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: heterocyst (nitrogen fixing) and vegetative cells (photosynthesis).
[2]Under the simplistic definition of multicellularity (at least two different cell types performing different specialized functions), this phenomenon has evolved in at least 25 separate lineages. However, when imposing stricter criteria (i.e., cell-to-cell connection, communication, and cooperation), multicellularity has evolved fewer times: at least three times in bacteria and at least six times in eukaryotic organisms. Within eukaryotes, multicellularity has evolved once in animals, twice in fungi, once in red algae, once in brown algae, and once in green algae which gave rise to land plants.
Hypotheses on Origin of Multicellularity
There are at least two hypotheses concerning how multicellularity evolved: the colonial hypothesis and the symbiotic hypothesis. The colonial hypothesis (Figure 1.16) is most widely accepted among biologists. This hypothesis was suggested by Ernst Haeckel, who stated multicellularity arose from colonies made from unicellular organisms of the same species. The proposed hypothesis begins with the aggregation of unicellular organisms into colonies that provide a mutual benefit to all members. As time passes, cells within the colony begin to develop specialized functions and slowly lose their independence; eventually, the colony forms a multicellular organism. [3]The Volvocales are an order of green algae, which range from unicellular to multicellular; this wide range of cellularity make them ideal model organisms for studying the colonial hypothesis (see Links to Learning – How Did Multicellularity Evolve). The symbiotic hypothesis suggests that multicellularity arose due to aggregation of unicellular cells from different species who all play a distinctive role within the organism.
Link to Learning
Watch How Did Multicellularity Evolve? – Volvocales as model organism
Advantages to Multicellularity
Multicellularity increases an organism’s overall fitness by creating biological advantages. One of the noteworthy benefits is the division of labor through cell specialization. This specialization allows the organism to utilize environmental resources better (e.g., nutrients), enabling them to inhabit new ecosystems. Multicellularity grants the organism the ability to modify their size independently of cell size. This helps eliminate the size constraint associated with individual cells; therefore, the organism can develop new connections with the physical and biological environment. The increase in size also helps organisms avoid becoming a predator’s prey. Furthermore, the size increase allows predators to capture larger prey, which assists them in moving up the food chain.
- Grosberg, R.K., and Strathmann, R.R. 2007. The Evolution of Multicellularity: A Minor Major Transition? Annu. Rev. Ecol. Evol. Syst. 38(1): 621–654. doi:10.1146/annurev.ecolsys.36.102403.114735. ↵
- Grosberg, R.K., and Strathmann, R.R. 2007. The Evolution of Multicellularity: A Minor Major Transition? Annu. Rev. Ecol. Evol. Syst. 38(1): 621–654. doi:10.1146/annurev.ecolsys.36.102403.114735. ↵
- Jiménez-Marín, B., and Olson, B.J.S.C. 2022. The Curious Case of Multicellularity in the Volvocine Algae. Front Genet 13: 787665. doi:10.3389/fgene.2022.787665. ↵