Happy hour started way earlier than we thought.
Roughly 10 million years ago -- long before humans began to store food harvested by agriculture, and long, long before they intentionally fermented food and made a delightfully relaxing ritual of consuming the result -- a new study finds that the guts of our primate ancestors evolved the ability to metabolize alcohol.
The latest research suggests that humankind has had far more evolutionary time to adapt itself to the presence of alcohol than has been previously thought. Earlier efforts to date mankind's practice of consuming alcohol had suggested that our relationship to fermented fruit began a mere 9,000 years ago. That is when humans were thought to have developed the technological wherewithal to jump-start the rotting process, separate the extraneous food bulk, and harvest the liquid inebriate known by the scientific name of ethanol.
If humankind's relationship with drink were truly that young, it would stand to reason that many among us might not yet have acquired the evolutionary means to tolerate alcohol. And that would likely color our understanding of a disease such as alcoholism -- in which an environmental factor that is benign or even salutary to most confers toxic effects on certain individuals. Alcoholics might be seen merely as evolutionary late bloomers, whose genetic ability to metabolize ethanol has not yet caught up with its ubiquitous availability.
If primates have been enjoying the effects of fermentation for 10 million years, by contrast, that would suggest that humans -- and even chimpanzees and gorillas before them -- have by now pretty fully evolved to consume alcohol. By that reading, any genetic predisposition to alcoholism might be viewed as the result of a more recent, or more random, genetic mutation.
To date our relationship to drink, a team led by biologist Matthew Carrigan of Santa Fe College in Gainesville, Fla., reached back into humans' ancestral lineage to detect where the ability to produce a gut enzyme key to metabolizing alcohol first appeared. They looked for an enzyme known as Class IV alcohol dehydrogenases -- abbreviated to ADH4 -- in the guts of primates from whose family trees humans diverged as long ago as 70 million years.
Their research was published Monday in the journal PNAS.
Carrigan's team found that the digestive enzyme ADH4, which begins to break down alcohol on the tongue and in the esophagus and continues to do so in the stomach, was first found plentifully in the gorilla, a primate ancestor from whose lineage humans diverged roughly 10 million years ago. It is similarly found in more recent evolutionary ancestors, including the chimpanzee and the bonobo.
Alas, without the genetic ability to produce ADH4, more distant ancestors of humans, including the orangutan, gibbon and baboon, would have developed no taste for alcohol. Long before they might have been drawn by the inebriating effects of ethanol, those primate ancestors would have rejected fermented foods because of the stomach pain, nausea, or physical discomfort associated with eating them.
Our 10-million-year-old relationship with alcohol should come as no surprise, the authors of the latest research suggest. It coincides with the middle Miocene climatic transition, a period of rapid environmental change in which the fragmented forest ecosystems of East Africa were giving way to grassland ecosystems.
In that period, our hominoid ancestors -- consumers of tree fruits all -- increasingly came down from the trees. As they dragged their knuckles along the ground, they would likely have found fallen fruit colonized by yeast and in various stages of fermentation. Those that could eat that spoilage would not only have fared better at living on the ground than those sustaining themselves on unripened fruit in the trees; they would have been more likely to survive lean growing seasons by virtue of their ability to eat fruits (and eventually meats) that were well past their prime.
Indeed, our human ancestors may even have enjoyed the gauzily pleasant effect that consuming such fruit had. So, when humans developed the tools to do so millions of years later, they took yeast into their own hands and became deliberate makers of beers, wines and spirits.
One small mystery emerged: researchers did find the ability to produce ADH4 in the aye aye, an endangered lemur species native to Madagascar. Close to 50 million years separates the aye aye from humans, so it's not likely that humans' ability to metabolize alcohol started that far back, and then somehow fell away in all the intervening species of ancestors. Aye ayes are known to eat tree fruits, sap and nectar, all of which can ferment naturally, and that probably accounts for their evolved avility to metabolize ethanol.
So if alcohol is served at this family reunion, humans, bonobos, chimps and gorillas might hoist a toast to their very distant tree-dwelling cousin, the aye aye.