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Thread: Why animal-like multicellularity evolved only once

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    Why animal-like multicellularity evolved only once

    Origin of animal multicellularity: precursors, causes, consequences -- the choanoflagellate/sponge transition, neurogenesis and the Cambrian explosion | Philosophical Transactions of the Royal Society B: Biological Sciences by Thomas Cavalier-Smith
    Evolving multicellularity is easy, especially in phototrophs and osmotrophs whose multicells feed like unicells. Evolving animals was much harder and unique; probably only one pathway via benthic ‘zoophytes’ with pelagic ciliated larvae allowed trophic continuity from phagocytic protozoa to gut-endowed animals. Choanoflagellate protozoa produced sponges. Converting sponge flask cells mediating larval settling to synaptically controlled nematocysts arguably made Cnidaria.
    Phototrophs -- organisms that get energy by capturing light -- photosynthesizers -- "algae" and land plants.

    Multicellularity evolved several times among photosynthesizers -- green algae, red algae, kelp, and among prokaryotes, cyanobacteria ("blue-green algae").

    Osmotrophs -- organisms that absorb their food from their environment -- fungi in the traditional sense.

    Multicellularity evolved at least three times there, among the "true" fungi, the oomycetes, relatives of kelp, and among prokaryotes, the actinobacteria or actinomycetes. These organisms take the form of thin strands, though they can make fruiting bodies, like mushrooms. These structures make spores to disperse the organisms.

    There are also slime-mold-like multicellular organisms. These are only part-time multicellular, living much of the time as separate one-celled organisms. They get together only to make a fruiting body for dispersing themselves. That has evolved several times, not only in Amoebozoa (the slime molds proper), but also in Excavata (acrasids), Rhizaria (Guttulinopsis vulgaris), Alveolata (Sorogena stoianovitchae), Stramenopiles (Sorodiplophrys stercorea, related to kelp and oomycetes), Opisthokonta (Fonticula alba), and some prokaryotes, the myxobacteria.

    Aggregative Multicellularity Evolved Independently in the Eukaryotic Supergroup Rhizaria - ScienceDirect - the slime-mold habit evolved at least 7 times.

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    Multicellularity is sometimes reversed into a one-celled state, as with yeasts.

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    Animal-like multicellularity evolved only once, and TCS asks why that might be the case. That is because it involves a lot of cooperation between the organism's cells, cooperation involving mechanisms that require a lot of evolution.

    He proposes that that is because it evolved from convenient precursors. The closest relatives of the animals are some protists called choanoflagellates or collar flagellates. They use their flagella to move water past them, and they feed on bacteria and the like that get caught in their collars.

    Multicelled choanoflagellates can have their cells cooperate to make water currents past them, and this can be elaborated into what sea sponges have. These animals make water currents through their bodies, and their cells filter out food -- cells that look much like choanoflagellates.

    As part of their adaptation for their size, sponges developed oogamy, reproduction with egg and sperm cells. They release sperm cells that swim to egg cells, and when they meet, the fertilized egg cells develop into a small ball that uses its cells' flagella to swim away and find a home for itself.

    Also part of their adaptation was the development of an interior layer or mesenchyme, underneath their outer layer or epithelium. The interior-layer cells do construction and other such tasks.


    Most present-day sponges have a difficulty with further progress. They take in water over most of their bodies and release it in large openings. TCS's scenario requires a sponge that goes in reverse, taking water in its large openings and releasing it in the rest of its surface.

    But once a sponge can do that, it can eat relatively large prey, like sponge larvae. TCS proposes that this predatory sponge may then develop feeding tentacles for capturing prey. These tentacles require coordination, and that would be handled by some cells specializing into neurons (nerve cells). Eventually, one gets to where coelenterates (cnidarians and ctenophores) are at.


    This single evolution of animals has some interesting astrobiological consequences. It could be that animals are difficult to evolve, unlike plants, fungi, and slime molds. So there might be planets with big forests and lots of mushrooms, but no animals.

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    Plants can communicate, perhaps that ability to sense and convey information may evolve on plant only planets to the point of intelligence.

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    Quote Originally Posted by lpetrich View Post
    This single evolution of animals has some interesting astrobiological consequences. It could be that animals are difficult to evolve, unlike plants, fungi, and slime molds. So there might be planets with big forests and lots of mushrooms, but no animals.
    My assumption is that the evolution of animal species would be a function of the planet/atmosphere where the evolution is happening, and the length of time that the planet is stable. IOW, given another planet that's nearly identical to earth, and enough time, the probability of multicellular life appearing continually approaches 1.

    The earth is 4.5 billion years old, and it's estimated that multi-cellular organisms took billions of years to appear. I don't know that this means animal life is difficult but rather that it has a very specific probability given certain parameters. Compared to single-celled life that is obviously a lower probability, but calling it 'difficult' requires context. Difficult compared to what?

    To me it's more accurate to say that life itself, and particularly complex life is an exotic feature of the universe. But given the vastness of the universe it's likely that if we could model animal life with a function, and earth-like planets with a function, that life is relatively common, just not as ubiquitous as stars or planets.

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    rousseau, that is a completely separate issue. How much time is available for evolution and how fast it goes is a separate issue from what might be more evolvable than something else.

    The Evolution of Multicellularity: A Minor Major Transition? | Annual Review of Ecology, Evolution, and Systematics - PDF reprint

    Claims that multicellularity evolved at least 25 times. Though it notes a lot of references, it does not make an explicit count. I looked at some of the references, and I could not find any overall counts in them either. The phylogeny diagram in that paper shows 16 branches with at least some multicellular organisms. Some multicellularity may be shared among neighbors, like animal-choanoflagellate and land-plant-charophyte-alga multicellularity, and multicellularity may have evolved more than once in some branches, like the chlorophytes -- Volvox, Ulva (sea lettuce), etc.

    The Multiple Origins of Complex Multicellularity | Annual Review of Earth and Planetary Sciences - PDF reprint

    Author Andrew Knoll tries to explain how to distringuish between simple and complex multicellularity. In simple multicellularity, "... communication between cells and the transfer of resources from one cell to another is commonly limited." Also, "... essentially every cell in simple multicellular organisms lies in direct contact with the external environment, at least during phases of the life cycle characterized by nutrient acquisition and active metabolism."
    Complex multicellular organisms show not only evidence of cell-cell adhesion but also intercellular communication and, commonly, tissue differentiation mediated by networks of regulatory genes. Programmed cell death occurs in a number of these groups, but unprogrammed cell or tissue loss can be lethal—perhaps more so in metazoans than in other groups with persistent stem cells. Notably, complex multicellular organisms display a three-dimensional organization in which only some cells are in direct contact with the environment. This organization is critically important for organismic function because it introduces transport problems for oxygen, nutrients, and signaling molecules that are required by internal as well as external cells (Schlichting 2003, Beaumont 2009, Knoll & Hewitt 2011). As discussed below, complex multicellular organisms have evolved structures that circumvent the limitations of diffusion, including both molecular conduits for cell-cell communication and tissues that facilitate bulk transport. Indeed, the circumvention of diffusion can be considered a physiological key to the evolutionary success of complex multicellular life (Knoll & Hewitt 2011).
    By that definition, at least, complex multicellularity evolved only six times: animals, twice in fungi (ascomycetes, basidiomycetes), land plants, florideophycean red algae, and laminarialean brown algae (kelp).

    The score: 1 animal-like, 3 plant-like, and 2 fungus-like.

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    Yea fair enough. I suppose that compared to other types of multi-cellularity animal life could be termed difficult. I just prefer reframing words like that in terms of probability.

    IOW, given the right conditions animal life may be very likely, just not as likely as other types of multi-cellularity, or slower to appear.

    So rather than the subjective 'difficult' instead a specific probability given inputs. If we have [x] planets with [y] composition and [z] star then animal life will almost assuredly have appeared [c] times.

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    Quote Originally Posted by lpetrich View Post
    By that definition, at least, complex multicellularity evolved only six times: animals, twice in fungi (ascomycetes, basidiomycetes), land plants, florideophycean red algae, and laminarialean brown algae (kelp).
    In TCS's scenario, animals evolved from the Ediacaran fauna. How good is the case for that? And even if we can identify an Ediacaran sponge ancestor, how sure can we be that the Ediacaran fauna wasn't polyphyletic?

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    Quote Originally Posted by lpetrich View Post
    rousseau, that is a completely separate issue. How much time is available for evolution and how fast it goes is a separate issue from what might be more evolvable than something else.
    I don't think it's a separate issue. Lets say Earth is absolutely average. It took 2 billion years for multi-cellular life to evolve, so we can conclude the odds of it evolving in a given year are 1 in 4 billion. What are the odds it evolved again on Earth? About even. Not observing it thus conveys no useful information.

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