As we think about how organisms can increase in complexity, there are really only a few strategies available. One way is to generate very complex unicellular organisms. This strategy is limited, however, and organisms of this type are generally small, only a few hundred micrometers in length. The alternative path to complexity is through multicellularity, which appears to have first occurred ~1 billion years ago. In true multicellular organisms (as opposed to colonial organisms), different cells become highly specialized. Most cells (somatic cells) are relieved of the need to produce a new organism; that task is taken up by specialized cells (germ cells). As noted above, this allows for the formation of cells with very limited, but highly useful abilities.
To get a better idea of the evolutionary history of multicellularity it is helpful to look in detail at the organization, both cellular and genomic, of current organisms. It has been estimated that multicellularity arose multiple times among the eukaryotes313. To begin to understand the steps in the process it is useful to consider those unicellular organisms most closely related to a particular metazoan lineage, known as a sister group. We can then speculate on the various steps between the unicellular and multicellular forms. In the case of the animals, it appears that their (our) unicellular sister roup are the choanoflagellates314. Choanoflagellates have cells that are characterized by a single flagellum surrounded by a distinctive collar structure315. Choanoflagellates exist in both unicellular and simple colonial forms.
Sponges (porifera) are among the simplest of the metazoans (multicellular animals). Fossils of extinct sponges, such as the Archaeocyathids, have been found in Cambrian rock over 500 million years old. Earlier sponge-like organisms have been found in even older Pre-cambrian rock. Sponges contain only a few different types of cells. These include the cells that form the outer layer of the organism (pinococytes) and those (porocytes) that form the pores in the organism's outer layer. The skeletal system of the sponge, the spicules, are produced by sclerocytes. A distinct type of cell (archaeocytes) function in digestion, gamete production, tissue repair and regeneration. Sponges also include cells, known as choanocytes, that move fluid through the body. It is the striking resemblance of these cells to the unicellular choanaflagellates (and subsequent genomic analyses) that led to the hypothesis that choanoflagellates and animals are sister groups316.
The next level of metazoan complexity is represented by hydra and related organisms, the hydrozoa, which include jellyfish. Some of these organisms alternate between a sessile (anchored) and benthic, or floating, lifestyles317. The hydrozoa contain more distinct cell types than the porifera. The most dramatic difference is their ability to produce coordinated movements associated with swimming and predation. While sponges behave as passive sieves, the hydrozoa have a single distinct mouth, an internal stomach-like cavity, and motile arms specialized to capture prey. Their mouth also serves as their anus, through which waste is released.
Hydrozoan movements are coordinated by a network of cells, known as a nerve net, that acts to regulate contractile muscle cells. Together the nerve net and muscles cells generate coordinated movements, even though there is no central brain (which in its simplest form is just a dense mass of nerve cells). A hydra can display movements complicated enough to capture and engulf small fish. Stinging cells, nematocysts, are located in the “arms". Triggered by touch, they explode outward, embedding themselves in prey and delivering a paralyzing poison318. Hydrozoans are complex enough to be true predators.
Questions to answer & ponder:
- What types of social signals do human send and receive?
- How would changes in the affinity of an auto-inducer receptor influence the behavior of an organism?
- Why might an organism grow well in a biofilm but not in isolation?
- In the case of a cellular slime mold, what is the advantage of multicellularity?
- Why do Dictyostelium stalk cells "sacrifice themselves" for fruiting body cells?
- What types of evidence suggest that choanoflagellates and sponges are related?
- Why is the presence of highly specialized cells considered evidence for common ancestry?
- In terms of cell types and functions, how do a hydra and a sponge differ from one another?
- What kind of evidence, in modern organisms, might lead you to conclude that the last common ancestor of plants and animals had flagella?
- What are the advantages of a closed gut versus a sieve?
- Does coordinated movement require a brain? Does having a brain equal self-awareness?
313 Multicellularity arose several times in the evolution of eukaryotes: http://www.ncbi.nlm.nih.gov/pubmed/23315654
315 Introduction to the Choanoflagellata: http://www.ucmp.berkeley.edu/protista/choanos.html
316 The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans: http://www.ncbi.nlm.nih.gov/
317 The live cycle of jellyfish: http://youtu.be/oHiVA9J_YIM
318 How do jellyfish sting: http://youtu.be/HyIwa7W-ZV8