When the first single-celled organisms left behind their loner ways and began existing as blobs of cells, it was a big step for life on this planet. Organisms could now grow orders of magnitude larger than each other. They could organize their cells into different types that performed different functions. Instead of drifting around with the other specks, multicellular organisms could grow, swim, crawl, fly, and evolve into everything else on Earth today.
It's nearly impossible to know how organisms first made the leap into multicellularity. But in an effort to look back in time, researchers in Japan bred collections of yeast cells that competed with each other for food. They found that the innovation of living as a group of cells would have given the first many-celled organisms a clear advantage--and that it may have been spurred by nothing more than sloppy wall building.
Yeast is a single-celled fungus, but in the wild it sometimes grows in clumps. This happens when a yeast cell clones itself, budding an identical "daughter cell," but fails to pinch the clone entirely free. These cells continue to reproduce, forming clumps. The researchers wondered if this incomplete separation of single-celled clones could have fostered the rise of multicellularity.
To find out, they first looked at individual yeast cells growing in an environment where their only food was sucrose, a sugar they have to break down before ingesting. To do this, the yeast secrete an enzyme called invertase that splits sucrose into its components, fructose and glucose, which the cell can then absorb. (One advantage of being a complex, multicellular animal is that we don't have to digest our food outside our bodies.) When the yeast cells were all alone, they struggled to survive in the all-sucrose environment. But when other yeast cells were nearby, they all benefited by absorbing the leftover sugars that escaped their neighbors.
Knowing that external digestion works better with friends around, the researchers hypothesized that clumps of yeast should be able to thrive where individual yeast cells can't. They genetically engineered yeast with a gene that encouraged clumping, rather than separation. As predicted, these clumpy yeast were able to grow in environments with a low concentration of food, where single yeast cells couldn't survive.
It takes work for yeast cells to make and secrete invertase, though. Cells that are unable to make invertase are called "cheaters" because they can sit back and enjoy (or absorb, at least) the fruits of their neighbors' external digestion. If they're all alone, these cheaters won't find enough food. But when they're around enough other single cells, the cheaters will outcompete the hard-working invertase producers.
To find out whether multicellularity would help yeast defend themselves against cheaters, the researchers arranged competitions between cheater yeast cells and non-cheaters, either alone or in clumps. At low food concentrations, the clumpy cells easily outcompeted the individual cells, ending up with higher numbers in their population. When cheaters were thrown into the mix, clumpy cells retained their advantage. All around, clumping was a better strategy.
The authors may have modeled the beginning of multicellularity--or one beginning, since the trait evolved multiple times. They showed that the need to digest your food externally is enough to give groups of cells an edge over single cells, and that the strategy of keeping your clones all in one place is enough to seize that advantage. When the first mutant cells failed to fully separate from each other, and found that their little family was now growing faster than the individual cells around them, it could have been the start of something big. (That is, visible without a microscope.)
They may be mere microscopic fungi, but a community of yeast cells has similarities to a community of animals, or even a city. Organisms live in groups when doing so helps them to get food, fend off predators, and pass on their genes. Even when our communities resemble disorganized, unsightly blobs, they're what keep our species alive.
H. Koschwanez, J., R. Foster, K., & W. Murray, A. (2011). Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity PLoS Biology, 9 (8) DOI: 10.1371/journal.pbio.1001122
