1.4 Evolution of Multicellularity
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Define multicellularity and contrast it with coloniality
- Understand the levels of biological organization of cells within a multicellular organism
- Summarize the proposed hypotheses regarding the evolution of multicellularity, and which major taxa exhibit multicellularity
- Explain some advantages of multicellularity
As discussed in Ch 1.3, there are physical limitations to the size of unicellular organisms. Over time, many organisms have overcome this challenge by developing multicellularity: the aggregation of cells that each perform a specialized function. Currently, a firm understanding of how multicellularity evolved is unclear. However, there are several proposed hypotheses. This section will examine who, when, how, and why specific organisms evolved multicellularity.
Multicellular vs Colonial Organisms
In simplistic terms, multicellularity refers to an organism consisting of more than one cell performing different specialized functions. However, some biologists insist that to be truly multicellular, there must be a physical connection between cells and a display of cell-to-cell commination. A common way to understand multicellularity is to contrast it with coloniality. A colony of cells refers to a collection of two or more individual cells that reside in close proximately. Cells normally form colonies to gain mutualistic benefits (e.g., protection from predation/environment). However, cells within a colony (e.g., in a biofilm) can work independently, meaning each cell is capable of surviving on its own. Conversely, multicellular organisms rely heavily on one another to survive; thus, if separated, the cells cannot survive on their own (e.g., human heart cells). For instance, the reproductive cells in Figure 1.13B rely on the motility cells for locomotion.
Levels of Organization within Multicellularity
Within a multicellular organism, cells can be organized into larger structures that work together. Let’s consider an example from an animal (Figure 1.14), although some of these terms can be applied to other types of multicellular organisms. A group of cells that share a similar function can be organized into a tissue (e.g., the layer of cells lining the stomach). Additionally, an organ is a structure consisting of multiple tissue layers that performs a specialized function (e.g., the stomach). Finally, when a group of organs works together to perform a similar function, an organ system is formed (e.g., the digestive system).
Evolution of Multicellularity
Multicellularity has evolved multiple times, both in eukaryotes and prokaryotes. [1]The first evidence of primitive multicellularity was observed from fossil records of cyanobacteria-like organisms that existed 3-3.5 billion years ago. Anabaena is a modern-day bacterium that exhibit primitive multicellularity (Figure 1.15). These bacteria possess two different cell types: he