A major step in the evolution of life forms was the progression from single-celled to multi-celled organisms, which is estimated to have occurred approximately 2 billion years ago. One organism that straddles this line is blue-green algae, or cyanobacteria. Cyanobacteria are photosynthetic bacteria that usually occur as single-celled organisms. However, these prokaryotes are often found in colonies and even as multicellular arrangements, called blue-green algae due to the presence of their photosynthetic pigments. Blue-green algae are considered the precursors to photosynthetic eukaryotes (plants), making them a helpful tool for studying the evolution to multicellularity.
The benefits of specialization and size have likely driven a number of single-celled organisms to multicellularity. The pros and cons of this alteration, and long-term sustainability of multicellular lineages, have been addressed by researchers. Due to genetic constraints, the first multicellular organisms probably only existed for their own lifespan, but they may have allowed the selection of features necessary for more complex forms, particularly cell adhesion, cell to cell communication, and programmed cell death.
As The Scientist explained in their January 2011 feature on the switch from simple to complex organisms, the recent genetic revolution has opened doors for evolutionary biologists, allowing direct links to be made between species, and even populations, when their genetic information is available. The one thing most can agree on is that the evolution to multicellularity was much more complex than the introduction of a single gene. The switch from single-celled to multi-celled probably occurred a number of times in a number of independent steps; rather than being a big step forward, it was a shuffle creating a web of genetic relationships among unicellular and multicellular organisms in a population.
Research from the University of Zurich published February 14, 2011 in the journal BMC Evolutionary Biology used genetic analysis of more than 1200 species of modern cyanobacteria to pinpoint early evolution of multicellularity in blue-green algae. In the 11,000 phylogenetic trees that resulted, they identified at least five reversals to unicellularity, confirming in one of the oldest phyla what was indicated by previous research. The results showed that the evolution of multicellularity was not linear, raising questions about just how difficult the leap from unicellularity really was.
Evolution has been envisioned as a special process, but the research on blue-green algae and other organisms like them has shown that it is a much more simple and reversible process, albeit one that creates a great deal of complexity. Ultimately, cyanobacteria gained the ability to form multicellular organisms via piecemeal genetic mutations that were shared over generations in unicellular organisms before actually maintaining multicellular populations.