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What is the meaning of multicellularity?

What is the meaning of multicellularity?



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I can't understand what multicellularity is. Wikipedia states that any organism having many cells is multicellular. By this definition bacteria can also be multicellular. For example, cyanobacteria can form filaments made up of specialised cells like spore cells and vegetative cells.

But aren't bacteria supposed to be unicellular only? I would appreciate it if someone could clear up my confusion.


Bacteria are, as a general rule, unicellular. However, there are some, like the cyanobacteria which you referenced, which are kind of border line. Wikipedia's definition of multicellularity is a very broad sweeping definition, and its statement that bacteria are unicellular is also a broad sweeping statement.

The fact is, that as a general rule, bacteria are unicellular, but at times, like you said, they can undergo filamentation. When they do, they end up with several connected cells which are generally considered functionally independent. In certain circumstances, however, the cells can be dependent on each other for survival, and because they didn't separate completely, it's hard to define whether we're looking at multicellularity or symbiosis.

It is generally accepted that cyaonbacteria are truly multicellular. In order to be multicellular, an organism generally has to fulfill 5 basic requirements:

  1. More than one cell
  2. Cells stick together
  3. Cells communicate
  4. Cells are dependent on each other
  5. Cells are differentiated

By these requirements, some cyaonbacteria technically are multicellular bacteria, defying frequent claims that all bacteria are unicellular. That said, wikipedia isn't totally wrong, since as a general rule, bacteria are unicellular. Cyanobacteria are an odd (debatably) exception to the rule.


Sources:

  • Bacteria with bodies - multicellular prokaryotes, Gould, S.E.
  • Multicellularity in Cyanobacteria, Paleontology Podcasts
  • Introduction to Cyaonbacteria, University of California Museum of Paleontology
  • Wikipedia: Multicellularity, Cyanobacteria, Filamentation

The Wikipedia article you mention actually gives cyanobacteria as an example of a multicellular organisms. So yes, there are some (primitive as they may be) multicellular species in the bacterial domain. This should not come as a surprise since a domain in biology is the highest taxonomic rank meaning that it encompasses a huge variety of different species. However, the misconception that all bacteria are single-celled is quite common because it is true for the vast majority of bacterial species.

Furthermore, it is quite common in evolution that similar traits evolve separately (for example flight evolved independently in bats and birds and the claw of the mole and the mole cricket look similar but evolved independently) this mechanism is called convergent evolution. In fact, multicellularity evolved independently at least 46 times in eukaryotes.


The Evolutionary Development of Multicellular Organisms

Multicellular organisms evolved. The first ones were likely in the form of sponges. Multicellularity led to the evolution of cell specializations that form tissues. Another major event was the evolution of sexual reproduction. The emergence of sex cells in the timeline provided a means for organisms to further diversify. Know more about these crucial events in geologic time in this tutorial.

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Geological Periods

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Genes are the blueprint of our bodies, a blueprint that creates a variety of proteins essential to any organism's survival. Find out in this lesson the various factors that affect growth. ..

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How did multicellular life evolve?

Cells of Dictyostelium purpureum, a common soil microbe, streaming to form a multicellular fruiting body. Credit: Natasha Mehdiabadi/Rice University

Scientists are discovering ways in which single cells might have evolved traits that entrenched them into group behavior, paving the way for multicellular life. These discoveries could shed light on how complex extraterrestrial life might evolve on alien worlds.

Researchers detailed these findings in the Oct. 24 issue of the journal Science.

The first known single-celled organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed. More complex forms of life took longer to evolve, with the first multicellular animals not appearing until about 600 million years ago.

The evolution of multicellular life from simpler, unicellular microbes was a pivotal moment in the history of biology on Earth and has drastically reshaped the planet's ecology. However, one mystery about multicellular organisms is why cells did not return back to single-celled life.

"Unicellularity is clearly successful—unicellular organisms are much more abundant than multicellular organisms, and have been around for at least an additional 2 billion years," said lead study author Eric Libby, a mathematical biologist at the Santa Fe Institute in New Mexico. "So what is the advantage to being multicellular and staying that way?"

The answer to this question is usually cooperation, as cells benefitted more from working together than they would from living alone. However, in scenarios of cooperation, there are constantly tempting opportunities "for cells to shirk their duties—that is, cheat," Libby said.

"As an example, consider an ant colony where only the queen is laying eggs and the workers, who cannot reproduce, must sacrifice themselves for the colony," Libby said. "What prevents the ant worker from leaving the colony and forming a new colony? Well, obviously the ant worker cannot reproduce, so it cannot start its own colony. But if it got a mutation that enabled it to do that, then this would be a real problem for the colony. This kind of struggle is prevalent in the evolution of multicellularity because the first multicellular organisms were only a mutation away from being strictly unicellular."

When social amoeba Dictyostelium discoideum starves, it forms a multicellular body. Credit: Scott Solomon

Experiments have shown that a group of microbes that secretes useful molecules that all members of the group can benefit from can grow faster than groups that do not. But within that group, freeloaders that do not expend resources or energy to secrete these molecules grow fastest of all. Another example of cells that grow in a way that harms other members of their groups are cancer cells, which are a potential problem for all multicellular organisms.

Indeed, many primitive multicellular organisms probably experienced both unicellular and multicellular states, providing opportunities to forego a group lifestyle. For example, the bacterium Pseudomonas fluorescens rapidly evolves to generate multicellular mats on surfaces to gain better access to oxygen. However, once a mat has formed, unicellular cheats have an incentive to not produce the glue responsible for mat formation, ultimately leading to the mat's destruction.

To solve the mystery of how multicellular life persisted, scientists are suggesting what they call "ratcheting mechanisms." Ratchets are devices that permit motion in just one direction. By analogy, ratcheting mechanisms are traits that provide benefits in a group context but are detrimental to loners, ultimately preventing a reversion to a single-celled state, said Libby and study co-author William Ratcliff at the Georgia Institute of Technology in Atlanta.

In general, the more a trait makes cells in a group mutually reliant, the more it serves as a ratchet. For instance, groups of cells may divide labor so that some cells grow one vital molecule while other cells grow a different essential compound, so these cells do better together than apart, an idea supported by recent experiments with bacteria.

Ratcheting can also explain the symbiosis between ancient microbes that led to symbionts living inside cells, such as the mitochondria and chloroplasts that respectively help their hosts make use of oxygen and sunlight. The single-celled organisms known as Paramecia do poorly when experimentally derived of photosynthetic symbionts, and in turn symbionts typically lose genes that are required for life outside their hosts.

These ratcheting mechanisms can lead to seemingly nonsensical results. For instance, apoptosis, or programmed cell death, is a process by which a cell essentially undergoes suicide. However, experiments show that higher rates of apoptosis can actually have benefits. In large clusters of yeast cells, apoptotic cells act like weak links whose death allows small clumps of yeast cells to break free and go on to spread elsewhere where they might have more room and nutrients to grow.

Groups of yeast cells. If key cells die a programmed death, these groups can separate. Credit: E. Libby et al., PLOS Computational Biology

"This advantage does not work for single cells, which meant that any cell that abandoned the group would suffer a disadvantage," Libby said. "This work shows that a cell living in a group can experience a fundamentally different environment than a cell living on its own. The environment can be so different that traits disastrous for a solitary organism, like increased rates of death, can become advantageous for cells in a group."

When it comes to what these findings mean in the search for alien life, Libby said this research suggests that extraterrestrial behavior might appear odd until one better understands that an organism may be a member of a group.

"Organisms in communities can adopt behaviors that would appear bizarre or counterintuitive without proper consideration of their communal context," Libby said. "It is essentially a reminder that a puzzle piece is a puzzle until you know how it fits into a larger context."

A fossil of a 600 million-year-old multicellular organism displays unexpected evidence of complexity. Credit: Virginia Tech

Libby and his colleagues plan to identify other ratcheting mechanisms.

"We also have some experiments in the works to calculate the stability provided by some possible ratcheting traits," Libby said.


Cells do not always part after mitosis, but sometimes stay together to form multicellular organisms. This increases their size, and hence provides a defense against predators. Unfortunately, it is not possible simply to increase the size of cell because the really big cell will have less surface (in relation to the volume), therefore it will have multiple difficulties with photosynthesis, respiration and other processes which relate with surface of cell. But many cells together will make surface big enough (Figure (PageIndex<1>)). Multicellular organism has two modes of growth: scaling the body and multiplying cells.

Figure (PageIndex<1>) Origin of multicellularity. It is not feasible just to enlarge cell, surface is too small. But if cells do not part after mitosis, they might form the body which is big enough to escape from predators. This also provide with new mode of growth and possibility of the division of labor (colored cells).

Multicellularity allows these cells also to divide the labor and cooperate. This is extremely important for the future evolution.

Cells in the multicellular body are not connected forever. Sometimes, one or few cells escape and start a new body. This body will be exact copy (clone) of the previous one (vegetative reproduction). It is also possible that when these &ldquoescaped cells&rdquo go the different route: they become &ldquosex delegates&rdquo, gametes. All gametes want syngamy, and these cells will search for the partner of the same species but with another genotype. In case of heterogamy and oogamy, it is easy to recognize because genders will provide a hint: male will search for the female. In case of isogamy, gametes search for the partner with different surface proteins. After they finally mate, a diploid cell (zygote) appears. Zygote may winter and then divide meiotically. This is the simplest life cycle of multicellular organism (Figure (PageIndex<2>)), quite similar to the cycle discussed above for unicellular organism.

Figure (PageIndex<2>) Most ancient life cycle of the multicellular organism. Zygote does not grow, it divides meiotically. Somatic (&ldquogrey&rdquo) cells are going to die, only germ cells transfer their DNA to future generations.

However, frequently zygote starts to grow and divide mitotically, making the diploid body. There are two reasons to make multicellular body out of zygote without meiosis: (a) because in can and (b) because diploid is better. &ldquoIt can&rdquo because zygote already contains DNA program about how to build multicellular body. Why diploid is better, explained in next section.

If multicellular organism consists of diploid cells ((2n)), we will use the neutral term diplont. Multicellular organisms with haploids cells ((n)) are haplonts.

&ldquoEscaped cells&rdquo, &ldquosex delegates&rdquo, or mother cells of gametes from the above is a first stage of the division of labor when cells are separating into two types, germ cells and somatic cells. Somatic cells are those which will eventually die, but germ cells are capable of giving offspring. Having germ cells is not absolutely necessary for multicellular organisms, but most of them have well separated germ lines. Thus, origin of death is directly connected with this separation: somatic cells are not needed for future generations. Unicellular organisms are potentially immortal, and same are cancer cells which also escape from organism (but they cannot make the new one).

Life cycle of multicellular organism could be described starting from haplont (Figure (PageIndex<3>)). When environment conditions are favorable, it has vegetative reproduction. One variant of vegetative reproduction is that cell (mitospore) separates itself from a haplont, then divides into more cells and becomes a new haplont. Sometimes, whole chunks are separated and grow into new haplonts. When conditions change, haplont may start the sexual reproduction: syngamy. In syngamy, one gamete separates from the haplont and unites with a gamete from another haplont. Together, gametes form a zygote. This zygote might go straight to meiosis (as it happens in unicellular eukaryotes) but more frequently, zygote will grow, divide mitotically and finally becomes a diplont. This diplont might be superficially almost identical to haplont but every cell of it contains diploid nucleus (every chromosome has a pair). Diplont (similarly to haplont) may reproduce itself vegetatively (make clones): cell separates itself from a diplont, then divides mitotically into more cells and becomes a new diplont.

The diplont is also capable for asexual reproduction: there could be a cell separates itself from a diplont and divides with meiosis creating four spores, each of them will grow into haplont.

Figure (PageIndex<3>) General life cycle. Haploid part is on the left, diploid on the right, syngamy on the top, meiosis on the bottom. "M" letter is used to label mitosis.

Sporic, Zygotic and Gametic Life Cycles

The life cycle described above is the sporic life cycle (Figure (PageIndex<4>)). Organisms with sporic life cycle have both diplont and haplont, equally or unequally developed.

Figure (PageIndex<4>) Sporic life cycle. Overview. Haploid part is on the left, diploid on the right, syngamy on the top, meiosis on the bottom.

In all, there are three types of life cycles: sporic, zygotic, which is the most similar to unicellular and most primitive and gametic, which is used by animals and a few protists (Figure (PageIndex<5>)). The zygotic life cycle starts with syngamy and goes to meiosis. It has no diplont. Gametic life cycle goes from meiosis to syngamy. It has no haplont.

Protists have all three types of life cycles whereas higher groups have only one. Animals exhibit gametic cycle, whereas plants(_2) retained the more primitive sporic cycle.

Evolution of Life Cycles

The most striking difference between unicellular and multicellular life cycles is that zygote of multicellular organism may start to make diploid body (diplont) which sometimes is visually almost identical to haplont. This is because in the evolutionary perspective, diplonts are &ldquobetter&rdquo than haplonts. Frequent situation of gene dominance allows only one variant (allele) of the gene to work, that may save organism from lethal mutations. An increased number of genes could help to make more proteins. A third reason is that diplonts&rsquo genomes are more diverse. One gene may be able to withstand one group of conditions, and the other variant may have a different set of possible conditions. Therefore, diplont is able to take advantage of the capabilities of both genetic variants.

Figure (PageIndex<5>) The evolution of life cycles (green arrows represent five evolutionary transitions) from unicellular zygotic to multicellular gametic through different variants of sporic cycles.

As a consequence, the evolution of life cycles goes from zygotic (similar to unicellular) to the sporic cycle (Figure (PageIndex<6>)), and then to the more and more expressed domination of diplont, and finally to the complete reduction of haplont, gametic life cycle. It is still an open question how zygotic protists evolved to the sporic side. Most probably, zygote (which is diploid by definition) did not want to divide meiotically. Instead, it grows (which is seen in some protists) and divides mitotically, giving birth to the diplont. This is how first sporic cycle started. The last step of this evolutionary chain was a complete reduction of haplont: after meiosis, spores were replaced with gametes which immediately go to syngamy.

Life Cycle of Vegetabilia

Ancestors of Vegetabilia (plants(_2)) were green algae with zygotic life cycle. It could be imagined that their zygote started to grow because these organisms inhabited shallow waters and want their spores to be distributed with a wind. One way for this to happen is to have the spores on the stalk of the plant. This is probably the reason

Figure (PageIndex<6>)The evolution of life cycles (green arrows represent five evolutionary transi- tions) from unicellular zygotic to multicellular gametic through different variants of sporic cycles.

of zygote growth: primordial diplonts of plants(_2) were simply sporangia, structures bearing spores. Then the benefits of diploid condition described above started to appear, and these primitive plants went onto the road of haplont reduction. However, some Vegetabilia (liverworts, mosses and hornworts), still have haplont domination. This is probably because their haplonts are poikilohydric (it is explained in next chapters), adaptation which is beneficial for small plants.

Life cycle of plants(_2) is sporic, but the science tradition uses plant-related names for the stages. The cycle (Figure (PageIndex<7>)) begins with a diplont called a sporophyte, which produces spores. Sporophyte bears a sporangium, inside which mother cell of spores uses meiosis to make spores. The spores germinate and grow into haplont called gametophyte. Gametophyte produces gametes, specifically a spermatozoa (or simply &ldquosperms&rdquo) and an oocyte (egg cell). These gametes are developed in special organs&mdashgametangia. Gametangium which contains male gametes (sperms) is called antheridium, and female gametangium is archegonium, the last normally contains only one egg cell (oocyte).

By syngamy (oogamy in this case), the two gametes form a zygote. Next, a young sporophyte grows on the gametophyte, and finally, the cycle starts again. Again, sporophyte of Vegetabilia starts its life as a parasite on gametophyte. Even flowering plants have this stage called embryo. Maybe, this is why the gametophyte of plants(_2) has never been reduced completely to transform their cycle into gametic. Even in most advanced plant lineages, their male (which makes only sperms) and female gametophytes have minimum 3 and 4 cells, respectively, but not 0!

Figure (PageIndex<7>) Life cycle of land plants. Red color is used for innovations, comparing with previous (general) life cycle scheme.


Biology in Life

My previous Pokémon post got be thinking philosophically about Pokémon. I jested last time about Combee being three different bees stuck together, when in reality one Combee is the set together because the two top bees are not sentient beings, and if you were to split them apart, you would not get three whole bees. Though this idea of Combee having three faces, yet only one brain is incredibly strange, I will dedicate that discussion topic to a future post, as it fascinates me.

But today, I want to discuss this strange set of Pokémon that are groups of Pokémon, something the writers did many times.

Dugtrio evolves from Diglett. But strangely, this is a case where all Dugtrio really is, is three Digletts stuck together. I quote, “a team of Diglett triplets….” though later on they mix it up slightly by saying, “Dugtrio are actually triplets, emerging from one body…” either way the fan art really did get interesting for this one.

Another fusion like Dugtrio, Magneton are just three Magnemite that are stuck together. Presumably, they are attached as a result of magnetic forces. Magneton does evolve, into Magnezone. Though this Pokemon looks very different, implying that each Magnemite then differentiates to serve different roles so that the final form can more efficiently get things done.

Metagross, though it is hard to see it, is actually 2 Metang stuck together, and Metang is actually 2 Beldum stuck together. So 1 metagross is actually 4 Beldum.

As the name suggests, this is a nose Pokemon. Probopass is the evolved form of Nosepass and is just a much larger amd mustachio-ed version of Nosepass. But, Probopass also has attached to it two Nosepass. These nosepass can actually detach and be live creatures. So the question really is, does probopass asexually reproduce by budding, thus once the Nosepass leave, another can be born in that spot, or are these Nosepass born by other means, and then just choose to attach themselves to a nearby Probopass upon evolution, much like Magneton and Dugtrio? To investigate this we could easily do some DNA testing of the Nosepass and Probopass to see if they are genetically identical.

I’m not sure if this one counts. Weezing is said to be two Koffing attached together, but their appearance does change a bit including size and coloration. Weezing is a really unsavory and useless Pokémon, so let’s just not discuss this further.

Sort of like Combee, Exeggcute is not 6 pokemon stuck together, but rather it is the initial Pokemon, and justconsists of 6 different heads that each express a different emotion. Strangely seeds are not attached in any way, so I wonder what would happen yet they separated…

Similar to Exeggcute, Klink is two faced objects stuck together, but it seems unlikely they are supposed to both be sentient. I’m not even really sure if the two bodies can be separated even. I don’t think this one really counts, but it sort of belongs in the Combee Exeggcute family.

The question I have about Dugtrio and Magneton is if they really are three Diglett stuck together or three magnemite stuck together, they should have then exactly three times the power of a single Diglett or a single magnemite. So for example, its hp, attack and special attack should all triple in the evolution if there are three working together. Actually, you would really expect all of the stats to go up with the exception of speed, which should stay the same or decrease. Yet for the most part, though they do gain some obvious strength and advantages, they are not 3 times as strong. That means this union has negative cooperativity, or in other words, by coming together they lose some powered potential power.


Contents

Because the first multicellular organisms would have lacked hard body parts, they are not well preserved in fossil records [1] . Until recently phylogenetic reconstruction has been through anatomical (particularly embryological) similarities. This is very inexact, as current multicellular organisms such as animals and plants are 500 million years removed from their single celled ancestors. This allows both divergent and convergent evolutionary processes a huge amount of time to mimic similarities and differences between groups of modern and ancestral species that don't actually exist. While modern phylogenetics uses more sophisticated techniques such as alloenzymes, satellite DNA and other molecular markers, they are still rather imprecise over such huge timescales. Nevertheless, it is hypothesized that the evolution of multicellularity in most, if not all, extant clades could have happened in one of three distinct ways:

Symbiotic Theory

This theory suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single celled organisms, each with different roles. Over time these organisms would become so dependent on each other they would not be able to survive independently, eventually leading to their genomes being incorporated into one, multicellular, organism. each respective organism would become a separate lineage of differentiated cells within the newly created species.

This kind of severely co-dependent symbiosis can be seen frequently, such as in the relationship between clown fish and Riterri sea anemones. In these cases it is extremely doubtful if either species would survive very long if the other became extinct. However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. Although such symbiosis is known to have occurred (e.g. mitochondria and chloroplasts in animal and plant cells - endosymbiosis) it has only happened extremely rarely and, even then, the genomes of the endosymbionts have retained an element of distinction, separately replicating their DNA during mitosis of the host species. For instance, the two or three symbiotic organisms forming the composite lichen, while dependent on each other for survival, have to separately reproduce and then re-form to create one individual organism once more.

Cellularisation (Syncytial) Theory

This theory states that a single unicellular organism could have developed internal membrane partitions around each of its nuclei. Many protists such as the ciliates or slime moulds can have several nuclei, lending support to this hypothesis. However, simple presence of multiple nuclei is not enough to support the theory. Multiple nuclei of ciliates are dissimilar and have clear differentiated functions: the macronucleus serves the organism's needs while the micronucleus is used for sexual-like reproduction with exchange of genetic material. Slime molds syncitia form from individual amoeboid cells, like syncitial tissues of some multicellular organisms, not the other way round. To be deemed valid, this theory needs a demonstrable example and mechanism of generation of a multicellular organism from a pre-existing syncytium.

The Colonial Theory

The third explanation of multicellularisation is the Colonial Theory which was proposed by Haeckel in 1874. The theory claims that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some, presumably land-evolved, multicellularity occurs by cells separating and then rejoining (e.g., cellular slime molds) whereas for the majority of multicellular types (those which evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division [2] . The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation [3]

The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently numerous times (in 16 different protoctistan phyla). For instance, Dictyostelium is an amoeba which groups together during times of food shortage, forming a colony that moves as one to a new location. Some of these amoeba then become slightly differentiated from each other. Other examples of colonial organisation in protozoa are Eudorina and Volvox (the latter of which consists of around 10,000 cells, only about 25-35 of which reproduce - 8 asexually and around 15-25 sexually). It can often be hard to tell, however, what is a colonial protist and what is a multicellular organism in its own right, as the two concepts are usually indistinguishable, and this problem plagues most hypotheses of how multicellularisation could have occurred.

However, most scientists accept that is by the Colonial theory that Multicellular organisms, from all phyla, evolved.


Multicellular organism

Our editors will review what you’ve submitted and determine whether to revise the article.

Multicellular organism, an organism composed of many cells, which are to varying degrees integrated and independent. The development of multicellular organisms is accompanied by cellular specialization and division of labour cells become efficient in one process and are dependent upon other cells for the necessities of life.

Specialization in single-celled organisms exists at the subcellular level i.e., the basic functions that are divided among the cells, tissues, and organs of the multicellular organism are collected within one cell. Unicellular organisms are sometimes grouped together and classified as the kingdom Protista. See protist.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn, Managing Editor, Reference Content.


Multicellularity, fitness decoupling, and the levels of selection

One of the major problems in evolutionary biology remains the study of earliest cooperative groups and their transition towards multicellular organisms. As one of the major transitions, the evolution of multicellularity provides one of the most fruitful research questions in biology to date. The basic problem is this: if the evolution of multicellularity involves the emergence of a higher level of selection then how is it that selection at the level of individual cells does not undermine any cooperation among cells? Okasha (2006) gives credit to Michod (1999) for stressing that:

“[M]ulticelled organisms did not come from nowhere, and a complete evolutionary theory must surely try to explain how they evolved, rather than just taking their existence for granted. So levels of selection other than that of the individual organism must have existed in the past, whether or not they still operate today.” (p. 17)

As creationists might put it “Explaining the evolution of certain traits is all well and fine, but if your theory cannot explain the origin of life [insert: major transitions] why should I accept it?” Certainly, the theory of evolution should still be widely accepted as a fact if its intended domain were limited to explaining particular evolutionary processes on levels of selection already persisting, i.e. evolutionary change over generations. But, evolutionary theorists are well-justified in aiming to achieve more. However, models already assuming a level in the ‘hierarchy of life’, the evolution of which we want to explain must conceptually prove insufficient for the purpose of explaining the evolution and persistence of new Darwinian individuals on a higher level. After all, one needs to explain the emergence of a mechanism of group reproduction, without such a mechanism already being present. As Libby and Rainey (2013) point out: “[v]ariation, heritability and reproduction are derived properties and their emergence at the group level requires an evolutionary explanation” (p. 2).

Building on the work of Buss (1987), Michod (1996, 1999) and Maynard Smith and Szathmary (1995) on the emergence of new units of selection, Paul Rainey was dissatisfied with debates on cooperation and cheater suppression dominating research on the major transitions. This led him, an experimental biologist, to engage in theorizing on the evolution of individuality, seeking contact with philosophers such as Kim Sterelny and Peter-Godfrey Smith. The result is a series of intra-disciplinary papers, ranging from philosophy of biology and theoretical biology to experimental biology, proposing a re-evaluation of the role cheats play in evolution (2003, 2007, 2010, 2013, 2014).

The major theme in Rainey’s work is a criticism of Michod (1996, 1999) and contemporary research on the evolution of multicellularity, for putting too much emphasis on cooperation and the suppression of cheats. Rainey argues that mere cooperation among cells, though a necessary condition, is simply not sufficient for a transition in Darwinian individuality and hence has been overemphasized. After all, there is more to the evolution of multicellularity than ‘solving’ an n-players prisoner’s dilemma or more accurately a public goods game. A group of cooperating individuals is still distinct from a group of cells constituting a new higher-level individual. As previously indicated, Rainey puts emphasis on at least two further conditions, beyond mere cooperation, which need to be instantiated for the emergence of individuality on the level of the multicellular organism: a mechanism of group reproduction and a mechanism to minimize the adverse effects of cheats, i.e. cancer control (2007, p. 616). The more or less simultaneous occurrence of these three conditions, Rainey suggests, is necessary for a major transition in individuality towards multicellularity to occur. I argue that without a mechanism of group reproduction it does not make sense to speak of a group organism at a higher level. Nevertheless, the importance of reproduction for natural selection has been treated at length (see Godfrey-Smith 2009 Libby and Rainey 2013). However, in order to grasp the importance of these two mechanisms and the role of multi-level selection in Rainey’s fertile work it shall prove useful to take a look at the group selection debate from which multi-level selection theory (MLS) was born, a debate that in fact continues to this day.

Once the idea that cancer is natural selection acting on the level of cells in a multicellular organism became widespread through conceptual arguments, largely driven by evolutionary game theory models, dispute has arisen over the degree to which selection at a lower level undermines [the conceptually possible] group selection acting at the higher level (see Frank et al. 1995, 2003 Axelrod et al. 2006). This is captured in the ongoing debate on the best explanatory account for the emergence and persistence of altruism and cooperation. Despite the now strong opposition to group selection acting on the level of multiple organisms, the evolution of multicellularity aims to explain selection starting to act on the level of multiple cells, which is essentially group selection lowered by one level. Multicellular organisms are groups of cells with fitness being best attributed to the whole organism rather than the individual cells. However, selection does not stop to act on the level of individual cells. Sometimes cells stop contributing to the good of the organism they ‘go rogue’ and become a tumour, potentially undermining the group organism. Humans are often taken as the paradigmatic example for a Darwinian individual, but as Godfrey-Smith (2009) emphasized, Darwinian properties come in degrees. Individuality is on a continuum, and the move on this continuum, from selection acting on the level of cells to selection acting on the level of groups, is according to Rainey and Kerr (2010) best described as fitness decoupling, i.e. fitness being prescribed not to the individual cells but independently to the multicellular organism, an idea first introduced by Michod (1996, 1999), see also Roze and Michod (2001). Given that Darwinian individuality always comes in degrees, after all, cancer is hardly avoidable it might be misleading to speak of selection acting on this or that level. A more appropriate way of interpreting fitness decoupling is to recognize a shift in causal strength between selection acting on the lower and higher level. It is here that multi-level selection becomes relevant when selection is acting on multiple levels.

How could multicellularity evolve and be sustained without group selection acting on the level of groups of cells when selection is acting on the level of individual cells? To cope with the problem of cheats, i.e. cancer in multicellular organisms, the opposition to group selection was in need of clarification. Otherwise, selection at the level of cells would have been an imminent threat potentially eliminating the property of individuality from multicellular organisms. These groups of cells, i.e. multicellular organisms, would lose the property of being distinguishable and taking part, independently of the fitness of their constitutive members, under natural selection. Clearly, many biologists argued that there is a difference between a multicellular organism and a herd of such individuals being subjected to natural selection. But with cancer, and Darwinian mechanisms acting on the levels of singular cells of multicellular organisms, the orthodox opposition to group selection was nevertheless, in need of re-evaluating their arguments.

Unlike group selection, selection acting on the level of the organism, was never in serious doubt. Reproduction on the level of groups of organisms was viewed as either marginal or completely reducible to individual reproduction. For a transition in Darwinian individuality from groups of co-operators to a genuinely new individual at a higher level, however, a mechanism of group reproduction becomes necessary. One of the most important additions made to biology was to bring clarity into the group-selection debate, by clarifying what group selection actually means. Thanks to Damuth and Heisler (1988), much conceptual confusion between multi-level selection and group selection was able to be resolved. They introduced the concept of multi-level selection [1], let us abbreviate with MLS1, and multi-level selection [2], MLS2 respectively, and a set of mathematical tools in order to partition the fitness effects of group and individual properties. Here, it is best to quote them directly:

“In multilevel selection [1]:In multilevel selection [2]:
(1) “Group selection” refers to the effects of group membership on individual fitness(1) “Group selection” refers to change in the frequencies of different kinds of groups
(2) Fitnesses are properties of individuals(2) Fitnesses are properties of groups
(3) Characters are values attributed to individuals (including both individual and contextual characters—see below)(3) Characters are values attributed to groups (including both aggregate and global characters—see below)
(4) Populations consist of individuals, organized into groups(4) Populations consist of groups, composed of individuals
(5) Explicit inferences can be made only about the changing proportions of different kinds of individuals in the whole population (the metapopulation)(5) Explicit inferences can be made only about the changing proportions of different kinds of groups in the population” (p. 410)

Though they omit discussion of heritability in groups, they grant that a complete account of multi-level selection would need to include such specifications. Rainey’s proposal as we shall see can be interpreted as a hypothesis capable of filling this gap. When we look at group selection taking place from an MLS1 point of view, mere cooperation between individuals would be sufficient—after all, group fitness is here simply defined as the average fitness of the group’s constituent cells. Footnote 8 Groups reaping the benefits of cooperation will do better than groups who fail to do so. However, from the MLS2 point of view, selection acts on genuinely new distinct units. Groups themselves are here Darwinian individuals themselves, taking part in Darwinian processes without having their fitness reducible to the fitness of their constituent members. Nevertheless, akin to matryoshka dolls lower level entities constitute the higher level entity with selection acting on both levels. This ambiguity lies at the heart of much of the dispute in the group selection debate and needs to be thoroughly understood in order to understand the major transitions (see Okasha 2006).

While Sober and Wilson (1998) defend group selection as a form of MLS1, Maynard Smith (1998) attacked their trait group model for not presenting a genuine case of group selection. In terms of Damuth and Heislers MLS account, Maynard Smith’s criticism concerns trait group model failing to satisfy the criteria of MLS2. Both sides of the debate were partly right. As already noted, philosophers should be able to ease conceptual confusions and make further theoretical work much more fruitful. Okasha’s (2006) Evolution and The Levels of Selection elegantly showed that much of the debate was ill-informed and could have been resolved if only there had been more clarity of what group selection is actually supposed to mean. As group selection has become an almost derogatory term for models, some proponents of group selection such as Wilson and Sober were led to use the term multi-level selection instead. In order to avoid confusion, Okasha (2006) proposes to refer to the group fitness at stake in group selection of the MLS1 type as fitness1 and group fitness of the MLS2 type as fitness2, a suggestion I follow here. Rainey’s critique of cooperation being insufficient, is merely him pointing out that researchers focusing on the evolution of individuality have focused too much on group selection at the MLS1 level, rather than the necessary transition to MLS2, where group fitness is not merely the mean fitness of the constituent cells. Both processes are at work when a transition in Darwinian individuality occurs. As Damuth and Heisler have illustrated, mere cooperation between group members is insufficient to transition from MLS1 to MLS2. Though game theory has been very successful in analysing MLS1 it might be insufficient for the analysis of fitness decoupling. This is not to say that an attempt to model Rainey’s hypothesis should not be undertaken, but that the orthodox tools for doing so may not be sufficient. Van Gestel and Tarnita (2017) criticize previous research into Darwinian individuality for mostly taking a “top-down” approach, i.e. looking at paradigm cases of Darwinian individuality and identifying a number of properties that are supposedly necessary for such a qualification, as being insufficient for understanding the major transitions. Instead they argue for a “bottom-up” approach in both theory and practice, which is nicely captured by the Hammerschmidt et al. (2014) experiment analysed in this paper. Footnote 9

When it comes to selection acting on multicellular organisms, Godfrey-Smith (2009) suggests a mechanistic account for Darwinian processes coming in degrees (Fig. 1). The graph above depicts a very useful illustration for theorizing about “bottom-up” approaches. His account posits three conditions that most groups of organisms do not satisfy for Darwinian mechanisms to fully act upon them: First, a bottleneck during which a propagule marks the beginning of a new life cycle. Second, a germ line being specialized for the reproduction of the collective. And third, the overall integration of the individuals forming a new individual rather than just a group. A single cell propagule marks the beginning of a new life cycle through a bottleneck and helps to easily demarcate individuals and their parent–offspring relationships (De Monte and Rainey 2014). But this criterion is not only useful for observing the variation from parent to offspring it is important precisely because the size of the bottleneck determines the causal strength natural selection can have. When biologists talk of multiple herds of deer and their swiftness, swiftness is usually thought of as an adaptation at the individual level and not the group (see Okasha 2006). However, when herds of deer go through very small bottlenecks, a case for group selection and collective fitness2 can be made much more easily, not only minimizing variance within the group but also accentuating variance between groups, further strengthening group selection. Hence, the causal strength of Darwinian processes comes once again in degrees. The same applies to the germ/soma distinction. The more cells are soma the more cells work for the survival and replication of the germ-line or rather the group. Hence, the higher the share of soma cells, the stronger the case to be made for these groups being individual units themselves. Overall integration entails the division of labour beyond the soma/germ-line distinction and “the maintenance of a boundary between a collective and what is outside it” (Godfrey-Smith 2009, p. 21). While his and Kerr’s (2010) proposal of a single cheating cell as a new propagule marking the beginning of a new life cycle already weakly satisfies the criteria of Godfrey-Smith, overall integration is still lacking. Their proposal of a mechanism for group reproduction and a mechanism to minimize the adverse effects of cheats seems even less restrictive than Godfrey-Smith’s account. That is because Rainey wants to capture the very beginning of MLS2 selection before the Lewontin conditions can even be applied to the higher level. Godfrey-Smith himself (2009) stresses the role of marginal cases that though not satisfying all criteria may nevertheless have natural selection acting upon them. And this is exactly what is needed to explain the major transitions in individuality.

From Groups to Individuals

This brings us back to the debate between Sober and Wilson (1998) and Maynard Smith (1998). Maynard Smith espoused Hamilton’s (1964) idea of kin selection and inclusive fitness as sufficient for explaining the ‘strange’ occurrences of altruism in groups of animals. If an organism acts altruistically and engages in costly behaviour towards its own fitness, but benefits others, the trait will spread as long as those others share the trait and the benefit to them outweighs the cost of the altruism. Without assuming asexual reproduction, we only need a sufficiently high degree of relatedness. Sober and Wilson, however, arguing for group selection argued that kin-selection is just one form of group selection generating a correlation between the trait in question and the probability of interaction between individuals possessing the same trait, as opposed to random encounters. Maynard Smith (1998) did not accept their version of ‘group selection’ as MLS2 and rightly so. Other possibilities would include the signalling of being a reciprocal altruist combined with higher interaction rates among altruists. Whether we accept it as ‘real’ group selection is more of a linguistic question rather than anything else. Unfortunately, considerable time and effort have been spent on this debate. Kin-selection and inclusive fitness, however, do not provide a genuine case of selection acting on groups that are individuals themselves. Footnote 10

De Monte and Rainey (2014) try to relax the Lewontin conditions in order to generate an even more minimalized account of evolution which can explain the evolution of individuality at the level of multicellular organisms. They take the argument of Sober and Wilson (1998) for the trait group model to show that such statistical bias can establish a genealogy already sufficient for Darwinian processes. Godfrey-Smith (2009), however, brought a halt to this line of argument for higher-level group selection by pointing out that such a correlation may happen in a mere two-dimensional spatial structure. The replacement of groups by neighbourhoods cannot satisfy the MLS2 criteria as acting on a higher level (pp. 117–118). Not only is it hard to distinguish groups in such a setting with borders being fluid but as Godfrey-Smith points out “there are as many groups as individuals” (2009, p. 118). Damuth and Heisler (1988) recognized as much, with MLS1 being “relevant to situations in which individual fitnesses are context-dependent, but there are no groups as such and individuals are more or less continuously distributed” (p. 411). What matters for cooperation among cells is then not as Sober and Wilson (1998) argued group selection, but rather any kind of correlated interaction (2009, p. 118). This correlation can be instantiated via appropriate ecological conditions that as we shall see play an important role in the formation of a group in the experimental work on Rainey’s hypothesis. Both kin selection and trait groups models are then subsumed under MLS1 and serve as appropriate tools for the evolution of cooperation, but as Rainey points out they are insufficient for explaining the transitions to MLS2.

Okasha (2006) formulates the question motivating the levels of selection debate as: “[W]hen is a character-fitness covariance indicative of direct selection at the level in question, and when is it a by-product of direct selection at another hierarchical level?” (p. 79). This is in effect a question of causal responsibility and not just a potential view one may or may not take for a specific research question in biology. For the pluralists, there is no such distinction possible, while the monist realist using the multi-level selection approach has a powerful tool at hand to settle questions of causal efficacy, while still having the benefit of different viewpoints. When it comes to explaining the major transitions in evolution, this realist picture may precisely be what is needed. Kin selection and inclusive fitness are not satisfactory for this purpose as they follow an MLS1 approach. There is just one major conceptual question left, and that is how we move from MLS1 and mere cooperation to MLS2 and having a genuine individual. Here the work of Michod (1996, 1999), Roze and Michod (2001) on a mechanism he termed fitness decoupling will be illuminating.

As already mentioned, fitness decoupling is the transition from MLS1 to MLS2. The fitness of the ‘group’ becomes independent of the average fitness of the constituent cells. It becomes a Darwinian individual, with multiple groups forming Darwinian populations themselves (Godfrey-Smith 2009). For Michod and Nedelcu (2003) conflict mediation between the higher level, i.e. the group, and the lower level, i.e. the cells, is the key to evolutionary transitions in individuality. The activities of the cells making up the organism affect the fitness2 of the whole rather than their own fitness. Footnote 11 But we need to remind ourselves that this is always a matter of degree, with cancer being a prevailing possibility. Michod proposes multiple ways in which such conflict mediation may occur. For the purpose of the Hammerschmidt et al. experiment, I will focus on the germ/soma distinction and the role of cheaters. Michod and Nedelcu (2003) argues for two crucial features a germ/soma distinction can accomplish: (1) reduced adverse effects of reproduction on the survival capacities of an organism and (2) “further increases in the survival component of group fitness” (p. 4). This division of labour opens the possibility of two distinct optimization problems to be solved, reproduction and survival. Once the ball gets rolling, the multicellular organism can become ever more specialized and integrated. This is what Rainey and Kerr (2010) argue for: cheats should be seen as a proto germ-line capable of saving the higher level entity rather than being their doom, a first step towards individuality.

Before the fitness at the multicellular organism-level is decoupled, adaptations at the group level are to be causally explained as a by-product of singular cell-level selection, i.e. MLS1 (Michod 1999). With a multi-level selection approach to the major transitions, a potentially large set of theoretical options that have been hitherto unexplored, such as a positive role for cheats, can come under serious scientific investigation. It is here that the environment will play an important role in enabling cheats to be interpreted differently. Paul Rainey refers to this special role of the environment making up for the lack of Darwinian properties as ecological scaffolding. Footnote 12 This new task of explaining the major transitions with seemingly non-Darwinian means then seems a far more adventurous enterprise than previous explanatory tasks of how already existing populations change over time with well-established methods readily available. Naturally, experiments are required in order to narrow this set of potential explanations down to the causal mechanisms behind fitness decoupling. As Godfrey-Smith (2009) stresses, Darwinian mechanisms can also act on marginal cases, and this is what the following experiment of Hammerschmidt et al. demonstrates: the fitness decoupling from MLS1 to MLS2, hence the evolution of individuality.


Origin of Multicellular Eukaryotes

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Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of AUEssays.com.

The beginning of multicellular Eukaryotes is one of the most important event in earth’s history regarding Eukaryotic life. As we can observe, most if not all animals, plants, and fungi are multicellular Eukaryotes therefore multicellularity is a key component of the early evolution regarding complex life. However, multicellularity nor Eukaryotic cell suddenly occur at once. In fact there is an order regarding the events that occurred and there are many theories as to how unicellularity evolved to multicellularity.

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To understand how multicellular Eukaryotes evolved, we first have to look at its ancestors. Multicellularity can be followed in this order unicellular ïƒ colonial ïƒ multicellular. The prefix “uni” in unicellular means one therefore unicellularity means “one cell”. Unicellular organism like Amoebas are made up of one cell, they are not dependent on any other cells and they reproduce asexually (Waggoner, 2001). Colonial organisms can be thought of as the connecting bridge between unicellularity and multicellularity. Colonial organisms such as Volvox are a collection of unicellular organisms coming together to form a structure that is beneficial to all of the cells. (Cooper GM, 2000) However if we remove one cell from this colony it is capable of fully surviving. This is the clear distinction between colonial and multicellular organisms. Lastly Multicellular organisms such as plants and animals contain many different specialized cells that work and communicate together within the organism and if we remove a cell that cell cannot survive on its own. (Cooper GM, 2000)

In addition, Eukaryotes have been thought to have evolved from Prokaryotes. The first Prokaryote fossils were discovered 3.8 billion years ago (Cooper GM, 2000) then the first Eukaryotic cells were thought to have evolved around 2.7 billion years ago in the Proterozoic time period and one billion years later multicellular Eukaryotes appeared. (Cooper GM, 2000) There are a few emerging theories as to how Prokaryotes evolved to Eukaryotes and most scientist agree it is through endosymbiosis. Endosymbiosis theory can be explained as one Prokaryotic cell such as bacteria engulfing another Prokaryotic cell and that cell escapes digestion and therefore lives inside the cell. Now this creates a symbiotic relationship where the engulfed cell provides energy and nutrients and in return the engulfed cell receives protection. (Waggoner, 2001)

The difference between Eukaryotes and Prokaryotes is that Eukaryotes have a nucleus, membrane bound organelles, linear DNA, and are typically multicellular. Conversely Prokaryotes lack a nucleus, organelles, linear DNA (have circular DNA), and are typically unicellular and smaller. (Waggoner, 2001)

After the events mentioned above, the next event that occurred was the change from unicellularity to multicellularity which is fundamental to all plants, animals, and fungi. The theories to how these unicellular cells evolved to multicellular cells is still debated but there are few emerging theories.

Theories are not the only thing that is being debated by scientists. The number of times multi cellularity has evolved is also debated based on the definition one uses. According to Karl Niklas (2014) a plant biologist from Cornell University If one defines multicellularity as an aggregation of cells “it can be said that it has been evolved at least in 25 lineages.” (Para.3) However if one defines it more strictly such as requiring the cells to communicate, cooperate and connect then it can be said to have “evolved once in animals, three times in fungi, six times in algae, and multiple times in bacteria.” (para.3) the reason for the variety of the number of times multicellularity has evolved is due to natural selection. The mechanism that was used to achieve multicellularity could have been in many ways but if the result is an evolved multicellular organism that functions better and has better fitness than the unicellular organism then this organism will be favored. In other words the traits are being naturally selected not the mechanism. (Niklas, 2014)

There is a certain criteria for multicellularity to evolve according to Karl Niklas. The first step for evolution of multicellularity is the cells must be genetically compatible to some extent. Secondly the group of cells that are formed must work together in a coordinated way to reproduce more cells until a distinct organism forms. The pathways of achieving multicellularity among the different kingdom differs which means the process of multicellularity is an example of convergent evolution. (Niklas, 2014)

The most plausible theory of the origins of multicellular Eukaryotes is the colonial theory which states that a unicellular organism of the same species comes together to form an aggregate and eventually a sphere which eventually folds to make tissue. (Niklas & Newman, 2013) In more detail, unicellular organisms of the same species exist and reproduce independently. Then they form an aggregate which is like a clump of unicellular organisms together because they benefit from coming together and cooperating. Then the cells line up in a more hollow shape with polarity on each side of a cell similar to the cell membrane in cells today. This helps keep the internal environments relatively stable. Lastly selective pressure leads to a separation and specialization of the somatic and germ cells in which through invagination of germ cells produce differentiated tissue. In fact this similar to the embryonic development of many multicellular organisms such as humans.

A second theory is called the cellularization theory which states the failure of cytokinesis following nuclear divisions is the cause of multicellularity. (Niklas & Newman, 2013) To be more specific cells exist and reproduce independently and then when they form an aggregate they form a syncytial (cell with multiple nuclei) because of incomplete cell division. Then after the Coenocyte, membranes form between the nuclei allowing one nucleus per cell and lastly these cells differentiate to form different types of cells such as germ cells. This is very similar to colonial theory except for the fact that it has a more complex transition in the beginning (single cell to multinucleate cell) compared to colonial theory and therefore it is less likely to have occurred. But this type of mechanism can be seen in algae, many fungi, and even fruit fly.

Viper is a quick and easy way to check your work for plagiarism. The online scanning system matches your work against over 5 Billion online sources within seconds.

Lastly the symbiotic theory states that different species of cells come together and work together because of mutual benefits from aggregation and evolve until they become codependent. (Waggoner, 2001) More specifically, cells from two different species first live and reproduce independently. Then they come in contact and they have a mutualistic relationship meaning they both benefit from being physically associated. After time they become codependent meaning they could not survive alone and at some point there is a fusion of the genomes which produces a new species and a new germ line. However the problem with this theory is there is no explanation of how the germ and somatic cells are produced nor any explanation of how fusion of two different genomes could occur without leaving any evidence. However the process of symbiosis of cells is observable in lichen.

Although the origin of multicellularity is still unknown, the advantages of multicellularity is the driving force behind the evolution of multicellularity. Multicellularity allows organism to exceed the size limits which helps defend against predators therefore increase survivability which would be favored. In addition, multicellularity increases the complexity of organisms by allowing specialization of cells which gives rise to other complex structures such as tissues and organs and it can help the organism keep a stable environment or homeostasis. (Waggoner, 2001)

Works cited:

Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates 2000. The Origin and Evolution of Cells. Web. 07 Apr. 2015.

Niklas K. J., Cobb E. D., Crawford D. R. (2013). The evo-devo of multinucleate cells, tissues, and organisms, and an alternative route to multicellularity.Evol. Dev.15466–474 10.1111/ede.12055

American Journal of Botany. (2014, January 25). From one cell to many: How did multicellularity evolve?.ScienceDaily. Retrieved April 06, 2015 from www.sciencedaily.com/releases/2014/01/140125172414.html

Niklas, K. J., & Newman, S. A. (2013). The origins of multicellular organisms.Evolution & Development,15(1), 41-52. doi:10.1111/ede.12013

The beginning of multicellular Eukaryotes is one of the most important event in earth’s history regarding Eukaryotic life. As we can observe, most if not all animals, plants, and fungi are multicellular Eukaryotes therefore multicellularity is a key component of the early evolution regarding complex life. However, multicellularity nor Eukaryotic cell suddenly occur at once. In fact there is an order regarding the events that occurred and there are many theories as to how unicellularity evolved to multicellularity.

To understand how multicellular Eukaryotes evolved, we first have to look at its ancestors. Multicellularity can be followed in this order unicellular ïƒ colonial ïƒ multicellular. The prefix “uni” in unicellular means one therefore unicellularity means “one cell”. Unicellular organism like Amoebas are made up of one cell, they are not dependent on any other cells and they reproduce asexually (Waggoner, 2001). Colonial organisms can be thought of as the connecting bridge between unicellularity and multicellularity. Colonial organisms such as Volvox are a collection of unicellular organisms coming together to form a structure that is beneficial to all of the cells. (Cooper GM, 2000) However if we remove one cell from this colony it is capable of fully surviving. This is the clear distinction between colonial and multicellular organisms. Lastly Multicellular organisms such as plants and animals contain many different specialized cells that work and communicate together within the organism and if we remove a cell that cell cannot survive on its own. (Cooper GM, 2000)

In addition, Eukaryotes have been thought to have evolved from Prokaryotes. The first Prokaryote fossils were discovered 3.8 billion years ago (Cooper GM, 2000) then the first Eukaryotic cells were thought to have evolved around 2.7 billion years ago in the Proterozoic time period and one billion years later multicellular Eukaryotes appeared. (Cooper GM, 2000) There are a few emerging theories as to how Prokaryotes evolved to Eukaryotes and most scientist agree it is through endosymbiosis. Endosymbiosis theory can be explained as one Prokaryotic cell such as bacteria engulfing another Prokaryotic cell and that cell escapes digestion and therefore lives inside the cell. Now this creates a symbiotic relationship where the engulfed cell provides energy and nutrients and in return the engulfed cell receives protection. (Waggoner, 2001)

The difference between Eukaryotes and Prokaryotes is that Eukaryotes have a nucleus, membrane bound organelles, linear DNA, and are typically multicellular. Conversely Prokaryotes lack a nucleus, organelles, linear DNA (have circular DNA), and are typically unicellular and smaller. (Waggoner, 2001)

After the events mentioned above, the next event that occurred was the change from unicellularity to multicellularity which is fundamental to all plants, animals, and fungi. The theories to how these unicellular cells evolved to multicellular cells is still debated but there are few emerging theories.

Theories are not the only thing that is being debated by scientists. The number of times multi cellularity has evolved is also debated based on the definition one uses. According to Karl Niklas (2014) a plant biologist from Cornell University If one defines multicellularity as an aggregation of cells “it can be said that it has been evolved at least in 25 lineages.” (Para.3) However if one defines it more strictly such as requiring the cells to communicate, cooperate and connect then it can be said to have “evolved once in animals, three times in fungi, six times in algae, and multiple times in bacteria.” (para.3) the reason for the variety of the number of times multicellularity has evolved is due to natural selection. The mechanism that was used to achieve multicellularity could have been in many ways but if the result is an evolved multicellular organism that functions better and has better fitness than the unicellular organism then this organism will be favored. In other words the traits are being naturally selected not the mechanism. (Niklas, 2014)

There is a certain criteria for multicellularity to evolve according to Karl Niklas. The first step for evolution of multicellularity is the cells must be genetically compatible to some extent. Secondly the group of cells that are formed must work together in a coordinated way to reproduce more cells until a distinct organism forms. The pathways of achieving multicellularity among the different kingdom differs which means the process of multicellularity is an example of convergent evolution. (Niklas, 2014)

The most plausible theory of the origins of multicellular Eukaryotes is the colonial theory which states that a unicellular organism of the same species comes together to form an aggregate and eventually a sphere which eventually folds to make tissue. (Niklas & Newman, 2013) In more detail, unicellular organisms of the same species exist and reproduce independently. Then they form an aggregate which is like a clump of unicellular organisms together because they benefit from coming together and cooperating. Then the cells line up in a more hollow shape with polarity on each side of a cell similar to the cell membrane in cells today. This helps keep the internal environments relatively stable. Lastly selective pressure leads to a separation and specialization of the somatic and germ cells in which through invagination of germ cells produce differentiated tissue. In fact this similar to the embryonic development of many multicellular organisms such as humans.

A second theory is called the cellularization theory which states the failure of cytokinesis following nuclear divisions is the cause of multicellularity. (Niklas & Newman, 2013) To be more specific cells exist and reproduce independently and then when they form an aggregate they form a syncytial (cell with multiple nuclei) because of incomplete cell division. Then after the Coenocyte, membranes form between the nuclei allowing one nucleus per cell and lastly these cells differentiate to form different types of cells such as germ cells. This is very similar to colonial theory except for the fact that it has a more complex transition in the beginning (single cell to multinucleate cell) compared to colonial theory and therefore it is less likely to have occurred. But this type of mechanism can be seen in algae, many fungi, and even fruit fly.

Lastly the symbiotic theory states that different species of cells come together and work together because of mutual benefits from aggregation and evolve until they become codependent. (Waggoner, 2001) More specifically, cells from two different species first live and reproduce independently. Then they come in contact and they have a mutualistic relationship meaning they both benefit from being physically associated. After time they become codependent meaning they could not survive alone and at some point there is a fusion of the genomes which produces a new species and a new germ line. However the problem with this theory is there is no explanation of how the germ and somatic cells are produced nor any explanation of how fusion of two different genomes could occur without leaving any evidence. However the process of symbiosis of cells is observable in lichen.

Although the origin of multicellularity is still unknown, the advantages of multicellularity is the driving force behind the evolution of multicellularity. Multicellularity allows organism to exceed the size limits which helps defend against predators therefore increase survivability which would be favored. In addition, multicellularity increases the complexity of organisms by allowing specialization of cells which gives rise to other complex structures such as tissues and organs and it can help the organism keep a stable environment or homeostasis. (Waggoner, 2001)

Works cited:

Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates 2000. The Origin and Evolution of Cells. Web. 07 Apr. 2015.

Niklas K. J., Cobb E. D., Crawford D. R. (2013). The evo-devo of multinucleate cells, tissues, and organisms, and an alternative route to multicellularity.Evol. Dev.15466–474 10.1111/ede.12055

American Journal of Botany. (2014, January 25). From one cell to many: How did multicellularity evolve?.ScienceDaily. Retrieved April 06, 2015 from www.sciencedaily.com/releases/2014/01/140125172414.html

Niklas, K. J., & Newman, S. A. (2013). The origins of multicellular organisms.Evolution & Development,15(1), 41-52. doi:10.1111/ede.12013


Rooted Bayesian consensus tree of 27 eubacterial species including five cyanobacterial species

Additional file 1: . Bayesian analysis of 16S rRNA gene sequences from 27 Eubacteria, based on GTR+I+G substitution model with an archaean outgroup. Posterior probabilities (black) and bootstrap values (red) from 100 re-samplings are displayed at the nodes. Cyanobacteria (blue-green box) are strongly supported as a monophyletic group with Gloeobacter violaceus being closest to other eubacterial species. (PDF 170 KB)

12862_2010_1652_MOESM2_ESM.txt

Additional file 2: Bayesian consensus trees of cyanobacterial subset and different outgroups - newick format. 22 Bayesian consensus trees with posterior probabilities of a cyanobacterial subset (58 taxa) and different eubacterial outgroups, displayed in newick format. Trees were run for 10,000,000 generations using a GTR+I+G substitution model with the first 3,000,000 generations being discarded as a burn-in. (TXT 44 KB)

Maximum likelihood tree of cyanobacterial subset

Additional file 3: . Maximum likelihood analysis of 16S rDNA sequences from 58 cyanobacteria, based on GTR+G+I substitution model, with Beggiatoa sp. as an outgroup. Posterior probabilities (> 0.9) in black and bootstrap values (> 50%) in red are shown at the nodes. Posterior probabilities were calculated from 265,858 trees inferred by Bayesian analysis. Bootstrap values were calculated from 500 re-samplings of the data set. Colors define groups: yellow are single-celled cyanobacteria of section I orange single-celled from section II green are multicellular, undifferentiated cyanobacteria from section III blue are multicellular and differentiated bacteria from section IV and pink from section V. Sections as described by Castenholz 2001 [9]. AC, B, C, E and E1 denote clades discussed in the text. (PDF 639 KB)

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Additional file 4: Results from the test of substitutional saturation. Substitutional saturation of the sequences was tested using DAMBE software. The index of substitutional saturation is smaller than the estimated critical value irrespective of the symmetry of the tree. The sequences are therefore not saturated. (PDF 34 KB)

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Additional file 5: Ancestral character state reconstruction using maximum parsimony. Summary of results over 10,000 randomly sampled trees from the Bayesian analysis. Uniquely best states were counted and are shown on the Bayesian consensus tree. Possible states are unicellular (yellow) and multicellular (black). At the nodes, probabilities for each character state are represented with a pie chart. The white part in the pie charts indicates fraction of trees where the node was absent, grey parts describe fraction of trees where both states were equally likely. Nodes where transitions occurred were labelled with an asterisk if they show strong support from the phylogenetic analyses. The maximum parsimony analysis produced a similar result compared to the maximum likelihood analysis. A unicellular ancestry for the most recent common ancestor of all cyanobacteria is supported. Nodes 3, 4 and 5 are most frequently optimized as multicellular. Multicellularity has been estimated for nodes 3 and 4 in 6800 trees and for node 5 in 6900 trees. In contrast, single celled states for these nodes have been reported, for node 3 in 13 out of 10,000 trees and for node 4 and 5 in 14 out of 10,000 trees. Five reversals to unicellularity can be detected and at least one reversal to multicellularity. (PDF 211 KB)

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Additional file 6: Phylogenetic tree of cyanobacteria - newick format. Phylogenetic tree of 1,254 cyanobacterial sequences including six chloroplasts and six Eubacteria analyzed using maximum likelihood analysis with a GTR+G+I estimated substitution model, conducted with the software RAxML. (TXT 108 KB)

Taxon names of the phylogenetic tree of cyanobacteria

Additional file 7: . Species names used in the phylogenetic analysis conducted with RAxML software. Taxon names are ordered by sub-groups as in Figure 1. (PDF 94 KB)