Humans were preadapted to dietary alcohol
consumption
Alcohol
has played a prominent role in human affairs throughout recorded history, but
how far does its use go back? One view is that humans were teetotal until the
advent of agriculture 9,000 years ago, when storage of food surpluses soon led
to the invention of fermentation techniques. This model attributes the social
problems associated with alcohol to the human metabolism not yet having had
enough time to fully adapt to its consumption.
An
alternative view is that primates became adapted to alcohol through eating
fruit that was partially fermented through yeast infestation. Such fleshy
fruits first appeared 80 million years ago, very early on in primate history
and before the dinosaurs became extinct. The ‘evolutionary hangover’ model
posits that arboreal primates foraging in trees became attracted to the smell
of slightly-fermented fruit that might otherwise have gone unnoticed. This
adaptation ceased to be beneficial once the attraction was turned to more
strongly alcoholic drinks.
To
resolve the issue, researchers resurrected digestive alcohol dehydrogenases (ADH4)
enzymes from our primate ancestors to explore the history of primate
interactions with ethanol. They reconstructed the evolutionary history of the
ADH4 family of enzymes using ADH4 genes from 28 different mammals, including 17
primates. They synthesised nine of the ADH4 enzymes, from which they deduced
that an ethanol-metabolising form was not yet appeared when orangutans diverged
from gorillas, chimps and humans, but was present in the last common ancestor
of gorillas, chimps and humans about ten million years ago.
The
timing coincides with a shift to apes spending more time on the ground. It is
likely that they began eating overripe, highly-fermented fruit that fell to the
forest floor. At this time, apes faced growing competition from monkeys due to
the ability of the latter to eat unripe fruit before it became suitable for
consumption by apes. The ability to eat overripe fruit without becoming inebriated
might have been an evolutionary adaptation to the problem.
It’s
something to think about over a few beers….
References:Carrigan, M. et al.,
Hominids adapted to metabolize ethanol long before human-directed fermentation.
PNAS 112 (2), 458-463 (2015).
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