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The transition from single-celled to multicellular organisms was one of the most significant developments in the history of life on Earth. Without it, all living things would still be microscopic and simple; there would be no such thing as a plant or a brain or a human. How exactly multicellularity arose is still a mystery, but a new study, published January 16 in Proceedings of the National Academy of Sciences, found that it may have been quicker and easier than many scientists expected.
"This is a significant paper that addresses one of the most fundamental questions in evolutionary and developmental biology," says Rick Grosberg, an evolutionary biologist at the University of California, Davis, who was not involved with the research.
Since evolution acts on individual cells, it pays off for a cell to be selfish. By hogging resources and hindering neighbors, a cell can increase the odds that more of its own genes get passed into the next generation. This logic is one of the reasons it has been challenging to imagine how multicellularity arose; it requires the subjugation of self-interest in favor of the group’s survival.
"Traditional theories make this out to be a difficult transition because you have to somehow turn off selection on the individual cells and turn it on for the collective," says Carl Simpson, a paleobiologist at the Museum für Naturkunde in Berlin, who also was not involved in the research. "The big result here is that these transitions can be super easy."
In the new paper, researchers at the University of Minnesota used a simple but elegant technique to artificially select for multicellularity in yeast. They dumped unicellular yeast into a tube of liquid food and waited a few minutes for the cells to settle. Then they extracted the lowest fraction of the liquid and allowed whatever cells it contained to form the next generation. Because the cells had to cluster together in order to sink to the bottom and survive, the artificial selection made it more advantageous for yeast to cooperate than to be solitary.
After just 60 generations, all of the surviving yeast populations had formed snowflake-shaped multicellular clusters. "Hence we know that simple conditions are sufficient to select for multicellularity," says biologist Michael Travisano, who led the research.
But at what point do the yeast become something more than a cluster of cells? When do they begin behaving as one organism?
In a true multicellular organism, such as a rabbit, evolution acts on the rabbit and not on each of the billions of cells that build it. So the researchers set out to determine whether artificial selection would act on the snowflake yeast as if they also were multicellular organisms. To test it, one batch of the multicellular yeast was allowed only five minutes to settle in a tube (representing a strong selection pressure), whereas another batch was given 25 minutes (a weaker selection pressure). After 35 generations, the yeast that were exposed to stronger selection evolved to have larger cluster sizes, whereas those in the weak selection group actually shrank in size. This indicated that each cluster of cells was evolving as one organism.
In addition, time-lapse photography (video below) revealed that, in order to reproduce, the multicellular yeast divides itself into branches that develop into the multicellular form as well. The daughter clusters did not create their own offspring until they had reached a similar size as their parents. The presence of this juvenile stage shows that the snowflake yeast had adopted a multicellular way of life, says William Ratcliff, a postdoctoral student in Travisano’s lab.
The researchers also found evidence of rudimentary division of labor, which is an essential characteristic for more complex multicellular life forms. In a human, for example, some cells may differentiate into blood cells, others may differentiate into immune cells, but only select egg or sperm cells help form the next generation.
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11 Comments
Add Comment"By hogging resources and hindering neighbors, a cell can increase the odds that more of its own genes get passed into the next generation."
Reply | Report Abuse | Link to thisApplies to more than just cells...
Although it would be unfair to discount the value of this research, I think that its observations are unremarkable. The processes of collective mutual benefit were likely very active in the earliest processes that created life as we know it. In the earliest stages of evolution, chemical evolution, the only mechanism that could account for any form of reproduction is one that produces reproduction by means of proxy. In energy rich environments of cyclic synthesis/destruction, chemical processes that favor the preservation of similar chemical species will mimic a process of reproduction that is key to any chain of events that will produce life. Reproduction by proxy was probably the most powerful mechanism for driving chemical communities to a state that would be more recognizable as life. Mechanisms for group cooperation were more likely to be in the salient of early life with individual/selfish mechanisms developing much later.
Reply | Report Abuse | Link to this"By hogging resources and hindering neighbors, a cell can increase the odds that more of its own genes get passed into the next generation. This logic is one of the reasons it has been challenging to imagine how multicellularity arose; it requires the subjugation of self-interest in favor of the group’s survival."
Reply | Report Abuse | Link to thisThis is an incorrect characterization that adds intent and purpose to selection. The challenge is not to imagine single cells intending to produce an outcome, but to note that out of all outcomes, the one that works continues.
Exactly - that representation of individual cells' self determinism is a projection of our own perspective of self-interests, primarily exhibited by males in mammalian social groups.
Reply | Report Abuse | Link to thisDon't singled cell organisms typically reproduce asexually, most often producing genetically similar or identical copies of itself? Are individual yeast cells genetically dissimilar from their neighbors? I think there is no benefit for single cells to compete for survival...
Brewers have long known that yeast that flocculate (settle) very well can be selected for by harvesting from the bottom of a fermentor right after fermentation is winding down. In just a few generations the brewer will have yeast that flocculate so strongly that fermentation will be incomplete.
Reply | Report Abuse | Link to thisI never thought of the resulting yeast clumps as multicellular organisms. Some brewer might have earned a doctorate centuries ago had they just thought about it a bit more (and not wanted to avoid the practice).
I also noticed that the author of the article implies intent on the part of the yeast. This sloppy description of selection is all too common in science writing. It helps promote misunderstanding of evolution and fuels creationist thought.
They exist in an environment with limited resources, which shows increasing signs of strain to cope with their growing numbers (the energy problem, the deforestation problem, the erosion problem, the pandemic problem, the biodiversity problem, and I could be here all day).
Reply | Report Abuse | Link to thisSo, from a long term point of view, breeding like rabbits may not be the best solution (euphemism).
This is huge. Does this not mean that fi in the drake equation must be significantly higher than any previous estimates, as the single-cell/ multi-cell boundry can be easily crossed.
Reply | Report Abuse | Link to thisYeast can not only flocculate (adhere to each other, grow in clumps) but also grow as pseudohyphae under nitrogen starvation, which is also multicelleluar-like. So the blueprint for multicellularity is already there.
Reply | Report Abuse | Link to thisWith over one billion years behind them, how do we know that single celled yeasts are not a reduction of a formerly multicellular "thing" that found life easier being single, (but stll retains the genetic know-how to become "married" again)?
Reply | Report Abuse | Link to thisThis is certainly interesting, but I would be wary of overinterpretation.
Reply | Report Abuse | Link to thisEven before the comments on flocculation in breweries, I thought of the possibility that this is an innate - and thus previously evolved - ability in yeast, that just takes a few generations to reach full expression when needed.
In fact, I'd say the very reproducibility of the experiment - usually the hallmark of good science - is kind of a weakness here.
If over many identical experiments the change starts happening after a random number of generations, it would indicate that something extraordinary had to happen - i.e. the occurrence of entirely new codings for multicellularity.
If over many experiments it happens at much the same time, it indicates an innate capacity, sort of like the size of a species changing over generations to adjust to a changing environment - populations of small animals have the genetic variability to grow bigger animals, but the current genetic majority codes for small. In particular, this experiment showed both multicellularity, specialization, and cellular "altruism." All of it in sixty generations? Or maybe a few hundred for all of it to appear, but still, an incredible lot of evolution in a very small sample over a very short time span.
Don't get me wrong, I have no more doubt that multicellularity arose naturally, than I do that I'm here, writing this now. Also the suggested hindrance, the selfishness of cells, isn't much of a hindrance considering that neighbouring cells are very likely near identical clones of each other. But if it all came as easy as this, sixty generations in a test tube, shouldn't the estimate be that it arose independently in nature around 25 trillion times rather than 25 times?
Scientists ascribing intention to yeast cells or multicellular units is absurd, but what is even more absurd and arrogant is ascribing intention to human beings! Insead of having intention, intention has us! Just look at any male for confirmation. Women may think they are superior and have intention, but all women have is relation, they hardly HAVE themselves, because women only "live" ("as if" they "live") in relation to others. Men only "live" in relation to their sexuality (and derivations of their sexuality: competition, comparison, as in sports, war, sex, visual, etc.,PERFORMANCES.
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