In mammals that eat a lot of fruit or nectar, genetic pressure to maintain a functional version of the ADH7 gene is stronger, likely because ethanol in these foods requires efficient metabolism. In...
Mechanism
Synthesis from 1 study
Animals that eat a lot of fermented fruit or nectar need a strong enzyme to break down the alcohol before it harms them. Those with a better version of this enzyme survive longer and have more babies, so the gene stays strong over time. Animals that don’t eat fermented food don’t need it, so the...
Most probable mechanism
When mammals eat fruit or nectar that has started to ferment, they swallow alcohol along with it. A special enzyme in their mouth and stomach breaks down that alcohol before it can harm them. If this enzyme works better, the animal is more likely to survive and have babies. Over many generations, animals that ate lots of fermented food kept or improved this enzyme, while animals that didn’t eat such food lost it because it wasn’t needed anymore.
Ethanol is ingested through consumption of fermenting fruits or nectars containing up to 8.1% alcohol.
Ethanol enters the upper gastrointestinal tract and is metabolized by the ADH IV enzyme, which is encoded by the ADH7 gene.
A specific change in the ADH IV enzyme at position 294 increases its efficiency in breaking down ethanol by up to 40-fold, allowing faster detoxification.
This improved enzyme function provides a survival advantage by reducing alcohol toxicity and enabling more efficient energy extraction from fermented food sources.
In populations consuming diets with more than 50% fruit or nectar, natural selection favors individuals with functional ADH7 and the enhanced enzyme variant, preventing gene loss.
In populations without significant dietary ethanol exposure, mutations that disable the ADH7 gene accumulate without penalty, leading to loss of enzyme function over generations.
Less supported by current evidence, but not ruled out
Some plant-eating animals lost the ability to break down alcohol in their stomachs, but they evolved a different way to handle toxic plant chemicals using bacteria in their guts, making the stomach enzyme unnecessary.
Herbivores consume plant material containing long-chain hydrophobic alcohols that are toxic.
The ADH7 gene becomes nonfunctional due to mutations, eliminating upper gastrointestinal detoxification.
Bacteria in the hindgut or foregut metabolize these toxic plant alcohols, replacing the need for the ADH IV enzyme.
Loss of ADH7 is evolutionarily tolerated because microbial detoxification provides sufficient protection against plant toxins.
Evidence from Studies
Supporting (1)
Community contributions welcome
Genetic evidence of widespread variation in ethanol metabolism among mammals: revisiting the ‘myth' of natural intoxication
Contradicting (0)
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