The available to arboreal primates. Paleogenetics researchers wish

The
article begins with telling the reader about the historical model of how humans
adapted to metabolize dietary ethanol also known as alcohol. This model states
that ethanol became a part of human’s diet after the ability to store food
supplies and intentionally ferment foods was used insinuating that humans were
not metabolizing alcohol until 9,000 years ago. Another model states that
ethanol consumption started 80 million years ago due to the small amounts of
ethanol in fruits available to arboreal primates. Paleogenetics researchers
wish to address these hypotheses by using the digestive enzyme ADH4 found in
the stomach, esophagus, and tongue of primates. The researchers compared the efficiencies
of modern and ancestral ADH4 enzymes with geraniol and ethanol. In their tests,
they found that nearly all enzymes were inactive against ethanol, but other
alcohols such as geraniol were oxidized. Once the divergence of orangutans
occurred, the common ancestor of humans, gorillas, and chimpanzees were able to
digest ethanol with an ADH4 active enzyme. The researchers discussed as to why
this change occurred after the divergence. They believe it occurred due to
paleontological and paleo-climatic evidence show a major climate shift that
caused mass extinction. The common ancestor was forced into a more terrestrial
lifestyle that would cause fruit with higher ethanol amounts to be a
significant part of their diet. With these results and evidence backing their
claim, the researchers state that the ability to metabolize ethanol goes back
roughly 10 million years effectively disproving the historical model.

Carrigan,
M. A., Uryasev, O., Frye, C. B., Eckman, B. L., Myers, C. R., Hurley, T. D.,
& Benner, S. A. (2015, January 13). Hominids adapted to metabolize ethanol
long before human-directed fermentation. Retrieved January 28, 2018, from http://www.pnas.org/content/112/2/458

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Frugivory
diets probably emerged in hominoids around 24 million years ago. Birds and
mammals have been attracted consuming ripe fruit. However, fruit is susceptible
to decay which means there is possibly less nutritional value in it, so it is a
race between the microbes like yeast and consumers to gain the nutritional
value. Sugar eating yeast, the primary fruit decaying agent, plus anti-fungal
agents naturally occurring in fruit creates a number of different alcohols, but
ethanol dominates. The conditions of tropical forests make it difficult for
fruits to stay ripe for an elongated time, thus nutritional requirements became
difficult to satisfy. While diverse diets are a main characteristic of hominids
(i.e. animal fat inclusion) frugivorous behaviors have deep reaching roots in
their diets. Ethanol contains a significant amount of calories that may have
helped the hominids combat these difficult times. However, most studies of
ethanol intake are focused on the behavioral effects and genetically expression
of addictive behavior. Researchers must also try and recreate the ethanol
seeking behavior of hominids in a natural setting (i.e. using fruit instead of
liquids). However, it is difficult as people prefer studies on alcoholism
rather than the natural intoxication process. The evolutionary perspectives are
missing when it comes to the subject of human alcoholism. Alcohol’s effects on
humans have been found to effect people of different ethnicities in interesting
ways. With this in mind, Dudley hypothesizes that natural selection acted on
human ancestors to associate ethanol with nutritional reward due to their
frugivorous roots.

Dudley,
R. (2000). Evolutionary Origins of Human Alcoholism in Primate Frugivory. The
Quarterly Review of Biology, 75(1), 3-15. Retrieved from http://www.jstor.org/stable/2664497

Apes
and humans share the mutation allowing them to metabolize ethanol. The Drunken
Monkey Hypothesis postulates that either by attraction, a cue to finding food,
or an increase to caloric gain, natural selection led to the ability to ingest
ethanol. While ethanol intake by non-human primates can occur naturally,
intoxication is basically non-existent in the wild. However, Hockings’
observations show the habitual and repeated consumption of ethanol by Bossou
chimpanzees. The Raffia Palms produces fermented sap year-round. Bossou
researchers witnessed the chimpanzees ingesting the sap daily. The researchers
state that they classified drinking events as when an individual used a tool to
obtain sap until they were done, and sessions as when the group was finished
drinking. The tools were made from the leaves which they crumpled to collect
sap and then drank. The ethanol content averaged at around 3%. Over a 17 year
period (1995-2012), the researchers observed 51 drinking events and 20
sessions, and the leaf tool was always used. Some chimpanzees could be seen
co-drinking with others while some monopolized a container. While one male
contributed 14 of the 51 events, sex was not a determinant of drinkers. This
was the 1st quantitative study of chimpanzees ingesting ethanol in
the wild, however rare it was. As fruit ferments naturally, consumption is
unavoidable, but the results show that it is not an absolute deterrent.
However, the chimpanzees sow no evidence of obtaining this without human
intervention. Further testing and observations are needed for the Drunken
Monkey Hypothesis.

Hockings,
Kimberley J., et al. (2015, June 01). Tools to tipple: ethanol ingestion by
wild chimpanzees using leaf-sponges. Retrieved January 28, 2018, from https://doi.org/10.1098/rsos.150150