Their approach was simple: they grew the yeast in a liquid and once each day gently centrifuged each culture, inoculating the next batch with the yeast that settled out on the bottom of each tube. Just as large sand particles settle faster than tiny silt, groups of cells settle faster than single ones, so the team effectively selected for yeast that clumped together.
Sure enough, within 60 days – about 350 generations – every one of their 10 culture lines had evolved a clumped, “snowflake” form. Crucially, the snowflakes formed not from unrelated cells banding together but from cells that remained connected to one another after division, so that all the cells in a snowflake were genetically identical relatives. This relatedness provides the conditions necessary for individual cells to cooperate for the good of the whole snowflake.
In addition to providing yet more overwhelming evidence for evolution, what’s astonishing about this research is just how quickly these cells made the structural leap from single cellularity to multicellularity. In a primordial soup that had millions of years to stew, the chances of multicellularity evolving seem pretty good if a group of scientists saw it happen in less than two months. For some, the experiment was a little too simple:
Sceptics, however, point out that many yeast strains naturally form colonies, and that their ancestors were multicellular tens or hundreds of millions of years ago. As a result, they may have retained some evolved mechanisms for cell adhesion and programmed cell death, effectively stacking the deck in favour of Ratcliff’s experiment.
“I bet that yeast, having once been multicellular, never lost it completely,” says Neil Blackstone, an evolutionary biologist at Northern Illinois University in DeKalb. “I don’t think if you took something that had never been multicellular you would get it so quickly.”
Ratcliff and his team will attempt to counter this criticism directly by replicating the experiment with a type of alga that has no multicellular ancestors.
As the article points out, in the end, it is still remarkable that the yeast cells responded so readily and specifically to what, at least to me, seemed like an imprecise evolutionary pressure. Simply creating an environment in which clumped cells were favored almost immediately prompted a genetic reaction. If DNA is this responsive (and flexible) a tool in such a short time frame, it’s easy to see how the incredible diversity of life on this planet could have evolved over millions of years.
In other news, only three of fifty one contestants on the Miss America pageant believe that evolution should be taught in schools. There are no words.