A global cooling 4,200 years ago led to the rise of rice as the world’s most popular food and the spread of new varieties across north and south Asia, a study has found.
Experts used genetic data from some 1,400 varieties of rice — including indica and japonica — to reconstruct the past movements of the grain across the continent.
They supported their analysis with archaeological, geographical and historical climate data from the region.
They found that rice farming was mainly confined to China for the first 4,000 years of its history — with japonica the main subspecies — until the cooling episode.
This event — which is believed to have caused the collapse of civilisations from Mesopotamia to China — saw japonica rice diversify.
Experts believe that the cooling was likely triggered by volcanic eruptions in Iceland and shifts in the pressure phenomenon known as the North Atlantic Oscillation.
Following the event, newly evolved temperate varieties spread in north China, Korea and Japan, the team concluded, while tropical varieties spread to Southeast Asia.
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A global cooling 4,200 years ago led to the rise of rice as the world’s most popular food and the spread of new varieties across north and south Asia, a study has found
‘This abrupt climate change forced plants, including crops, to adapt,’ said paper author and biologist Rafal Gutaker of New York University.
‘Our genomic data, as well as palaeoclimate modelling 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,’ added paper author and biologist Michael Purugganan, also of the New York University.
This, he said, ‘showed that after the [event 4,200 years ago], tropical rice migrated south while rice also adapted to northern latitudes as temperate varieties.’
Following the global cooling event, it is thought that the tropical japonica rice continued to diversify — with trade and migration allowing it to reach islands in Southeast Asia around 2,500 years ago.
Experts used genetic data from some 1,400 varieties of rice — including indica and japonica — to reconstruct the past movements of the grain across the continent, pictured
Although the team had expected rainfall and water to be key in limiting the diversification of rice, they found instead that temperature was the controlling factor — especially in forging genetic differences between japonica rice varieties.
The team also found that the spread of indica rice occurred more recently and was more complicated — with the variety originating in India’s lower Ganges Valley roughly 4,000 years ago before spreading into China around 2,000 years ago.
‘This study illustrates the value of multidisciplinary research,’ said Professor Purugganan.
‘Our genomic data gave us a model for where and when rice spread to different parts of Asia, archaeology told us when and where rice showed up at various places, and the environmental and climate modelling gave us the ecological context.’
‘Together, this approach allows us to write a first draft of the story of how rice dispersed across Asia.’
Although the team had expected rainfall and water to be key in limiting the diversification of rice, they found instead that temperature was the controlling factor — especially in forging genetic differences between japonica rice varieties
A staple food for more than half of the world’s population, rice was first cultivated around 9,000 years ago in China’s Yangtze Valley — from where it spread across Asia and on to the Middle East, Africa, Europe, and the Americas.
During this journey, rice evolved to adapt to these new environments — but scientists had known little about the routes, timings and environmental forces involved in this spread.
Filling in these gaps could help researchers to cultivate new varieties of rice that are better equipped to face droughts and wider climate changes — and mitigate humanity’s looming food security issues.
‘Armed with knowledge of the pattern of rice dispersal and environmental factors that influenced its migration, we can examine the evolutionary adaptations of rice as it spread to new environments,’ said Dr Gutaker.
This, he added, ‘could allow us to identify traits and genes to help future breeding efforts.’
The full findings of the study were published in the journal Nature Plants.