January 19, 2012
New Study Sheds Light On The Evolution Of Multicellularity
One of the greatest mysteries of evolutionary biology revolves around the question of how Earth´s original single-celled ancestors – the predecessors of all life on the planet – first made the critical transition to multicellularity. A new study published this week in Proceedings of the National Academy of Sciences (PNAS), however, points out that the switch may not have been as difficult as most scientists have speculated.
The traditional paradigm for attempting to understand the emergence of multi-cellular organisms has involved trying to explain how fundamentally ℠selfish´ single-celled organisms – that is, organisms that ensure their survival by hogging as many resources as possible and hindering their neighbors from doing the same – could suddenly begin working together.
“Traditional theories make this out to be a difficult transition because you have to somehow turn off [natural] selection on the individual cells and turn it on for the collective,” explained Carl Simpson, a paleobiologist at the Museum fÃ¼r Naturkunde in Berlin, Germany, to Sarah Fecht of Scientific American.
“The big result here is that these transitions can be super easy,” added Simpson, who was not involved with the study.
For their experiment, researchers at the University of Minnesota devised a relatively simple yet elegant method of artificially selecting for multicellularity in yeast colonies. Millions of single-celled yeast cells were simply dropped into a tube full of liquid nutrients and then given several minutes to settle in the solution. The researchers then extracted the smallest portion of the nutrient-yeast mixture from the very bottom of the tube and used the cells in it to grow the next generation of yeast.
In order for cells to make it to the ℠next round´ they had to be able to clump together with other yeast cells so that they would be heavy enough to sink to the bottom of the tube.
Repeating this process numerous times, researchers were astounded by what they found.
After just a couple of weeks of repeating the process once a day, the scientists observed that all of the surviving yeast cells had formed small cooperative populations, each cell linked together with others in snowflake-like clusters. The individual cells still functioned as individuals but they easily formed tight networks with their neighbors.
Yet after two months and 60 repeats of the selective process, the researchers say that the yeast had already attained a sort of true - if primitive - multicellularity, displaying a division of labor with different cells specializing in different tasks, different life stages, and even reproducing multicellular offspring rather than individual cells.
“Multicellularity is the ultimate in cooperation,” Michael Travisano, the evolutionary biologist who led the study, told Brandon Keim of Wired News.
“Multiple cells make up an individual that cooperates for the benefit of the whole. Sometimes cells give up their ability to reproduce for the benefit of close kin.”
The problem with earlier studies looking for the origin of multicellularity, explained Travisano, was that they tended to focus on finding some kind of genetic root for complexity.
He notes that the results of his team´s study shift the emphasis to environment factors as being the cornerstone of inter-cellular cooperation and eventual multicelluarity. Simply stated, if the environment offers cells an incentive to go multicellular, they´ll find a way to do it.
Indeed, far more significant than the specific results of this study is what the experiment says in general about the evolution of complex organisms: It´s probably not as complex or slow as biologists have traditionally thought.
“We [now] know that simple conditions are sufficient to select for multicellularity,” says Travisano.
“This is actually simple. It doesn´t need mystical complexity or a lot of the things that people have hypothesized – special genes, a huge genome, very unnatural conditions.”
Mother Nature sets the stage, and life rises to play the part.
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