza sativa) is a significant milestone in agricultural history, with origins deeply rooted in the geographical distribution and evolution of wild rice species. The genus Oryza includes both cultivated and wild species, believed to have originated approximately 130 million years ago on the ancient supercontinent Gondwana. As Gondwana split apart, different types of rice dispersed across various continents. The primary wild ancestors of Asian cultivated rice (Oryza sativa) are Oryza rufipogon and Oryza nivara. These species are distributed throughout Asia, with Oryza rufipogon being particularly important as it is considered the direct ancestor of cultivated rice. Genetic studies suggest that japonica rice was first domesticated from O. rufipogon populations in the central region of the Pearl River in southern China. Subsequently, as the initial varieties spread to Southeast Asia and South Asia, japonica (Oryza sativa subsp. Japonica) hybridized with local wild rice, leading to the development of indica rice (Oryza sativa subsp. Indica) (Wang et al., 2017). Further genetic analysis indicates that modern wild rice populations have extensively mixed with domesticated rice through gene flow mediated by pollen and seeds. This extensive gene flow suggests that many assumed wild rice varieties may actually represent different stages of feral domesticated rice. The gene flow from domesticated to wild rice forms a hybrid swarm, indicating ongoing and dynamic interactions between wild and cultivated rice populations. Debates about the origins of domesticated rice have led to two major hypotheses: the single-origin model and the multiple independent domestications model. The single-origin model posits that both indica and japonica subspecies were domesticated fromO. rufipogon. In contrast, the multiple independent domestications model suggests different domestication events for indica and japonica in different regions. However, recent molecular evidence supports the single-origin model, indicating that rice was first cultivated in the Yangtze River basin in China around 8 200 to 13 500 years ago (Choi et al., 2017). 2.2 Early evidence of rice cultivation The domestication of rice (Oryza sativa) marks a pivotal event in agricultural history, signifying a major shift from foraging societies to agricultural ones. The earliest evidence of rice cultivation dates back to the middle reaches of the Yangtze River, where archaeological and archaeobotanical findings suggest that rice cultivation began around 8500 to 8000 years ago. This region is considered one of the primary centers of rice domestication, from which it spread to southern China and Southeast Asia (Wu et al., 2016). In the lower Yangtze River region of China, early rice farming projects identified a critical turning point around 4600 BC, when the number of domesticated rice varieties began to surpass wild varieties. This period marks the beginning of systematic rice cultivation, where rice was grown in small, irregularly shaped paddies, allowing for precise water management and control (Zhao, 2010). The Shangshan site, dating back approximately 10 000 years, provides some of the earliest direct evidence of rice cultivation, though it remains unclear whether the rice at that time was fully domesticated (Gross and Zhao, 2014). Further evidence from the Kuahuqiao site in eastern China, dating to about 7 700 years ago, indicates that early Neolithic communities selected lowland marshes for rice cultivation. These communities used fire to clear wetland shrubs and maintain vegetation suitable for rice paddies, demonstrating a sophisticated level of environmental management. Additionally, the Hemudu culture, around 5 000 BCE, is noted for its advanced rice cultivation techniques, including the use of seedbeds and transplantation into flooded fields (Wang et al., 2022). The domestication process of rice in China was lengthy, with significant developments occurring approximately 6 500 to 6 000 years ago. This period marked the peak of early domestication traits in Yangtze japonica rice, which later hybridized with proto-indica varieties from India. The transition from wild rice to domesticated rice
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