4.7: The appearance of multicellular organisms
- Page ID
- 181670
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)What drove the appearance of multicellular organisms? A number of theoretical and empirically supported and mechanistically plausible models have been proposed. Some suggest that predation was an important driver, either enabling the organisms to become better predators or to avoid predation. In an experimental study, when the small unicellular algae Chlorella vulgaris (5 to 6 μm in diameter) was grown together with a unicellular predator Ochromonas vallescia, which engulfs its prey. Over time Chlorella formed multicellular colonies that Ochromonas could not ingest.[199] At this point what we have is more like a colony of organisms than a colonial organism or a true multicellular organism.
The change from a multi-individual colony to a multicellular organism involves cellular specializations, so that different types of cells within the organism come to carry out different functions. The most dramatic specialization involves differentiations between somatic and germ line cells. At the other extreme, instead of producing distinct types of specialized cells to carry out distinct functions, a number of unicellular eukaryotes, such as the protist Paramecium (→), have developed complex cells that display specialized behaviors such as directed motility, predation, osmotic regulation, and digestion. Cellular specialization can be carried out even further in multicellular organisms. The stinging cells of jellyfish provide a classic example; highly specialized cells deliver poison to any organism that touches them through a harpoon-like mechanism (←). The structural specialization of these cells can make processes such as cell division impossible and typically a stinging cell dies after it discharges. Presumably, it is simpler to generate a new stinging cell than it is to reset a discharged cell. The production of these new cells involves both cell division and differentiation, which we will consider in more detail later on. While we are used to thinking about individual organisms, the same logic can apply to groups of organisms. The presence of cooperation can extend beyond a single species, leading to ecological interactions in which organisms work together to various degrees to achieve something that would be difficult or impossible to achieve on their own, while maintaining their ability to reproduce.
Based on the study of a range of organisms, much progress has been made in clarifying the origins of multicellular organisms. Such studies indicate that multicellularity has arisen independently in a number of eukaryotic lineages. This indicates that becoming multicellular is a successful way to establish an effective relationship with the environment.
Questions to answer
- What type(s) of mutation would enable an organism to escape a cell death module?
- Make a model for the process that could lead to the evolution of social interactions.
- What factors limit the complexity of a unicellular organism?
- Is the schooling or herd behavior seen in various types of animals a homologous or an analogous trait?
Questions to ponder:
- What strategies might be used to defend against the effects of cheaters in a population?
- Why is r (the relationship between organisms) never 0.
- What are some of the advantages of multicellularity? What are the drawbacks? Why aren’t all organisms unicellular or multicellular?