The Origins and Dispersal of Rice Cultivation

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Domesticated rice (Oryza sativa) is one of the five major crops in the world and a staple food for more than 30% of the world population. Yet the question of where, when, why and how the domestication of rice originated has been, and still is, a question under debate. However, as more archaeological and archaeobotanic discoveries have recently come to light, the question of the origin of rice cultivation now seems less elusive than it was a few decades ago. To date, both archaeological and archaeobotanic discoveries seem to indicate that rice cultivation first began in the middle Yangzi Valley by 8500-8000 years BP, and subsequently expanded to south China and Southeast Asia.

The Yangzi Valley generally refers to the area within longitudes approximately between 103 [degrees] and 123 [degrees] E, and latitudes between 24 [degrees] and 33 [degrees]N The Yangzi Valley is divided into the upper, the middle and the lower reaches. The upper reach is defined as the western part of longitude 111 [degrees] 19[minutes]E; the middle reach is from 111 [degrees] 20[minutes]E to 116 [degrees] 14[minutes]E, and the eastern end is the lower reach [ILLUSTRATION FOR FIGURE 1 OMITTED]. Today, the middle and lower Yangzi Valley is a landscape of alluvial plains with isolated hills and three large lakes. The climate is temperate to subtropical. The present mean precipitation ranges from 1000 to 1600 ram, and mean annual temperature ranges from 14[degrees] to 16 [degrees]C (Editing Committee of the Physiography of China 1984). The flora in this region is a mixture of deciduous and evergreen trees, meadows and woods, as well as cultivated crops, with a subtropical fauna.

However, the climate and environment in the Yangzi Valley during the Last Glacial Maximum (LGM) of the terminal Pleistocene differed significantly from that of the present. According to geomorphological and geological studies, the region was a landscape of high platforms and terraces during the LGM, formed by the down-cutting of the rivers. A loess horizon called the 'Xiashu loess' widely accumulated in the region, dated approximately between 20,000 and 13,000 BP (Yang 1986). Very meagre faunal and floral remains were found in this Xiashu loess. Mineralogical analysis suggests that the Xiashu loess was formed in a cold and dry palaeoclimate (Yang 1986), possibly with precipitation as low as 300-600 mm (Wu et al. 1991). This would have been a decrease of 7001000 mm from the present.

Pollen profiles also reveal that the vegetation between 21,000 and 18,000 BP in the middle and lower Yangzi Valley was a mixture of deciduous and coniferous trees, drought-resistant herbs and ferns (Xu et al. 1987). Oak, pine and fir were major trees, followed by spruce and elm; Artemisia and Gramineae were the major herbs (Liu 1991). This combination indicates a temperate but dry and cool climate. In general, estimates of mean annual temperature during the LGM in the region vary from 4 [degrees] to 10 [degrees] C lower than at the present (Xu 1985; Zhang et al. 1985), and the precipitation from 400 to 1000 mm less than the current level (Wang et al. 1995).

From 15,000 to 13,700 BP the palaeoclimate moderated, as trees increased to 46-47% of the total pollen composition, with oak becoming the major component, followed by pine, elm and willow (Xu & Zhu 1984). Herbs accounted for 36-39%, the rest were ferns. Drought-resistant herbs and hygrophilous plants were both present, indicating that the palaeoclimate was meister and warmer than that in the previous stage.

After 13,700 BP the palaeoclimate became colder and drier again. Both the quantity and the variety of the pollen profile were substantially reduced. Drought-resistant herbs dominated, indicating a steppe environment with sparse deciduous tree cover (Xu et al. 1987). It is estimated that the average annual temperature of this period was about 6-7 [degrees] C lower than that at the present, with a precipitation of 500600 mm (Xu et al. …