Microorganisms are small, both by definition and mostly in reality as well. With their small size comes a relative lack of morphological complexity. Consistent with their small size and relative simplicity, the dominant lifestyle of microorganisms, especially for bacteria, is one of absorption. That is, carbon sources, chemical energy sources, organic along with inorganic enzyme cofactors, electron acceptor molecules, or light come into contact with cells, cross their plasma membranes, and are subsequently utilized or incorporated. There exist complications on this basic idea, but they are not extensive. One can imagine that the original cellular life forms followed a basically similar approach to nutrient acquisition. That is, the first organisms presumably were small and simple and relied on chance encounters with especially chemical resources to survive. Consistent with this view – that small and simple represents something of a default form among organisms – the predominance of genetic diversity among cellular organisms is found in small, simple, and especially unicellular species. In addition, simpler organisms pre-date more complex organisms in the fossil record. Given these observations, then clearly larger, more complex, and multicellular organisms must have evolved from smaller, simpler, and unicellular ancestors.

Also consistent with the above hypothesis that complex organisms, particularly those displaying true multicellularity, evolved from simpler, unicellular microorganisms, we can consider a variety of intermediate forms between the unicellular and the multicellular. These intermediate forms include bacteria which form cellular arrangements (e.g., staphylococcus), bacterial microcolonies, biofilms, microbial consortia, relatively complex colonial forms (e.g., various colonial eukaryotic algae), normally single-celled organisms that can also take on multicellular forms (i.e., dimorphic fungi), cells which aggregate to form differentiated colonies (i.e., the fruiting bodies of cellular slime molds or Myxococcus bacteria), and various bacteria that display seemingly differentiated cellular forms such as sheathed bacteria as well as diazotrophic cyanobacteria that form heterocysts. Notwithstanding these indications that even microorganisms can exist predominantly other than as unicellular organisms, important remaining questions are both how such forms can maintain a stable existence, that is in the face of a potential for cheating in their midst, as well as why they display other than small, unicellular forms. That is, what is the advantage of being larger or more complex and how are organisms able to be larger or more complex in order to obtain those advantages? An understanding of microbial evolution is helpful towards addressing both of these questions. In fact, molecular mechanisms aside, much of this evolution can be framed in terms of cooperation and defection within populations consisting of mostly clonally related individual cells.


Table: Important Terms and Concepts Concerning the Evolution of Greater Organism Size and Complexity.


Strict adherence to a developmental program despite variation in initial or ongoing conditions.
A very basic form of canalization during the development of organisms is germ line sequestration. Otherwise, what canalization most fundamentally can achieve is an impetus for the cells making up the bodies of multicellular organisms to display behaviors of cooperation rather than defection. Indeed, various policing mechanisms exist within bodies that serve to canalize development and indeed individual cells typically will display a self policing such that the potential to deviate from developmental pathways or differentiated functionality is relatively difficult, such as following mutation. The cells making up bodies thus tend to have redundant mechanisms that serve to canalize their development along with their subsequent functionality, and a basic consequence of these mechanisms is that development of body cells into unchecked cancers is relatively rare.
Cellular differentiation

Cells making up the same organism having dissimilar functions and appearances despite being genetically identical.
Not only is the differentiation of the cells making up an organism into phenotypically different types both useful and an advantage associated with multicellularity, cellular differentiation also is a key aspect of conflict mediation within certain multicellular organisms, particularly in the differentiation of reproductive cells from those cells responsible, essentially, for the support as well as ongoing survival of those reproductive cells.
Cheating Display of one or more defection behaviors especially within the context of unilateral defection.
Cheating is what ideally is curtailed in the course of a transfer of fitness such as from that of individual cells to that of a multicellular body. Cheating is the basic phenomenon that is occurring in the course of the development of tumors and cancers (that is, besides the various genetic, molecular, and physiological underpinnings of the transformation of normal body cells into cancerous cells).
Cheating load Extent to which defection negatively impacts those individuals that have been defected against.
In a sense, parasite load can be viewed as a subset of cheating load, with parasite load representing an aspect of defection 'from without'. Also relevant therefore, and contributing to cheating load, is defection 'from within'. As with cheating generally, within multicellular organisms it is especially cancers that can be viewed as an example of defection (or cheating) 'from within', with cancers if left untreated generating the familiar cheating load of body functional decline.

Possession by a single cell of multiple nuclei where especially an entire or at least large fraction of a whole organism consists of that single cell.
Coenocytic organisms include plasmodial slime molds, so-called coenocytic fungi, one of the life stages of the genus Plasmodium (which is the cause of malaria), and certain macroscopic algae. It represents a means by which organisms or simply individual cells can attain relatively large sizes while retaining intracellular transport mechanisms as a means of material movement rather than needing to switch to extracellular mechanisms of body or cellular coordination.

Single, moderately well-delineated entity possessing multiple cells but little or no cellular differentiation.
Colonies of cells consisting of a single genotype – that is, which are clonal – thus represent a form of transitional complexity, one that is found intermediate between that of unicellular and that of multicellular. Colonies themselves vary in terms of both size and degree of cellular differentiation. The algae known as volvox, for example, represent fairly substantial sophistication, for a colony (or, instead, fairly simple make up for a multicellular organism). Alternatively, the simplest of colonial forms consist of two or only a few cells in combination with what would appears to be a lack of cellular differentiation (e.g., a streptococcus). Note that colonies also can consist of multiple multicellular organism, though this is not the meaning that is being employed here, where colony instead is being used a shorthand for "Colony of cells".
Emergent property

Aspects of organisms that are derived from lower level aspects but in ways that are difficult to predict from knowledge of lower-level aspects alone.
Numerous aspects of multicellularity represent emergent properties, essentially new functions that are consequences of cells banding together into relatively complex forms. Importantly, these emergent properties can represent advantages associated with multicellularity, and also aspects that are difficult to predict either given study of, for example, individual cells in isolation, or instead in studying the behavior of unicellular organisms. Endosymbioses, too, possess emergent properties, as do many other systems as complexity increases. On the other hand, numerous trends can be seen as well where processes found at lower levels of complexity appear to be elaborated upon at higher levels, with perhaps only changes or improvements in these processes designated as emergent.
Extended phenotype

Genetically controlled aspects associated with organisms that are found literally outside of an organism's body.
The 'classic' example of an extended phenotype is the beaver's dam. Extracellular polymeric substances (i.e., EPS), as produced by microorganisms such as in the course of microcolony or biofilm formation, also represent a form of an extended phenotype. Generally extended phenotypes serve both as a means of increasing organism complexity as well as the size of an organism's impact on its environment. Extended phenotypes also can serve as public goods and thus can represent aspects of organisms that may be relatively easily exploited by cheaters. Extracellular enzymes, particularly those that are employed outside of an organism's body, such as by fungi or bacteria, serve as another example of an extended phenotype.
Free living

Especially organisms that do not serve as symbionts associated with other organisms.
Organisms that exist as symbionts and/or endosymbionts tend to have been derived (or are thought to have been derived) from free-living ancestors. One aspect of increasing organismal complexity thus tends to involve the conversion at some point in the past of free-living organisms into no longer free-living organisms. This occurs either through coopting of useful other organisms as symbionts, or as endosymbionts, or instead simply the coopting of what in principle otherwise could be free-living individual cells into a multicellular body.
Germ line

Cell lineages within multicellular organisms that directly give rise to the next generation of multicellular organisms.
In animals, germ line cells, a.k.a., germline, are those lineages that either undergo meiosis to generate gametes or undergo mitosis to generate those cells that undergo meiosis. Key to appreciation of the role of germ line cells in the evolution of multicellularity is that not all cells within a multicellular organism are necessarily germ line cells. Therefore, not all cells may be capable of generating or giving rise to the next generation of organisms. Indeed, a very basic aspect of cellular differentiation is distinction between germ line cells and what otherwise is known as soma, that is, the non-germ line cells that make up most of an organism's body. Note that though germ line cells are specialized in terms of their morphology and physiology, they nonetheless are not sequestered early on from the soma in all organisms.
Germ line sequestration


The act of cellular differentiation within multicellular organism into the lineage that is responsible for directly giving rise to organism reproduction such as towards differentiation into gametes.
Often germ line sequestration is a somewhat sophisticated process that can be seen as analogous to specialized processes that symbiotic organisms can go through to effect their transmission to new hosts. Antagonistic pleiotropy therefore can come into play, with cheater cells in principle potentially less able to undergo these differentiation processes and therefore potentially less able to give rise to a next generation of organisms. In addition, in various organisms, such as animals, germ line sequestration is initiated relatively early during organism development. As a consequence, soma is fully barred from giving rise to germ line cells, consequently fully barred from directly contributing to the next generation, and as a result unable to reproduce except indirectly by fostering the health, survival, and functionality of the germ line.
Group selection

Biased reproductive success of associations of individual organisms that may or may not counter biases in the reproductive success of the individual organisms themselves.
Group selection unquestionably operates for many species. Nevertheless, it can be questionable whether the strength of group selection is comparable to the strength of natural selection acting on individual organisms. When the strength of group selection is much stronger than that selection acting on the components making up a group, however, then one can say that a transfer of fitness has occurred, that is, where these group components are no longer necessarily the 'individual' but instead that the group has become the 'individual'. This is precisely what can happen given closely associated populations of clonally related cells such as can make up the bodies of multicellular organisms. Note, regardless, that for group selection to operate then groups must have both some potential to reproduce and some potential to compete intraspecifically with equivalent groups.
Individual Especially an organism that in some manner is not simply a portion of a larger organism.
What actually constitutes an individual can be difficult in many circumstances to say. One perspective is that a transfer of fitness involves the conversion of many otherwise somewhat associated individuals into a single, larger individual. For instance, the cells making up a multicellular organism are not individuals while the multicellular organism itself is. Cells making up a colony of cells, by contrast, may or may not represent individuals (and the colony itself, by the same token, may or may not represent an individual).
Microbial consortium

Intimate, mutualistic symbiosis between microorganisms.
Microbial consortia are one means by which living systems can increase their size and complexity, though this involves multiple interacting organisms rather than increases in the size and complexity of individual organisms. Microbial consortia, in other words, though certainly consisting of multiple cells, are not multicellular organisms nor strictly even colonial organisms. Microbial consortia instead can display a degree of complexity that is similar to that seen with endosymbiotic relationships, though not necessarily with equivalent degrees of physiological integration.
Multicellular organism


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Individual that displays cellular differentiation along with greater sophistication than a colony.
Multicellular organisms typically reproduce via a single-celled stage consisting of various combinations of gametes, zygotes, and/or spores. The resulting organism – disregarding normal microbiota – thus tends to consist of clonally related cells. Colonies too in many cases can be generated from a single-celled stage, however, and thus it is especially cellular differentiation, which is relatively lacking within colonies of cells, that differentiates multicellular organisms from merely colonial forms.
Multilevel selection

Biased reproductive success as considered in terms of a combination of components of individuals, individuals, and groups of individuals.
Thus, one can consider natural selection as operating at multiple levels of organization ranging from genes or groups of genes competing within cells (e.g., genetic parasites) to individual cells competing within multicellular organisms or colonies to individuals competing within populations, and also populations competing against one another and even communities competing against other communities. In terms of the evolution of multicellularity, natural selection without question will operate on individual cells in terms of their relative reproductive success. Cancers thus represent excessive reproductive success on the part of a particular clonal lineage within otherwise mutually cooperating cells making up bodies. Individual organisms themselves, as multicellular bodies, similarly will be subject to natural selection such as in terms of their competition with other individual organisms.
Mutual policing

Interference with defection behaviors by cheaters or would-be cheaters as mediated by non-specialized cells or individuals.
Mutual policing is mediated by ordinary individuals making up a population and is accomplished at some cost to those individuals. It serves as a means of increasing the potential for a population to display mutual cooperation. In a sense, mutual policing can be viewed as ordinary individuals ganging up on defectors or otherwise not tolerating defection in their midst.

Living entity that is capable of both sequestering resources and reproducing.
Individual cells making up multicellular organisms are not organisms unto themselves. The cells making up unicellular organisms, by contrast, are organisms. Individual cells making up a multicellular organism, while certainly in most cases capable of division, in particular are not necessarily capable of directly generating an entire organism. It is this low potential to directly generate individual organisms that results in these cells not representing individual organisms themselves. Specialized reproductive cells – including gametes, zygotes, or spores – by contrast can readily give rise to individual organisms and therefore, particularly in the case of zygotes and spores, can be viewed as immature organisms themselves.
Policing Various mechanisms of interference with defection behaviors by cheaters or would-be cheaters especially towards increasing a population's potential to display mutual cooperation.
Interference, or punishments, as mediated against such cheaters can include reciprocal defection (meaning defection in response to defect, i.e., Tit for Tat) as well as ostracism (as a special kind of reciprocation, that is, blocking an individual's ability to defect by eliminating an individual from one's midst). Alternatively, individuals can be blocked in some manner from being able to display potential defection behaviors at all, including by blocking entrance into locations in which defection is either possible or profitable. Spatial segregation of defectors from cooperators in combination with various forms of punishment or coercive blocking of defection behaviors thus represents much of the arsenal that may be employed to achieve mutual cooperation within groups, and a subset of these measures can be described as various forms of policing.
Quorum sensing

Chemical means of intercellular signaling as observed particularly within closely physically associated groups of bacteria.
Quorum sensing at a minimum can be viewed as a means by which bacterial populations can gauge their density within environments. More fundamentally, however, quorum sensing represents a form of communication among cells that allows groups of individuals to behave in a more coordinated and thereby cooperative manner. In addition, the chemicals involved in quorum sensing can be viewed as public goods with cheating thereby possible, such as in terms of making use of this public good but not contributing to its formation.
Self policing Interference with defection behaviors within a population as mediated by would-be cheaters themselves.
These are various internal blocks on cheating, such as the various genes carried by animal cells that serve to limit a cell's potential to become cancerous.
Transfer of fitness

Change of a primary unit of selection especially from a less inclusive smaller unit to a more inclusive larger unit
Transfer of fitness is seen for example in going from a collection of especially clonally related cells, where each cell represents a primary unit of selection (that is, primary meaning what unit that selection acts most strongly upon) to a cell group that comes to represent instead the primary unit of selection. This is a key change in going from a unicellular existence to one of multicellularity. In other words, natural selection primarily acts upon you, that is, your body, and less so on the individual cells that make up your body. Assuming that your ancestors were unicellular organisms, this means that at some point in the evolution of you (our lineage) there was a transfer of fitness from emphasis on individual cells to instead an emphasis on the collective fitness of multiple cells. This is accomplished in part by progeny organisms being generated solely by specialized cells rather than by just any cell making up a body.
Unit of selection

Entities possessing genetic variation in combination with differential reproductive success but not necessarily limited to individual organisms.
In the evolution of multicellularity a key concern is that individual bodies themselves consist of individuals that, though not organisms, nonetheless in the evolutionary past would have served as organisms unto themselves. Cells, that is, also can serve as units of selection, in addition to bodies as units of selection, and key to the evolution of multicellularity is what is known as a transfer of fitness from primarily being associated with individual cells to instead being associated with the groups of individual cells making up individual bodies.