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To study life's origins, creating some snowflakes to see how they evolve.

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 to 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 multi-cellular. 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 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.

He was inspired by Richard Lenski, a biologist at Michigan State University, and his colleagues who have grown 12 vials of E. coli through more than 75,000 generations, documenting since 1988 how the populations have changed.

Dr. Ratcliff wondered if an evolution study encouraging cells to stick together could shed light on the origin 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 most quickly, then used them to grow the next day's population of yeast.

He reasoned that if he selected for the heaviest individuals of clumps of cells, there would be an incentive for the yeast to evolve a way to stick together.

It worked : Within 60 days, the snowflake yeast appeared.

When these yeast divide, thanks to a mutation, they do not fully separate from one another.  Instead they form branching structures of genetically identical cells. 

The yeast had become multicellular. [ Veronique Greenwood ].


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