Rice farming and gene-culture coevolution

 

Why didn’t alcohol intolerance evolve the same way in China as it did in southern Europe? One reason was that the Chinese have a long tradition of boiling water for drinking. So they didn’t need wine as a safe, pathogen-free alternative to water. Facial flushing, before and after (Wikicommons, Brooks et al. 2009)

 

 

In a study of 1,162 Han Chinese, a team led by Thomas Talhelm (2014) found certain psychological differences between Chinese from rice-farming regions and those from wheat-farming regions. The latter tended to be more individualistic, more analytic in their thinking, and less likely to make a big distinction between friends and strangers. Interestingly, these findings were no less true for subjects who had no connection to farming of either sort.

 

Talhelm et al. (2014) attributed these psychological differences to differences between rice farming and wheat farming:

 

The two biggest differences between farming rice and wheat are irrigation and labor. Because rice paddies need standing water, people in rice regions build elaborate irrigation systems that require farmers to cooperate. In irrigation networks, one family's water use can affect their neighbors, so rice farmers have to coordinate their water use. Irrigation networks also require many hours each year to build, dredge, and drain—a burden that often falls on villages, not isolated individuals. 

 

In comparison, wheat is easier to grow. Wheat does not need to be irrigated, so wheat farmers can rely on rainfall, which they do not coordinate with their neighbors. Planting and harvesting wheat certainly takes work, but only half as much as rice. The lighter burden means farmers can look after their own plots without relying as much on their neighbors. (Talhelm et al., 2014)

 

The term “gene-culture coevolution” isn’t mentioned, but it did cross my mind when I read that paper, and I suspect it also crossed Talhelm’s. Did rice farming favor the success of individuals who were better at rice farming? And were those individuals sometimes better because they had certain mental and behavioral tendencies?

 

That hypothesis has recently been tested in a study by Chen Zhu and other co-researchers, including Thomas Talhelm:

 

Here, we report evidence that the advent of rice domestication and cultivation may have shaped humans not only culturally but also genetically. Leveraging recent findings from molecular genetics, we construct a number of polygenic scores (PGSs) of behavioural traits and examine their associations with rice cultivation based on a sample of 4101 individuals recently collected from mainland China. (Zhu et al. 2021)

 

The polygenic scores were for height, body mass index, depression, time discounting, age at first birth, educational attainment, risk preference, and tolerance for alcohol (as measured by the ability to break it down via two enzymes: alcohol dehydrogenase and aldehyde dehydrogenase). For each mental or behavioral trait, the polygenic scores were compared with the subjects’ county of birth, specifically the county’s proportion of farmland devoted to rice paddies.

 

The trait/county correlations were then investigated for possible confounds. Because rice farming is more common in southern China, there may be confounds with latitude, temperature, certain minority groups, and so on. After controlling for these confounds, the authors found that two negative correlations remained significant: (1) age at first birth and (2) tolerance for alcohol, as measured by the ability to produce aldehyde dehydrogenase.

 

It seems, then, that people from rice-farming regions are more likely to feel sick after drinking alcohol, and that women are more likely to begin having children at a younger age. Let’s take a closer look at both findings:

 

Age at first birth

 

Rice-farming regions are associated with a younger age at first birth. Historically, rice was far more labor-intensive than wheat. The labor demands were so high that a husband and wife could not farm a large enough plot to support their family through their labor alone. Life expectancy may have also been shorter in rice-farming regions than in wheat-farming ones, with the result that the reproductive schedule was shifted toward younger ages wherever rice was primarily grown (Zhu et al. 2021). 

 

Alcohol tolerance

 

Rice-farming regions are associated with defective enzymes for alcohol tolerance. This seems to be much truer for aldehyde dehydrogenase than for alcohol dehydrogenase. Both defects reduce clearance of alcohol by the liver, but the first one has a stronger effect. In about 36% of East Asians, it causes facial flushing, nausea, and tachycardia after alcohol consumption. Not surprisingly, carriers appear to suffer less from alcoholism and alcohol-related liver disease (Brooks et al. 2009; SNPedia 2020).

 

Zhu et al. (2021) argue that selection against alcohol tolerance was stronger in rice-farming regions because rice farmers had surplus rice for wine production at an earlier date. This argument assumes that alcohol consumption was always maladaptive: once it became common, selection immediately favored individuals who couldn’t tolerate alcohol. Yet we know that alcohol consumption used to be adaptive in many parts of the world, such as southern Europe, where fermented beverages were safer to drink than water because they had no water-borne pathogens.

 

I would argue that selection against alcohol tolerance began in China when fermented beverages, notably rice wine, ceased to be the best alternative to water. Consumption of rice wine was widespread at first but later gave way to consumption of boiled water, particularly in the form of tea, during the Tang dynasty (7th to 10th centuries). There then followed a period of relative abstinence until the 19th century:

 

Following the Yangtze's incorporation into the Chinese state during the Qin dynasty, beer progressively disappeared from use over the course of the Han dynasty in favor of the stronger huangjiu and the rice wines of the southern Chinese. […] The Dutch historian Frank Dikötter describes the period between the Han and Tang dynasties as a "golden age" for alcohol […] The "rise of a tea culture during the Tang was a significant shift away from heavier patterns of intoxication".  […] Wine was reintroduced to China at Macao by Portuguese traders and missionaries, who produced small batches for communion. […] The production and its effect was minor, prior to the opening of the country by the 19th-century First and Second Opium Wars, after which European alcoholic beverages and methods of alcohol production were introduced throughout China. (Wikipedia 2022)

 

With the replacement of rice wine by tea during the Tang Dynasty, people no longer had the same need for enzymes that break down alcohol. The advantages of alcohol, in terms of preventing water-borne diseases, became outweighed by the disadvantages, in terms of alcoholism and alcohol-related diseases.

 

Thus began a long Chinese tradition of drinking hot water (Ye 2017). Rice wine, with its high alcohol content of 18-25%, became seen not only as unnecessary but also as problematic, due to the risk of alcoholism and alcohol-related diseases. Rice-farming regions may have therefore experienced a steeper decline in alcohol consumption during China’s long period of relative abstinence between the 7th and 19th centuries. If so, alcohol intolerance would be a recent evolutionary development in China’s rice-farming regions.

 

Elsewhere, as in southern Europe, people continued to drink wine as a safe alternative to water. There was thus selection for willingness to consume alcohol, but not to excess.

 

What about individualism?

 

Why didn’t Zhu’s research team look at genetic markers for individualism? Wasn’t that the main behavioral difference between rice-farming and wheat-farming regions in Talhelm’s study? The oversight is all the more puzzling because Talhelm was one of Zhu’s co-authors.

 

There is, in fact, a genetic marker for individualism: the short allele of 5-HTTLPR (Chiao and Blizinsky 2010; Schroeder et al., 2016). In addition, I would look at genetic markers for the propensity to identify social norms, to comply with them, and to ensure that others comply (Frost 2020).

 

What about cognitive ability and future orientation?

 

Zhu et al. (2021) also looked at cognitive ability (as measured by educational attainment) and future orientation (as measured, inversely, by time discounting). Cognitive ability and future orientation are both high in East Asians, but neither seem to differ significantly between Chinese from rice-farming regions and those from wheat-farming regions. In the initial results (i.e., before they were controlled for possible confounds), the first group was less future-oriented than the second, but this difference seems to be due to southern Chinese being less future-oriented than northern Chinese.

 

Things are rarely simple. No single theory can explain all the mental and behavioral differences that exist within our species. Yes, some researchers have proposed various unified theories of human biodiversity: r/k selection, selection in tropical vs. northern environments, and selection among trading vs. non-trading peoples, etc.  Each of them explains part of the picture, but none explains the whole picture.

 

There will never be a unified theory of human biodiversity, other than Darwin’s theory of evolution by natural selection. Human nature is shaped by many different selection pressures with many different causes.

 

References

 

Brooks, P.J., M-A. Enoch, D. Goldman, T-K Li, and A. Yokoyama. (2009). The Alcohol Flushing Response: An Unrecognized Risk Factor for Esophageal Cancer from Alcohol Consumption. PLoS Med 6(3): e1000050. https://doi.org/10.1371/journal.pmed.1000050  

 

Chiao, J. Y., & K.D. Blizinsky. (2010). Culture-gene coevolution of individualism-collectivism and the serotonin transporter gene. Proceedings of the Royal Society B: Biological Sciences 277: 529-537. https://doi.org/10.1098/rspb.2009.1650   

 

Frost, P. (2014). Rice farming and gene-culture co-evolution. Evo and Proud, May 31

https://evoandproud.blogspot.com/2014/05/   

 

Frost, P. (2020). The large society problem in Northwest Europe and East Asia. Advances in Anthropology 10(3): 214-134.

https://doi.org/10.4236/aa.2020.103012    

 

SNPedia (2020). rs671

https://www.snpedia.com/index.php/Rs671   

 

Talhelm, T., X. Zhang, S. Oishi, C. Shimin, D. Duan, X. Lan, and S. Kitayama. (2014). Large-scale psychological differences within China explained by rice versus wheat agriculture. Science 344: 603-607.

https://doi.org/10.1126/science.1246850   

 

Wikipedia (2022). Alcoholic drinks in China.

https://en.wikipedia.org/wiki/Alcoholic_drinks_in_China

 

Ye, F. (2017). The History of Drinking Hot Water in China (from a Cold-Water-Drinking Chinese). Radii, June 23

https://radiichina.com/the-history-of-drinking-hot-water-in-china/   

 

Zhu, C., T. Talhelm, Y. Li, G. Chen, J. Zhu, and J. Wang. (2021). Relationship between rice farming and polygenic scores potentially linked to agriculture in China. Royal Society Open Science 8(8):210382. https://dx.doi.org/10.1098%2Frsos.210382