Multicellular evolution not linear

Multicellular blue-green algae made the transition from single-celled to multi-celled not once, but several times over the course of history, according to a study published last week (February 14) in BMC Evolutionary Biology, giving support to the idea that the evolution of multicellularity may not have been as big of an evolutionary leap as scientists once believed.

Cyanobacteria bloom
Image: Wikimedia commons, Lamiot

"Simple multicellularity has evolved a number of times within the bacteria and as many as two dozen times within the eukaryotes," paleobiologist linkurl:Andrew Knoll;http://www.oeb.harvard.edu/faculty/knoll/knoll-oeb.html of Harvard University, who was not involved in the research, said in an email, but relatively little is known about how that transition occurs. This paper provides an "explicit phylogenetic reconstruction" of one group that has evolved multicellular forms, and shows that it's not a simple linear progression of complexity.In cyanobacteria, "multicellularity is easy to lose and regain," agreed linkurl:Bettina Schirrmeister;http://www.ieu.uzh.ch/staff/phd/schirrmeister.html of the University of Zurich, who co-authored the study. "It's not this classical transition from unicellular to multicellular to more complex forms as we might have expected in the past."Blue-green algae, photosynthetic prokaryotes also known as cyanobacteria, first appeared in the fossil record almost 2.5 billion years ago, and have since populated most of the world in a variety of unicellular and multicellular forms. Using gene sequences from 1,254 species of modern cyanobacteria, a team of researchers led by Bettina Schirrmeister of the University of Zurich created over 11,000 different phylogenetic trees that helped pinpoint when multicellularity evolved in this lineage. Their first surprise was that this transition happened numerous times in the distant past more than 2 billion years ago, an unexpected finding if the appearance of multicellularity is a complex evolutionary phenomenon. The researchers also found that many modern unicellular cyanobacteria are descended from multicellular predecessors, suggesting that once multicellularity evolves, it doesn't always stick around. If organisms always evolved towards greater complexity, as evolutionary biologists have traditionally thought, the unicellular cyanobacteria should be more similar to each other than to the multicellular cyanobacteria, Schirrmeister said. But this isn't what they found. Instead, the relationship between uni- and multi-cellular cyanobacteria is a complicated evolutionary web. But even multicellular cyanobacteria are relatively simple organisms, Knoll said, and there are still a lot of open questions about the evolution of complex multicellular life in eukaryotes. (See The Scientist's linkurl:recent feature;http://www.the-scientist.com/2011/1/1/38/1/ on this topic.)Even so, Schirrmeister said, cyanobacteria are some of the most ancient life forms on Earth, "so to understand how multicellularity evolved in cyanobacteria will help us understand how complexity evolves in organisms generally."
**__Related stories:__***linkurl:From Simple To Complex;http://www.the-scientist.com/2011/1/1/38/1/
[January 2011]*linkurl:Top 7 papers in multicellularity;http://www.the-scientist.com/news/display/57906/
[5th January 2011]*linkurl:Earlier start to multicellular life?;http://www.the-scientist.com/blog/display/57539/
[30th June 2010]*linkurl:Related F1000 evaluations;http://f1000.com/search/evaluations?query=multicellularity
[22nd February 2011]