What are the key ingredients for making a multicellular animal, or metazoan? A couple of the fundamental elements are:
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A mechanism to allow informative interactions between cells. You don’t want all the cells to be the same, you want them to communicate with one another and set up different fates. This is a process called cell signaling and the underlying process of turning a signal into a different pattern of gene or metabolic activity is called signal transduction.
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Patterns of differing cell adhesion. But of course! The cells of your multicellular animal better stick together, or the whole creature will fall apart. This can also be an important component of morphogenesis: switching on a particular adhesion molecule (by way of cell signaling, naturally) can cause one subset of cells to stick to one another more strongly than to their neighbors, and mechanical forces will then sort them out into different tissues.
These are extremely basic functions, sort of a minimal set of cellular activities that we need to have in place in order to even begin to consider evolving a metazoan. Fortunately for our evolutionary history, these are also useful functions for a single celled organism, and while the metazoa may have elaborated upon them to a high degree, there’s nothing novel about the general processes in our make-up. The principles of signaling and transduction were first worked out in bacteria, and anyone who has a passing acquaintance with immunology will know about the adhesive properties of bacteria, and their propensity for modulating that adhesion to build complexes called biofilms.
So let’s take a look at the distribution of signaling and adhesion molecules in single-celled organisms, multicellular animals, and most interestingly, a group that is close to the division between the two (although more on the side of multicellularity), the sponges.
