Yeast, Genes, and Intoxication

I will bet that many students will raise their glasses in celebration at the end of exams.  Humans experience the intoxication of alcohol all thanks to Saccharomyces cerevisiae, otherwise known as brewers yeast.   Brewers yeast is a single-celled fungus that plays an important role in the lives of humans from our distant past through to today.   

The Genome News Network states that brewers yeast, and its evolution, is often studied because many genes that control the yeast's function are important in humans.  Brewers yeast provides the first clear example that whole-genome duplication plays a role in evolution not just.  This is important for understanding how gene duplication works, and its role in evolution. 

Its good to understand how brewers yeast produces alcohol.  Here are the basics, as taken from a  December 2006 New Scientist article.  "S. cerevisiae ... converts sugars into ethanol, generating a meagre two molecules of ATP per glucose molecule. Most cells resort to anaerobic respiration only when oxygen is in short supply, but give S. cerevisiae some sugar and it will churn out alcohol even when oxygen is plentiful - sacrificing huge amounts of energy in the process".  It is amazing how evolution works.  If you were to look at this process on its own, it would seem absurd and wasteful for brewers yeast to function as it does.  A person may conclude that this was a horrible evolutionary mistake.  Yet, we must not forget that every factor, and every possible element that influences brewers yeast survival, is essential to its evolutionary path.  Holism is key to addressing evolutionary forces.  

The gene duplication which occurred would have been a response to competition from other similar organisms.  "Ethanol is toxic to most microbes, so acquiring the ability to turn all the glucose available in a fruit in to a sea of the stuff gave S. cerevisiae's ancestors a big competitive advantage" (New scientist).  Ripened fruit contains higher levels of alcohol and is a food source for other larger organisms such as primates.   Heres where human ancestors came in.   New Scientist notes that "primates have long feasted on fruit, and many researchers think that our ancestors evolved to love the scent of alcohol that helped them discover ripe fruit". Throughout human prehistory and history, we have influenced plant growth by purposefully selecting for certain traits.  This may be an early example of the human imprint on evolution in other organisms.  Again, everything is connected.  Skeptics point out that tests conducted as of April 2004 have found only very small doses of alcohol in fruits, perhaps one-tenth or one-hundredth of that found in a single cocktail.  To learn more about human ancestors role, scientists are studying how much ethanol primates consume in the wild (New Scientist).  The comparative approach between chimpanzees and humans is a common technique to help gain insight to human ancestors.  Any results will help understand the effects brewers yeast may have had on human evolution.  

According to New Scientist, we, like other primates, have an enzyme called adlehyde dehydrogenase that clears acetaldehyde, a toxic breakdown product of alcohol, from our bodies.  As we know, where there are genes, there are gene mutations.  There is a mutant gene for aldehyde dehydrogenase that makes the enzyme slower at clearing acetaldehyde from the body (New Scientist).  This causes hot flushes and nausea after drinking alcohol (New Scientist).  Up to 80% of Han Chinese, Korean, and Japanese people, however, have at least one copy of [the mutant gene]" (New Scientist).  This article also notes that although the effects may appear as a disadvantage, they may be an asset since alcoholism is less common among those with the mutant adlehyde degydrogenase gene.  Humans today enjoy and abuse alcohol in a variety of ways.  Perhaps this mutant gene will mean more to us in the future, or maybe not.  Time will tell. 

Currently, human evolution is effected by evolutionary forces in different ways than our distant and not so distant ancestors. Today, gene flow, genetic drift, and mutations are more influential than natural selection on human evolution in many areas of the world.  There is the possibility, and likelihood of change in frequency of the mutant aldehyde dehydrogenase enzyme.  Whether the effects will be disruptive, stabilizing, or directional in the human population is the question.  I expect gene flow to play the most important role in this case. Increased migration, travel, and interactions between people all over the world result in the movement of genes throughout the global human population.  

Darwin's Bottle http://findarticles.com/p/articles/mi_m3488/is_9_85/ai_n6230587/pg_1

Genome News Network http://www.genomenewsnetwork.org/articles/2004/03/19/evolution.php

New Scientist December 2006 http://www.newscientist.com/channel/life/mg19225831.100-festive-special-the-brewers-tale.html

The Missing Drink April 2004 http://www.signonsandiego.com/uniontrib/20040407/news_lz1c7biology.html 

Images: 

Chimp Banana

Beer Bottles