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