Multicellular life has evolved independently at least 25 times, but these transitions are so ancient that they have been hard to study.
The researchers wanted to see if they could evolve multicellularity in a single-celled organism, using gravity as the selective pressure. In a tube of liquid, clusters of yeast cells settle at the bottom more quickly than single cells. By culturing only the cells that sank, they selected for those that stick together. After many rounds of selection over 60 days, the yeast had evolved into 'snowflakes' comprising dozens of cells.
Many single-celled organisms, including yeast, often form clumps of genetically distinct cells. But Ratcliff’s snowflakes were made up of genetically identical cells that had budded off and stuck together. Many other multicellular organisms may well have evolved through a similar 'divide-and-stick' process.