An experiment repeated 600 times finds hints at the secrets of evolution

In a laboratory in Atlanta, thousands of yeast cells fight for their lives every day. Those that live another day grow the fastest, reproduce the fastest and form the largest clumps. For about ten years, the cells have evolved to hang together and form branching snowflake shapes.

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

While multicellularity is believed to have evolved at least 20 times in the history of life on Earth, how living things progress from a single cell to many sharing a fate is far from clear. But in a paper published Wednesday in the journal Nature, researchers reveal a clue to how cells can build themselves in 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. Gradually they evolved from a soft, squishy substance to something with the toughness of wood.

Will Ratcliff, a professor at Georgia Tech, started 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 over more than 75,000 generations and have been documenting how populations have changed since 1988. Dr. Ratcliff wondered if an evolutionary study that encourages cells to stick together might shed light on the origins of multicellularity.

“All the lineages we know of that evolved multicellularity made this step hundreds of millions of years ago,” he said. “And we don’t have a lot of information about how individual cells form groups.”

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

And it worked: Within 60 days, the snowflake yeast appeared. When these yeasts divide thanks to a mutation, they do not completely separate from each other. Instead, they form branching structures of genetically identical cells. The yeast had become multicellular.

But the snowflakes, Dr. Ratcliff discovered as he continued to investigate, never seemed to grow very large and remained stubbornly microscopic. He credits Ozan Bozdag, a postdoctoral researcher in his group, with a breakthrough with oxygen, or lack thereof.

For many organisms, oxygen functions as a kind of rocket fuel. It makes it easier to access the energy stored in sugars.

Dr. During the experiment, Bozdag gave oxygen to some yeasts and grew others that had a mutation that prevented them from using them. He found that over the course of 600 transfers, the yeast exploded in size without oxygen. Their snowflakes grew and grew, eventually becoming visible to the naked eye. Closer examination of the structures revealed that the yeast cells were much longer than normal. The branches had become entangled and formed a dense mass.

That density could explain why oxygen seems to have been a hindrance to the yeast getting big, the scientists think. For yeast that could use oxygen, growing large had significant drawbacks.

As long as snowflakes remained small, the cells generally had equal access to oxygen. But big, dense plugs meant cells in each clump were cut off from oxygen.

Yeast, on the other hand, which couldn’t use oxygen, had nothing to lose, so they grew big. The finding suggests that feeding all cells in a cluster is a critical part of the trade-offs an organism faces as it becomes multicellular.

The clusters formed are also tough.

“The amount of energy it takes to break these things has increased by more than a factor of a million,” said Peter Yunker, a Georgia Tech professor and co-author of the paper.

That power may be the key to a new step in the development of multicellularity, says Dr. Ratcliff – the development of such a thing as a circulatory system. If cells lining a large clump need help accessing nutrients, a body strong enough to channel a flow of fluid is essential.

“It’s like shooting a fire hose into a yeast cluster,” said Dr. Junker. If the clump of cells is weak, that stream of nutrients will destroy it before each cell receives nourishment.

The team is now investigating whether dense clumps of snowflake yeast could evolve ways to get nutrients to their inner members. If they do, perhaps this yeast in their test tubes in Atlanta can tell us something about what it was like centuries ago when your ancestors and many living beings around you first began building bodies out of cells.