When we think of life on Earth, most of us think of multicellular organisms, like large mammals or massive trees. But we're only aware of three groups of complex, multicellular organisms, which suggested it might be a major hurdle. Now, a new study describes how researchers evolved a multicellular form of yeast (the same species that contributes to bread and beer), and were able to see specialized cell behaviors and reproduction in as little as 60 days.

The authors lay out the problem very simply in their introduction, stating that, "Multicellularity was one of the most significant innovations in the history of life, but its initial evolution remains poorly understood." There is some evidence that it can be a favorable trait—research shows that clusters of cells evolve when a single-celled organism is kept in culture with a predator that can only swallow one cell at a time.

But that's about as far as these experiments went. It wasn't clear how these clusters of cells formed, whether they were genetically related, or whether they engaged in any sort of specialized behavior. More significantly, it wasn't obvious whether these clusters took a sort of "every cell for itself" approach to reproduction. So, although this work showed a multicellular lifestyle could be selected for, the researchers didn't look into how far down the road towards specialization those cells would go.

The new study attempts to follow more the behavior of simple multicellular groups more closely. It uses baker's yeast (Saccharomyces cerevisiae), an organism that normally grows as single cells. The authors grew these in culture and, once a day, transferred them in a way that favored multicellular growth.

Their method was pretty simple. Normally, yeast are grown in a culture that's shaken, and the single cells will only slowly settle to the bottom when that's stopped. The authors only transferred the cells at the bottom of the culture to fresh food, so that they selected for those cells that settled to the bottom quickly. This favors large clusters of cells, instead of single ones.

With only 60 daily transfers, all of their experimental populations were dominated by yeast cells that grew as clusters, which the authors describe as "roughly spherical snowflake-like." These were formed because, instead of separating after they divided, cells would remain attached, expanding the cluster with each division. Although this comes at a cost compared to individual cells—the authors calculate that individual cells in the cluster are 10 percent less fit than their single-celled relatives when they're not selecting for things on the bottom. But, with the selection in place, the clusters had a huge advantage.

But the clusters didn't simply keep growing indefinitely. Instead, the yeast quickly evolved a form of reproduction by splitting off what the authors call "propagules," or smaller clusters that break off and go on to develop on their own.

With more generations, this form of reproduction began to include specialized cell behavior. A small percentage of cells in the cluster would start committing suicide through a process called apoptosis. This death would allow the propagule to split off cleanly at the site of the dead cell, improving the efficiency of reproduction. Normally, there's no evolutionary advantage to a cell ending up dead but, since the cells in the propagule are genetically identical, this behavior can be selected for.

This new form of growth and reproduction is still a long way off from the complex, specialized tissues found in most multicellular organisms. But the ease with which this behavior evolved suggests that the foundations of multicellularity may evolve very easily, and don't present the barrier to complexity that many people have assumed it was.

PNAS, 2012. DOI: 10.1073/pnas.1115323109 (About DOIs).

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