There are two alcohol-metabolizing enzymes: alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). They regulate the production of acetaldehyde, a toxic byproduct of alcohol metabolism, which causes facial flushing, nausea, and rapid heart beats. Aldehyde dehydrogenase metabolizes acetaldehyde into acetate, which is removed from the body easily.

African descent

One-fourth of people with African descent have a variation of ADH1B that increases rate of alcohol metabolism and reduces likelihood of alcoholism, less positive response to alcohol, and protects against alcohol-related birth defects.

Native Americans

"Native Americans and Alaskan Natives are five times more likely than other ethnicities in the United States to die of alcohol-related causes"*

Asians

Koreans and Korean-Americans have relatively high rates of alcohol dependence, whereas Chinese and Chinese-Americans have low rates. East Asians have a variation that makes it harder to convert aldehyde into acetate, leaving it in the system longer. This was selected recently, during the last 20,000 years, roughly coinciding with rice domestication 10,000 years ago, possibly to stop over indulging in rice wine. It has also been suggested that acetaldehyde offered protection from parasites that were unable to stomach the toxin*.

Early evolution

Yeast produced ethanol from sugar as a defense mechanism, so rotting fruits would make them toxic to other microbes. 10 m.y.a., a common ancestor of humans, chimpanzees, and gorillas evolved a version of ADH4 that was 40x more efficient at ethanol metabolism than the one found in primates going back 50 m.y.a. Lemurs and Baboons, on the other hand, have a less effective version of ADH4.

The mutation enabled eating fruits off the ground, which may explain why it gives us pleasure.

Agriculture

Fermentation started around 9,000 years ago, according to chemical residues from pottery jars.

A primitive cereal, einkorn, cultivated in the Middle East during the Neolithic, is harder to make bread from, so it's possible that the main motivation for agriculture was alcohol.

Alcohol expectancy theory

One-fourth to one-half of emerging adults consumed alcohol before their most recent sexual occasion.*