Why Some Like it Hot: Food, Genetics, and Cultural Diversity Gary Paul Nabhan Island Press, 2004

Ethnobotanist Gary Paul Nabhan suggests that diet had a key role in human evolution, specifically, that human genetic diversity is predominately a product of regional differences in ancestral diets. Chemical compounds found within animals and plants varied depending on climate. These compounds induced changes in gene expression, which can vary depending on the amount within the particular food and its availability. The Agricultural Age led to further diet-based genetic diversity. Cultivation of foods led to the development of novel plants and animals that were not available in the ancestral environment. Rather than natural selection based on competition between individuals or groups, Nabhan's thinking is similar to that of Alfred Wallace, in that natural selection is driven by environmental stress - those who adapted to what was edible within a given environment survived and propagated their genes.

There are several examples that Nabhan uses to illustrate a gene-diet interaction, and perhaps the most well-known is lactose intolerance. In the ancestral environment, young children but not adults used milk as food. Production of the enzyme that metabolizes the milk sugar lactose diminishes following weaning. Without this enzyme, consumption of most dairy products leads to gastro-intestinal distress. Most adult Native Americans, Asians and Africans cannot digest lactose, whereas peoples from Europe, Asia Minor and North Africa, wherein herding of animals was common practice, are lactose tolerant. Nabhan suggests that animal herding relaxed the selection for lactose intolerance. Through diet, genes can be selected for over a period of a few thousand years, a short period of time relative to the total duration of hominid existence on Earth.

There are other fascinating examples of gene-diet interaction. Culturally specific recipes, semi-quantitative blending of locally available foods and herbs, and cooking directions needed in order to reduce toxins present in plants, emerged over time through a process of trial-anderror and were transmitted through the ages. The effects on genes by foods can be extremely complex given the range of plant-derived compounds available within a given region. The advent of agriculture is suggested to have overridden natural selection by random changes in the environment. The results of human-driven selection can be highly unexpected.

One other interesting example of gene-diet interaction is alcohol tolerance, which again points out the relatively rapid effect of diet on genes on the human evolutionary time-scale. In sedentary herding societies, drinking water was frequently contaminated by livestock waste. The author suggests in order to avoid contaminated water, beverages made with fermented grains or fruit were drunk instead. Thus, alcohol resistance was selected for in populations that herded animals, such as Europeans. By contrast, those groups which did not practice herding, such as East Asians and Native Americans, did not need to utilize alcohol as a water substitute and are highly sensitive to the effects of alcohol. Nabhan does not further examine if there is an association between alcohol sensitivity and dependence. Although sensitivity to the effects of alcohol might suggest that one is also prone to dependence, such an association appears to exist for Native Americans but not for East Asians. How an adaptation which facilitated survival could turn into a problem (more so in some groups than others) is not addressed in the book.

Drinking alcoholic beverages is an "acquired" taste, which leads to inevitable question of how humans developed a taste for things with unpleasant flavors in the first place. Part of the answer lies in the mechanism of taste perception, of tasting bitter substances in particular. An industrial chemist, Arthur Fox, was working with a thiourea compound phenylthiocarbamide (PTC) when some of it accidentally aerosolized. …