multicellular evolution not linear

The Scientist | 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.

"Simple multicellularity has evolved a number of times within the bacteria and as many as two dozen times within the eukaryotes," paleobiologist Andrew Knoll 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 Bettina Schirrmeister 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.