How and When Did Humans Start Consuming Alcohol?

As winter deepens (in the North — hello, summery South!), many people's minds are turning towards celebrations of the solstice, many of which are accompanied by (sometimes copious) alcohol consumption. But when did we first acquire a taste for alcohol? Apparently, humans have always had it. According to a wonderful study appearing in PNAS, alcohol metabolism appeared in our primate ancestors between 7 and 21 million years ago, long before the human species existed. The primate population that evolved to metabolize alcohol eventually gave rise, not only to humans, but also to chimps, bonobos, and gorillas, all of which share our ability to break down booze. It turns out that our kind has been able to tolerate alcohol for longer than we've been human.

Metabolizing alcohol is a complex process involving many enzymes, but the researchers focused their efforts on just one, ADH4. ADH4 is expressed in primates' stomachs and tongues, and has been shown to play a significant role in alcohol metabolism. Of course, a full understanding of how primates evolved to metabolize alcohol will only emerge after we've studied the other enzymes, too, but ADH4 is a good start.

I think my favourite thing about this study may have been the surprisingly straightforward, direct approach the researchers chose. They started by making a phylogeny of the ADH4 gene based on its sequence in modern primates. Each node in the tree represents a hypothetical common ancestor, and the team could infer the structure of the ancestral versions of the protein in each one — what it looked like in the common ancestor of humans, chimps, bonobos, and gorillas, for example, or in the common ancestor of that group and orangutans. So far, this is just standard phylogenetics. The cool part is what the team did next. They engineered bacteria to express the ancestral versions of ADH4, extracted these proteins from the bacterial cultures, and tested their ability to metabolize alcohol. In other words, they resurrected proteins that haven't been seen for millions of years — ADH4 went on changing in each ancestor's descendants — just to find out if they could break down alcohol. Amazing!

The researchers found that the mutation responsible for alcohol metabolism appeared in our common ancestor with bonobos, chimps, and gorillas. Orangutans can't break down alcohol; nor can gibbons, baboons, or a range of other primates. The natural question, of course, is "why then?". Why in that group of animals, at that time? It's a common question in evolutionary stories, and it's always a tough one to answer with certainty. In this case, the researchers point out that the evolution of alcohol metabolism coincided with a major climate disruption around the middle of the Miocene; one of its consequences was the transformation of East African forest ecosystems into fragmented forests and grasslands. Our ancestors, who may have been knuckle-walking through these grasslands, may have started eating more fruit they found on the ground, rather than in trees. Fruit sitting on the ground rots, and part of that process is fermentation of the sugars into ethanol. Many hominoids went extinct during the transition from forest to grassland, so the ability to eat fermented fruit might have been quite an advantage. As is so often the case, it's hard to be sure if that's actually what happened, but it's an attractive and plausible explanation.

Ref
Carrigan et al. Hominids adapted to metabolize ethanol long before human-directed fermentation. PNAS Early Edition. (2014) doi: 10.1073/pnas.1404167111

Image credits
The grape photo is by Hannele Luhtasela-el Showk and is used with permission.