A Bronze Age Climate Event Was Pivotal To Rice Evolution

A climate event around 2200 BCE dramatically changed the course of history in ways that are still deeply felt today. 

This climate event led to the collapse of civilizations from Europe to China. It played a key role in causing the Sarasvati river, which has a prominent role in the Rig Veda, and home to the core of Harappan civilization, to dry up. It lead to the demise of Sumerian culture and the end of a major ancient Egyptian dynasty. 

The power vacuum and unrest it created facilitated Indo-European expansion in multiple directions from a homeland in the vicinity of modern Ukraine. It also played important roles in the expansion and demise of other language families including the Austroasiatic language family.

This climate event also pushed the rice farming ancestors of the Austroasiatic Munda people of India to South Asia from Southeast Asia, and before that also pushed their ancestors to migrate from Southern China to Southeast Asia. In a related development, the climate event was pivotal in the evolution of domesticate rice crops.

A major global cooling event that occurred 4,200 years ago may have led to the evolution of new rice varieties and the spread of rice into both northern and southern Asia, an international team of researchers has found. . . . [Rice] was first cultivated 9,000 years ago in the Yangtze Valley in China. . . .  In their study, the researchers reconstructed the historical movement of rice across Asia using whole-genome sequences of more than 1,400 varieties of rice -- including varieties of japonica and indica, two main subspecies of Asian rice -- coupled with geography, archaeology, and historical climate data. 

For the first 4,000 years of its history, farming rice was largely confined to China, and japonica was the subspecies grown. Then, a global cooling event 4,200 years ago -- also known as the 4.2k event, which is thought to have had widespread consequences, including the collapse of civilizations from Mesopotamia to China -- coincided with japonica rice diversifying into temperate and tropical varieties. The newly evolved temperate varieties spread in northern China, Korea and Japan, while the tropical varieties and spread to Southeast Asia. 

"This abrupt climate change forced plants, including crops, to adapt," said Rafal M. Gutaker, a postdoctoral associate at the NYU Center for Genomics and Systems Biology and the study's lead author. "Our genomic data, as well as paleoclimate modeling by our collaborators, show that the cooling event occurred at the same time as the rise of temperate japonica, which grows in milder regions. This cooling event also may have led to the migration of rice agriculture and farmer communities into Southeast Asia." 

"These findings were then backed up by data from archaeological rice remains excavated in Asia, which also showed that after the 4.2k event, tropical rice migrated south while rice also adapted to northern latitudes as temperate varieties," said Michael D. Purugganan, the Silver Professor of Biology at NYU, who led the study. 

After the global cooling event, tropical japonica rice continued to diversify. It reached islands in Southeast Asia about 2,500 years ago, likely due to extensive trade networks and the movement of goods and peoples in the region -- a finding also supported by archeological data. 

The spread of indica rice was more recent and more complicated; after originating in India's lower Ganges Valley roughly 4,000 years ago, the researchers traced its migration from India into China approximately 2,000 years ago. 

While the researchers had thought that rainfall and water would be the most limiting environmental factor in rice diversity, they found temperature to be the key factor instead. Their analyses revealed that heat accumulation and temperature were very strongly associated with the genomic differences between tropical and temperate japonica rice varieties. 

From Science Daily (which predominantly publishes press releases from the institutions where people publishing scientific papers are employed). The paper and its abstract are as follows:

Rice (Oryza sativa) is one of the world’s most important food crops, and is comprised largely of japonica and indica subspecies. Here, we reconstruct the history of rice dispersal in Asia using whole-genome sequences of more than 1,400 landraces, coupled with geographic, environmental, archaeobotanical and paleoclimate data. Originating around 9,000 yr ago in the Yangtze Valley, rice diversified into temperate and tropical japonica rice during a global cooling event about 4,200 yr ago. Soon after, tropical japonica rice reached Southeast Asia, where it rapidly diversified, starting about 2,500 yr BP. The history of indica rice dispersal appears more complicated, moving into China around 2,000 yr BP. We also identify extrinsic factors that influence genome diversity, with temperature being a leading abiotic factor. Reconstructing the dispersal history of rice and its climatic correlates may help identify genetic adaptations associated with the spread of a key domesticated species.

Rafal M. Gutaker, et al., "Genomic history and ecology of the geographic spread of rice." 6(5) Nature Plants 492 (May 15, 2020). DOI: 10.1038/s41477-020-0659-6