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How multicellular creatures evolved

Washington, Wed, 18 Jan 2012 ANI

Washington, Jan 18 (ANI): Using natural selection and common single-celled brewer's yeast, researchers have now bridged the famous bridging the famous multi-cellularity gap by recreating an important evolutionary step in the lab.


More than 500 million years ago, single-celled organisms on the Earth's surface began forming multicellular clusters that ultimately became plants and animals. Just how that happened is a question that has eluded evolutionary biologists.


According to the scientists in the University of Minnesota's College of Biological Sciences, the yeast "evolved" into multicellular clusters that work together cooperatively, reproduce and adapt to their environment - in essence, precursors to life on Earth as it is today.


The study only took them one experiment conducted over about 60 days, asserted associate professor Michael Travisano, who is a senior author of the paper.


"I don't think anyone had ever tried it before," said lead author Will Ratcliff.


"There aren't many scientists doing experimental evolution, and they're trying to answer questions about evolution, not recreate it."


The achievement has earned praise and admiration from evolutionary biologists around the world.


"To understand why the world is full of plants and animals, including humans, we need to know how one-celled organisms made the switch to living as a group, as multicelled organisms," said Sam Scheiner, program director in the National Science Foundation (NSF)'s Division of Environmental Biology.


"This study is the first to experimentally observe that transition, providing a look at an event that took place hundreds of millions of years ago."


The two scientists, both from the Department of Ecology, Evolution and Behavior, chose brewer's yeast or Saccharomyces cerevisiae, a species of yeast used since ancient times to make bread and beer, because it is abundant in nature and grows easily.


They added it to a nutrient-rich culture media and allowed the cells to grow for a day in test tubes. Then they used a centrifuge to stratify the contents by weight. As the mixture settled, cell clusters landed on the bottom of the tubes faster because they are heavier.


They removed the clusters, transferred them to fresh media, and grew them up again. Sixty cycles later, the clusters - now hundreds of cells - looked roughly like spherical snowflakes.


Analysis showed that the clusters were not just groups of random cells that adhered to each other, but related cells that remained attached following cell division.


That was significant because it meant they were genetically similar, which promotes cooperation. When the clusters reached a critical size, some cells essentially committed suicide (apoptosis) to allow offspring to separate. The offspring reproduced only after they attained the size of their parents.


"A cluster alone isn't multiellular," Ratcliff said.


"But when cells in a cluster cooperate, make sacrifices for the common good, and adapt to change, that's an evolutionary transition to multicellularity," Ratcliff added.


The study has been published in the Proceedings of the National Academy of Sciences. (ANI)


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