The clusters worked together cooperatively, reproduced and adapted to their environment - just like the precursors to life on Earth today.
"The first step toward multi-cellular complexity seems to be less of an evolutionary hurdle than theory would suggest," says George Gilchrist, acting deputy division director of the National Science Foundation's (NSF) Division of Environmental Biology. "This will stimulate a lot of important research questions."
The team says it wasn't even difficult. Using yeast cells, culture media and a centrifuge, it took just a single experiment conducted over about 60 days.
"I don't think anyone had ever tried it before," says Will Ratcliff of the University of Minnesota. "There aren't many scientists doing experimental evolution, and they're trying to answer questions about evolution, not recreate it."
The scientists added brewer's yeast, or Saccharomyces cerevisiae, to a nutrient-rich culture media and allowed the cells to grow for a day in test tubes. They then used a centrifuge to separate the contents by weight.
The heavier cell clusters were removed, transferred to fresh media, and agitated again.
And, sixty cycles later, the scientists had clusters that consisted of hundreds of cells. They weren't just groups of random cells sticking together, analysis showed, but were related cells that stayed attached even after cell division.
When the clusters reached a critical size, some cells died off in a process known as apoptosis to allow offspring to separate; and the offspring reproduced only after reaching the size of their parents.
"A cluster alone isn't multi-cellular," Ratcliff says. "But when cells in a cluster cooperate, make sacrifices for the common good, and adapt to change, that's an evolutionary transition to multi-cellularity."
In order for multi-cellular organisms to form, most cells need to sacrifice their ability to reproduce, an altruistic action that favors
Evolutionary biologists have estimated that multi-cellularity evolved independently in about 25 groups - and the scientists say they wonder why it didn't happen more often, given that the process seems so simple and that trillions of one-celled organisms have lived on Earth for millions of years.
Next, though, the team plans to look at the role of multi-cellularity in cancer, aging and other areas of biology.
"Cancer was recently described as a fossil from the origin of multi-cellularity, which can be directly investigated with the yeast system," says Michael Travisano of the University of Minnesota.
"Similarly, the origins of aging, development and the evolution of complex morphologies are open to direct experimental investigation that would otherwise be difficult or impossible."