What are the key innovations that led to the evolution of multicellularity, and what were their precursors in the single-celled microbial life that existed before the metazoa? We can hypothesize at least two distinct kinds of features that had to have preceded true multicellularity.PZ goes on to describe the genes in a single-cell eukaryote that diverged near the base of the animal phylum. The species is a choanoflagellate called Monosiga brevicollis.
The obvious feature is that cells must stick together; specific adhesion molecules must be present that link cells together, that aren't generically sticky and bind the organism to everything. So we need molecules that link cell to cell. Another feature of multicellular animals is that they secrete extracellular matrix, a feltwork of molecules outside the cells to which they can also adhere.
A feature that distinguishes true multicellular animals from colonial organisms is division of labor — cells within the organism specialize and follow different functional roles. This requires cell signaling, in which information beyond simple stickiness is communicated to cells, and signal transduction mechanisms which translate the signals into different patterns of gene activity.
It's important to note that this single-cell organism and its multicellular animal relatives form a distinct clade that is separated from the fungi and plants. Since there are multicellular fungi and multicellular plants, the evolution of multicellularity must have occurred many times.
PZ notes that choanoflagellates have primitive forms of adhesion molecules—one of the prerequisites for multicellularity in animals—but they lack some of the standard animal signalling pathways.
PZ Myers is a fan of evo-devo. There are many problems with this approach to biology but one of the most irksome is the emphasis on animals as models for all of evolution and development. I've referred to this as Animal Chauvinism. In his recent posting PZ is careful not to claim that the evolution of multicellularity in animals is the model for all forms of multicellular species but unsophisticated readers might easily get the wrong impression. Let's try and make the generalization that PZ might have wanted to make.
We can agree with his statement that two requirements of multicellularity are the ability of cells to stick together and the division of labor where cells differentiate to carry out specialized functions. Lest anyone imagines that these properties were invented by animals—or even by eukaryotes—let's look at some simple multicellular bacteria.
The first example is cyanobacteria. That's a filament of Anabaena sphaerica at the top of this posting. The cells adhere to each other through a common cell wall, forming long multicellular filaments. Other species of cyanobacteria form different groups of cells; for example, Glaucocystis (upper right) has four cells together in a single sheath.
Look carefully at the Anabaena filament. Do you see the fat round cell in the middle of the filament? That's a heterocyst. It's a differentiated cell that has become specialized for nitrogen fixation. All the other cells are capable of photosynthesis but the heterocyst specializes in fixing nitrogen. This species is a bacterial example of a multicellular organism with two types of cells.
The specialization of the heterocyst means that the two types of cells have to communicate. This communication takes place via small pores in the cell wall between the cells in the filament. Signaling involves transfer of small molecules such as ATP and glutamine between the various cells. What this means is that some cynaobacteria meet the two criteria that PZ Myers lays out for the evolution of multicellularity. There's no doubt about the fact that this version of a multicellular organism predates the evolution of metazoa by about 2-3 billion years.
The myxobacteria are dramatic example of multicellular bacteria. That's Chondromyces crocatus shown in the photograph above left.
Under certain conditions the single cells of myxobacteria come together to form fruiting bodies that consist of hundreds of cells. In the most extreme examples, some cells form the stalk, some cells form sprangia and others form spores. These are multicellular bacteria with specialized differentiated cells.
There are many other multicelluar bacteria but these two are sufficient to illustrate the point. Cell differentiation and multicellularity are not inventions of animals. There weren't even invented by eukaryotes. Differentiation and multicellularity were invented by bacteria long before the true eukaryotes ever appeared on this planet.
[Photo Credits: Anabaena sphaerica from Wikipedia: Glaucocystis from Cyanobacteria slides: Chondromyces crocatus from The Myxobacteria Web page]