AN EXPERIMENT REPEATED 600 TIMES FINDS HINTS TO EVOLUTION’S SECRETS

In a lab in Atlanta, thousands of yeast cells fight for their lives every day. The ones that live another day grow fastest, reproduce quickest and form the biggest clumps. For about a decade, the cells have evolved to hang onto one another, forming branching snowflake shapes.

These snowflakes are at the heart of experiments exploring what might have happened when single-celled creatures first banded together to become multicellular. That process, however it went down, eventually resulted in fabulously weird organisms like octopuses and ostriches and humans.

Although multicellularity is thought to have evolved at least 20 times in the history of life on Earth, it is far from obvious how living things go from a single cell to many that share a fate. But, in a new paper, researchers reveal one clue to how cells could start building themselves into a body. The team that produced the snowflake yeast found that over 3,000 generations, the yeast clumps grew so large that they could be seen with the naked eye.

Will Ratcliff, a professor at Georgia Tech, began the yeast experiments when he was in graduate school. Ratcliff wondered if an evolution study encouraging cells to stick together could shed light on the origins of multicellularity.

So he set up a simple experiment. Every day, he swirled yeast cells in a test tube, sucked up the ones that sank to the bottom quickest, then used them to grow the next day’s population of yeast. He reasoned that if he selected for the heaviest individuals or clumps of cells, there would be an incentive for the yeast to evolve a way to stick together.

And it worked: Within 60 days, the snowflake yeast appeared. When these yeast divide, thanks to a mutation, they don’t fully separate from one another. Instead they form branching structures of genetically identical cells. The yeast had become multicellular.

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