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Scientists have uncovered the mystery of the origin of rice with microscopic fossils 10,000 years ago...
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Time:2017-08-10 14:55
Source: 中国科普博览
White rice, black rice, glutinous rice, brown rice, Thai fragrant rice, Indian fragrant rice, japonica rice, indica rice...In the modern supermarket container or grain and oil market, we might be confused by these various names. However, from the perspective of botany, all these rices actually belong to the same plant-rice, which can be divided into two subspecies, namely japonica and indica. The rice grains of the former are short and round in shape, while the latter are more slender and taste different. As for other complicated names, they are either different varieties or local varieties within these two subspecies, or different titles given due to different processing steps.
In today’s world’s food system, the output of rice is second only to corn, and the consumer groups are concentrated in East Asia, Southeast Asia, and South Asia, the three most densely populated regions in the world. Rice has become the world’s most-supported population. Plant crops. Therefore, the origin of rice has aroused the attention and interest of researchers very early, but the entire research process is long and tortuous, and it has been debated for nearly a hundred years just to confirm the origin.
Where did rice originate? India or China?
The Soviet scholar Vavilov at the beginning of the 20th century was the most well-known early scholar in the field of the origin of cultivated crops. An important theoretical basis for his research on the origin of major crops worldwide was that "the origin of cultivated crops should be in the existing cultivation. The region with the highest genetic diversity of species and wild relatives". Based on this theory, he proposed that the origin of rice should be in India. This view is consistent with the formulation of the Swiss botanist De Candor in the 19th century. Therefore, the "Indian origin theory" has been the dominant view on this issue from the 19th to the early 20th century.
The earliest Chinese scholar to systematically study this issue was the famous agronomist Mr. Ding Ying. Since the discovery of wild rice in the outskirts of Guangzhou in 1926, he has made a series of fruitful contributions in the field of rice origin, evolution and breeding, and clearly proposed In response to the view that "rice originated in southern China", he responded positively and refuted the "theory of Indian origin". His systematic work has been valued by many international colleagues, and the two factions have been in a state of incompetence for decades.
Since the 1950s, many Neolithic sites in the Yangtze River Basin in my country have successively discovered the remains of rice. Among them, the Hemudu site in Yuyao, Zhejiang, excavated in 1973 is the most famous of the many sites. The discovery of rice here about 7000 years ago has attracted attention from all over the world and has also triggered another climax in the discussion of the origin of rice. Since this period, the main body of research on the origin of rice has gradually changed from agronomists to archaeologists, and the research thinking has also changed from identifying areas with the highest genetic diversity of modern wild rice to finding the earliest evidence of rice cultivation and utilization.
With the accumulation of materials, in the early 1990s, the famous archaeologist Mr. Yan Wenming, based on the evidence of early rice cultivation unearthed at archaeological sites and the distribution range of wild rice in history, proposed "Rice originated in the middle and lower reaches of the Yangtze River." the opinion of. In this process, although India and even Thailand and other places claimed to have discovered rice 8,000 or even 9,000 years ago, and caused some controversy, these discoveries were later proved to be problematic. In fact, most of them did not exceed the age. 4000 years ago. Since then, "the theory of the origin of rice in the middle and lower reaches of the Yangtze River" has gradually become the consensus of the international academic community, and the dispute between "India" and "China" that has lasted for more than 100 years has basically settled.
When did rice originate?
However, the exploration of the origin of rice did not stop there. The confirmation of the place of origin is only one aspect of this issue, and another important aspect is the time of origin. Strictly speaking, these two issues are unrealistic to talk about either side of the other, but at different stages of research, the focus will always be on one side. Since the 1990s, with the initial settlement of the dispute over the place of origin, the time of origin has become a major issue that academia needs to face urgently.
It happened from this period that plant archaeology, as an important branch of archaeology, grew rapidly in China. A series of purposeful and targeted research work continued under this background, and the macroscopic picture of the origin of rice became gradually clear.
Rice remains were found in a large number of early and mid Neolithic sites, to name a few, such as the Wannian Xianren Cave in Jiangxi, Pengtou Mountain and Bashidang in Li County, Hunan, Jiahu Lake in Wuyang, Henan, Baligang in Dengzhou, and Yuyao, Zhejiang Tianluo Mountain, Xiaoshan Lake Crossing Bridge, Xiaohuangshan Mountain in Shengzhou, Shangshan Pujiang Mountain, Sihongshan Mountain in Jiangsu, etc. Most of these sites are more than 7,000 years old, and the earliest sites are nearly 10,000 years old. A large number of new materials have made the exploration of the origin of rice a basis for research and analysis.
There are two key points for the confirmation of the origin of rice. One is when humans began to consciously manage and use wild rice, and the other is when rice has been domesticated. The former focuses on changes in human behavior, while the latter focuses on changes in plants' biological characteristics. It is precisely because of the continuous management and utilization of wild rice resources by humans that some of its biological characteristics have changed, which finally led to the emergence of domesticated rice.
Therefore, in order to solve the problem of the origin of rice, plant archaeologists need to establish a time series of Oryza resources from appearance to complete domestication in a region. In this way, based on the materials provided by new archaeological discoveries, credible dating results and reliable identification standards have become technical requirements to achieve this goal.
There are a series of differences in biological characteristics between wild rice and domesticated rice. The two most important ones are related to the plant's own reproduction mode. One is the shattering property and the other is the seed dormancy. The seeds of wild rice will scatter naturally after maturity, enter the soil, and then grow new plants under suitable climatic conditions. If the climatic conditions are not good, the seeds can stay dormant in the soil for a long time and will not rot until the external conditions are suitable for germination. However, humans grow rice to obtain grains for their own consumption. Therefore, domesticated rice does not automatically shatter after maturity, but requires humans to thresh after harvesting. This is a huge change based on long-term human selection. . In addition, in order to ensure the germination rate after sowing, the dormancy of domesticated rice seeds is greatly reduced compared with wild rice under long-term artificial selection.
Although there is currently no reliable research method for the dormancy of rice unearthed at archaeological sites, the research on rice shattering properties of a series of sites has preliminarily clarified the process of rice domestication in the middle and lower reaches of the Yangtze River.
From the perspective of shattering, the middle reaches of the Yangtze River had already domesticated japonica rice roughly 8000 years ago, while the changes in this biological trait in the lower reaches of the Yangtze River roughly occurred around 6000 years ago or slightly earlier. In the following thousands of years, the japonica rice spread out from the middle and lower reaches of the Yangtze River. It was introduced to India around 4000 years ago, and after crossing with the wild rice used by humans, indica rice appeared. As for the indica rice, it will come back and spread. Back to China happened very late.
The current understanding of the process of rice domestication is also supported by DNA research. In recent years, studies on the DNA of domesticated rice have shown that some major genetic mutations related to rice domestication have only appeared once. Such genes carried by indica rice, such as those that control grain shattering, all come from japonica rice.
Despite this, our understanding of the initial stage of human use of rice is still very limited. Because the scale of rice utilization at that time was much smaller than that of a mature agricultural society, the seeds and other materials that could survive our research were extremely limited or almost missing. Therefore, there is a lack of effective research materials whether it is for dating or exploring the changes of early rice biological characteristics. So, are there other materials or methods that can be used to determine the age of rice? The latest research work published on PNAS by Lu Houyuan from the Institute of Geology and Geophysics of the Chinese Academy of Sciences is an important breakthrough for this problem.
Phytolith research and rice ten thousand years ago
Pollen, seeds, etc. are the research materials we often hear used for plant archaeology, and this time researcher Lu Houyuan’s team used a substance called phytolith. Different from large fossils such as seeds, phytoliths are siliceous particles deposited in the cavities of higher plant cells or between cells. Its morphology is directly related to the morphology of plant cells, so it has certain plant taxonomic significance.
Compared with plant seeds and other organic matter, this siliceous micro-fossil is easier to be preserved for a long time under different burial conditions, and has certain advantages in exploring early plant utilization.
In addition, during the formation of phytoliths, the original organic carbon in some plant cells will be sequestered, and its content can reach about 0.3-6% of the weight of the phytoliths. It is a new material that can be used for carbon fourteen dating.
The peculiar phytoliths of the genus Oryza include rice fan-shaped, bimodal and side-by-side dumbbell-shaped. Researcher Lu Houyuan’s team has carried out a large number of taxonomic studies on modern plant silicates for a long time. Their analysis of modern samples shows that the number of fish scales on the edge of the rice fan can be used to distinguish wild rice from domesticated rice. In wild rice, the proportion of rice fan-shaped fish scales greater than or equal to 9 is about 17%, while the proportion in domesticated rice is about 63%.
In addition, they have also established a new method to purify phytoliths from soil samples and use the organic carbon stored in them for carbon fourteen dating. In order to verify the reliability of the phytolith C14 dating, the research team conducted a comparative analysis of the phytolith and charcoal dating results from many domestic sites, and the results showed that the dating data obtained by this method is reliable.
On the basis of the aforementioned work, the research team applied these two new research methods to the exploration of early rice utilization at the Shangshan site in Pujiang, Zhejiang. The Shangshan Site is currently known as the earliest Neolithic site in the lower reaches of the Yangtze River. Archaeological excavations from 2001 to 2008 indicate that the ancients living here used rice husks and rice leaves as admixtures to make pottery. Micro observation confirmed the existence of rice. In addition, a very small amount of charred rice was unearthed at the site, but such remains were lacking in the earliest stages of the site. According to the previous dating data, the earliest age of the site can be as early as 11,000 years ago, and the discovery of this rice remains 10,000 years ago also caused a sensation.
However, due to the lack of plant relics such as seeds and charcoal that can be used for dating at the earliest stage of the Shangshan site, the previous dating data are all determined by the "charcoal" pottery pieces mixed with rice husks and rice leaves. Because these carbon pottery may contain "old charcoal" from other sources such as clay, there has been controversy over the earliest age of the Shangshan site and the age of the rice remains related to it.
Against this background, scientists conducted phytolith dating of soil samples from the earliest stage of the site. Test results show that the earliest age of the site can be at least as early as 9,400 years ago. In addition, the identification and analysis of rice fans among them showed that in the earliest stage of the Shangshan site, the proportion of rice fans with fish scales greater than or equal to 9 was as high as 36%, much higher than the 17% of modern wild rice. In the Huxi site around 8,400 years ago, this proportion has risen to about 57%.
Considering that the earliest evidence of fan-shaped phytoliths found so far is quite different from that of wild rice, and the rate of rice domestication inferred from known data, the start of rice utilization in the lower Yangtze River must be earlier than the currently known 9,400 years. It is likely to be earlier than 10,000 years ago. This not only confirmed the use of rice in the lower reaches of the Yangtze River around 10,000 years ago, but also revealed the early process of local rice domestication.
This research by researcher Lu Houyuan’s team also shows that the time when rice in the lower reaches of the Yangtze River in China began to be domesticated and the world’s major crops (wheat in West Asia, maize in Central America) were basically synchronized, and both occurred from the end of the Pleistocene to the beginning of the Holocene. During the transitional period, the time when rice began to domesticate corresponds to the climate environment background when the East Asian monsoon began to strengthen and the climate gradually warmed and humidified about 10,000 years ago. This is closely related to the important changes in the global climate pattern during this period. However, the climate-environmental mechanisms that contributed to this event are still unresolved issues that need to be explored through multidisciplinary collaboration in the future.
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