Evolving Yeast Shows How Complex Life May Have Arisen

Over two years, clumps of single-celled yeast grew into a multicellular structure that could explain how living organisms developed on early Earth

Snowflake yeast
In researching the formation of these multicellular organisms, Ratcliff used a strain of snowflake yeast with budding “daughters” that tend to cling to their parents, allowing the creation of small clumps of connected yeast cells. A. Zamani, S. Cao and W. Ratcliff/Georgia Tech via Twitter

To many, it’s a familiar story—the simple, single-celled organisms living in the ancient Earth’s proverbial “primordial stew” slowly evolved into complex, multicellular organisms that today includes modern humans. But that crucial leap from unicellular to multicellular is poorly understood, in part due to scientists today having no real way to witness it happening. Now, new research that’s been released as a preprint explains how scientists have observed hundreds of thousands of yeast cells start to create multicellular groups, possibly modeling how this process played out.

“This is the coolest paper we’ve ever written,” evolutionary biologist and lead author Will Ratcliff of Georgia Tech told Michael Greshko of National Geographic.

Ratcliff has devoted the last decade working with yeast to better understand multicellular life. Some single-celled organisms such as yeast reproduce through the process of budding, in which a cell grows a small copy of itself protruding from its surface. That copy typically splits off from its parent cell when it reaches maturity, creating two independent, single-celled organisms.

 

While multicellular life comprises the most visible organisms on this planet today, it’s worth keeping in mind that for much of life’s existence on Earth, single-celled organisms were the only game in town, reports Veronique Greenwood of Quanta. It was only about 2 billion years after the first life on Earth is suspected to have formed that the first evidence of multicellular organisms exists in fossil records.

What motivated the evolution of single-celled organisms into multicellular organisms is still hotly debated, with some scientists suspecting that cells that clumped together could have better avoided being consumed by unicellular predators or more efficiently found resources.

In researching the formation of these multicellular organisms, Ratcliff used a strain of snowflake yeast with budding “daughters” that tend to cling to their parents, allowing the creation of small clumps of connected yeast cells. However, these clumps appeared to reach a maximum size when they grew to a few hundred cells in number.

To figure out why the yeast stopped growing, Ratcliff and his collaborators recalled that the early Earth had little oxygen compared to the modern day. After a few years of running experiments with several different mutations of yeast in varying levels of oxygen, the scientists noticed that the strains that consumed no oxygen started to grow into clumps large enough to be visible to the naked eye. It appeared that yeast clumps consuming oxygen would intentionally limit their size, likely so the cells inside the clump could have access to the rich energy source provided by the gas.

Remarkably, the large yeast structures become firm like gelatin as a result of their cellular structures becoming entangled with each other.

Inspired by a famous, decades-long experiment observing colonies of E. coli bacteria growing, the scientists behind the experiment hope to continue allowing the yeast in this study to evolve and observe how it changes.

“Not a lot of people want to do a 30-year-long evolutionary experiment,” Ratcliff told Greshko. “But I think the payoff here is huge.”