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Domestic Paddy Rice Expansion Was Affected by Climate Change

On September 14, 2020, archaeologists and geographers led by Barry Rolett and Zhou Zheng (Ting Ma and colleagues 2020) reported the results of a sediment core-based study of the southeast Asian coastline. The researchers were attempting to understand the mechanics of the spread of rice domestication, a process which was delayed for thousands of years. This Background Check discusses sediment core analysis and some of the findings surrounding the domestication of rice and its eventual spread out of the heartland.
Woman working in Japanese rice paddies in springtime
Woman working in the rice paddies of Sawara, Chiba, Japan. Angie, Wikimedia Commons

Many of the earliest domesticated plants developed on the planet were cereals such as wheat, barley, rye, millet, maize, and rice. Of those, only rice originated in a wetland environment, which posed unique problems hindering its spread. What caused the long delay of domesticated rice production in Southeast Asia and how does sediment core analysis help to understand that?

Sediment Core Analysis

Sediment core analysis refers to the use of a thin, narrow tube that scientists poke down into the oceans and seas to sample the soils within the seafloor. A venerable science, having been first been attempted in the late 18th century, sediment core studies were kick-started by deep-sea soundings ahead of the laying of the first functioning submarine telegraph cable in 1851. The first global sea-floor map was produced using sediment cores by the HMS Challenger expedition of 1872–1876, led by Scotsman Charles Wyville Thomson and Canadian John Murray.

Over time, the technology improved: core tubes became longer and less prone to disturbing the soil in them, increasing the value of what might seem odd prying into the ocean's darkest secrets. Today, the technique is used in many different disciplines: to study ancient ecology, sedimentology, climate change, pollution. Because organic material is brought up in the cores, the soils are datable cross-sections of whatever was being deposited into the oceans over time.

One well-known sediment core study is the Cape Roberts Project, which used soil cores to examine the tectonic and climatic history of the Ross Sea in Antarctica.
Sediment cores from the Cape Roberts Project in Antarctica
Illustration of sediment cores taken for the Cape Roberts Project in the Ross Sea to reconstruct the glaciation history of Antarctica. Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany

Studying Ecology with Sediment Cores

Coastal and wetland areas are of particular interest to archaeologists because the soil cores include pollen and opal phytoliths, microscopic plant parts that are precipitated in the air by wind action, float on the water, and then settle down into layers on the seafloor.

A phytolith is a tiny, three-dimensional copy of a plant cell created by the plant as a product of taking in water with dissolved silica. The silica hardens and creates a copy of the cell, and is one of the ways in which a plant supports its structure—the crunch is celery is phytoliths in action. These are nearly indestructible, surviving long after the plant has died, and thus very useful to people interested in plant domestication. Domestic rice phytoliths have shapes that are distinct from wild ones.
Rice Bulliform Phytoliths
The 'fish scale' decorations on the edges of these rice bulliform (fan-shaped) phytoliths occur in higher frequency in wild forms. In this image, a and b phytoliths have more fish scale decorations; while c and d have fewer fish scale decorations. Black bar = 20µm. Image by Lu Houyuan.

History of Rice Development

In general, scholars agree that rice developed out of wild rice which grew in the Lower and Middle Yangtze River floodplains of central China. Rice was first used between 13,000—10,000 years ago (BP). The oldest field systems—rice paddies—formed to nurture the crop date to about 10,000 BP, but the accomplishment of true rice domestication wasn't complete until 6,500—5,000 BP. Recent studies have shown that the rate of domestication for any cereal (wheat, rye, barley, maize) is a protracted process, but the spread of domesticated rice was definitely hindered even more.

Paddy rice was in use on Taiwan by about 4,800 BP, in the Indonesian archipelago by 3,500 BP, on the Korean Peninsula by 2,500 BP, and on Japan by 2,100 BP. But in each of these cases, rice was not a particularly large part of the economy until much later: Southeast Asian societies simply added small gardens of rice into their broadly variable diets.

A Dynamic Environment

The Southeast Asia coastline is and has been subject to marine transgression (changes in sea level that impacted the wetlands) and typhoon events for the last 10,000 years, as reported in Keyang He and colleagues (2020). According to Ting Ma and colleagues, after 5,000 BP, coastal Asian lands suitable for rice planting increased in area from 16,000 sq km to 96,000 sq km, a result of the Holocene era sea-level changes and sediment supply. Those changes modified what had been estuaries into delta formations.

Immigrants from the Yangtse valley in China were also central to the adaptation of wetland rice as the main subsistence crop. During the Warring States period (480–221 BCE), Han Chinese farmers fled southward, bringing with them the technology of socketed metal tools and domestic water buffalo. And that enabled large-scale rice farming in southern China and Southeast Asia to become established.

Selected Sources and Further Reading

Created by KKris. Last Modification: Monday 14 of September, 2020 15:13:17 EDT by KKris.