Yeast: How a Mighty Little Fungus Evolved To Change the World

Yeast: How a Mighty Little Fungus Evolved To Change the World

Without alcohol, it’s fair to say that modern society would look quite different. Beer, for instance, is believed by some archeologists to be the reason humans transitioned from hunter-gatherers to farmers. And wine has been an integral part of life around the world for centuries.

But beer would just be barley-flavored tea and wine would remain as grape juice if it weren’t for one key ingredient: yeast. These little organisms comprise one part of 120,000 identified species of fungi, according to the Bradbury Science Museum. Yet, they’ve evolved for millions of years to produce alcohol, a trait fairly unique to these microbes. This evolutionary development not only allowed yeast to survive, but forever shaped civilization.

What is yeast exactly and when did humans first notice it?

Yeast are “single-celled fungal organisms,” who consume sugar and turn it into alcohol and carbon dioxide.

They were first observed in the 17th century by Antonie van Leeuwenhoek, a Dutch scientist, who observed specks of yeast through a microscope, according to Ancient Brews: Rediscovered and Re-Created by Dr. Patrick McGovern, the scientific director of the biomolecular archaeology project and adjunct professor of anthropology at the University of Pennsylvania Museum. However, Leeuwenhoek didn’t realize what he saw was alive.

But long before Leeuwenhoek observed and documented these fungi, humans had reaped the benefits of yeast for thousands of years without realizing it.

“You have the monks who literally thought that their stirring stick was divinely inspired, and that’s why they would get fermentation,” says Travis Rupp, research and development manager/beer archaeologist at Avery Brewing Co. and lecturer of classics at the University of Colorado-Boulder. “When in fact, all they were doing was hanging [the stick] up and all this wild yeast and bacteria fell on it before they would stick it in the pot for the next brew.”

It wasn’t until the 19th century that scientists began to recognize that not only were yeast very much alive, but that the fungus was responsible for fermentation.

Evolution and fermentation

When the Cretaceous period began around 145 million years ago, flowering plants started to propagate all over the planet.

It was during this period when “a couple of yeast cells that had been budding in tree sap nudged against each other and mated,” writes Dr. Nicholas P. Money in The Rise of Yeast: How the Sugar Fungus Shaped Civilization. “This liaison caused a genetic eruption called whole-genome duplication.”

When yeasts are introduced to the sugary base used for beer or wine, they break down glucose molecules through a process called glycolysis. In the presence of oxygen, this can also accompany a citric acid cycle, known as the Krebs Cycle. Together, these processes allow the yeast to break down glucose in the most efficient manner, though produce no alcohol.

“But yeast cells in beer wort and grape juice soon deplete the oxygen because the dissolved gas diffuses slowly through these sugary fluids,” writes Money.

The fungi eventually gained the ability to break down glucose through a process of “anaerobic burn,” which requires very little oxygen. It also creates alcohol as a byproduct—a process better known as fermentation—which give yeast an evolutionary leg-up over other microbes.

While anaerobic burn requires more energy, the alcohol created allows the yeast to destroy “every other fungus and bacterium that would like to compete for the sugars that allow yeast to thrive,” says Money. Some yeast can tolerate alcohol levels up to 20%, though most strains die when levels reach between 12–15% alcohol by volume (abv). In comparison, most harmful, competing microbes perish at around 5% abv.

As a result, this defensive chemical reaction has been utilized by humans to preserve their own food and drink through fermentation for thousands of years.

So, the next time you pop open a bottle of wine or beer, remember it’s the product of a mighty little fungus and millions of years of evolution.

Published on December 2, 2020




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