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Thread: Was it inevitable-ish that something like fungi would have evolved from multicellular life?

  1. #1 Was it inevitable-ish that something like fungi would have evolved from multicellular life? 
    Forum Freshman GreatBigBore's Avatar
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    I have no formal knowledge of anything, so apologies in advance if I'm totally not making sense, or making a bunch of technical errors.

    I'm taking a guess that once multicellular life got started, it was all but inevitable that something like plants would eventually appear, and something like animals would eventually appear. If that's complete BS, someone please tell me. But it seems that if there were no huge obstacle, it would just happen once multicellular organisms arrived. Same for animal-like things, I would think. I'm wondering whether something like fungi would be equally inevitable.


    The most exciting phrase to hear in science is not “Eureka” but “That’s funny...” -- Isaac Asimov
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    New Member Vmedvil's Avatar
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    No, evolution is based on the effect the environment has on things, if the environment had been different then the living system would have evolved differently, The Environment at that time lent itself toward multi-cellular life thus multi-cellular life is what we saw evolve.


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    Forum Freshman NoCoPilot's Avatar
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    If you can find it, the book "Rare Earth: Why Complex Life is Uncommon in the Universe" by Peter Ward and Donald Brownlee is exactly on this subject. They present a lot of evidence that life was perfectly happy to muddle along at the bacterial level for 2.9 of the 3.9 billion years it has existed on Earth. Evolution is not inevitable.
    Last edited by NoCoPilot; August 6th, 2018 at 06:58 PM.
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    Vmedvil makes a salient point. And it is one that I am constantly surprised that so few seem to understand. Plants and animals do not seek change; they do not pursue evolution; it is forced on them by the changes in the environment.

    NoCoPilot is correct in that evolution is not inevitable. Without environmental changes, it would most likely not occur. But when the environment changes significantly, then the existing life forms will either adapt to the changes and evolve or they will likely become extinct.

    The book that NoCoPilot referenced is available online as a pdf file here
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    Forum Freshman NoCoPilot's Avatar
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    Quote Originally Posted by GreatBigBore View Post
    I'm taking a guess that once multicellular life got started, it was all but inevitable that something like plants would eventually appear, and something like animals would eventually appear. If that's complete BS, someone please tell me. But it seems that if there were no huge obstacle, it would just happen once multicellular organisms arrived. Same for animal-like things, I would think.
    And let's not minimize the leap from primitive prokaryotes -- such as thermophiles that exist in undersea thermal vents -- to your "multicellular life." That leap is HUGE, and not particularly well understood. Multicellular life is not a starting point; it's a way station very far down the line in the evolution of life.

    The beginning, the leap from non-life to primitive prokaryotes (so-called archaebacteria), is also totally a mystery, to be fair.
    Last edited by NoCoPilot; August 18th, 2018 at 12:19 AM.
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    You used the word "leap" three times in describing changes that, as you say, are mysteries.
    "Leap" describes a single major change. I would suggest that, rather than leaps, what transpired was a countless series if very small changes. Baby Steps, if you will.
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    Forum Freshman NoCoPilot's Avatar
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    Very true, Smite. The progression from free elements to organic compounds to enzymes to amino acids to coacervates to proteins to AEG molecules to viruses to bacteria to archaeans to prokaryotes to eukaryotes to multi-celled organisms (I might not have that chain exactly right -- and one does not follow the other in this "progression" -- but hopefully you get the idea) is a stepwise link up in complexity.

    However, there are distinct discontinuities within the progression that I don't think it is an exaggeration to call "leaps."


    • The first RNA?
      Perhaps the strongest evidence for the RNA World Hypothesis is the fact that the ribosome, a large molecular complex that assembles proteins, is a ribozyme. Although the ribosome is made up of both RNA and protein components, structural and biochemical analyses revealed that the mechanisms central for translation (the process of assembling a peptide chain based on a RNA sequence) is catalyzed by RNA, not protein. This suggests that the use of RNA by early lifeforms to carry out chemical reactions preceded the use of proteins. Source
    • The first cell membrane?
      Amphiphilic compounds spontaneously self-assemble into more complex structures such as bimolecular layers, which in turn form closed membranous vesicles. The first forms of cellular life required self-assembled membranes that were likely to have been produced from amphiphilic compounds on the prebiotic Earth. Source
    • The first eukaryote with chloroplasts to allow photosynthesis?
      Overwhelming evidence indicates that eukaryotic photosynthesis originated from endosymbiosis of cyanobacterial-like organisms, which ultimately became chloroplasts (Margulis, 1992). So the evolutionary origin of photosynthesis is to be found in the bacterial domain. Source
    • The first multi-cellular organisms with shared DNA?
      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. Source

    There may well be intermediate steps between having RNA and not having RNA. Between having a cell membrane and not having a cell membrane. Between photosynthesizing and not photosynthesizing. Between sponges and babies. But these intermediate steps have not survived the fossil record and we don't understand them.

    The origin of life is hairy stuff. There's no way I'd claim to understand it. But I think the fact that the efforts of science, going back to Miller and Urey in 1953, to create life in the laboratory under ideal conditions have so far proven unsuccessful, justify the use of the word "leap" to describe processes we cannot yet duplicate.
    Last edited by NoCoPilot; August 28th, 2018 at 04:24 PM.
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    Forum Freshman NoCoPilot's Avatar
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    Incidentally, here's a summary of "Rare Earth" I wrote for myself as I read it, to try to understand the concepts being presented. The last third deals with the emergence of evolution, and why. Oops, too long. Will break in half.
    Quote Originally Posted by NoCoPilot

    • 13.799 (± 0.021) billion years ago (bya) - Big Bang. Universe starts (Thursday, 4:00 pm sharp).
    • 13.8 to 4.6 bya - Early universe is dominated by chaos. Stars come and go, entirely hydrogen and helium fusion. Only late in the game are enough heavy elements created for planets to start forming. Many collisions and annihilations keep things popping.
    • 4.54 (± 0.05) bya - Our solar system coalesces out of a giant molecular cloud. Our system is unusual in its prevalence of heavy elements and the size of our sun (95% of all stars are smaller). Our system is between arms of a spiral galaxy, out near the tips, in an area of low density so we experience fewer collisions and annihilations than normal.
    • The rocky inner planets of our solar system grow by accretion, sweeping up dust and debris in their path. The outer gas planets grow much slower from lighter elements farther from the sun, and end up far larger because more gravity is required to hold gasses together. The gas giants together account for 99% of all the mass circling the sun -- but the sun itself contains 99.8% of all mass in the solar system.
    • Earth's distance from the sun is unique in our solar system (the so-called "habitable zone") -- far enough from the sun to allow nitrogen and carbon and water to occur (the basic elements of life), but not so far that these elements are gaseous (inside Earth's orbit) or permanently bound up in the composition of the planet (beyond Earth's orbit, before the gas giants). By some estimates, this delicate balance has only a 0.0001% variance possible. Venus and Mars show what happens outside the HZ.
    • 4.6 to 3.9 bya - Earth is constantly bombarded by debris and planetoids as it sweeps the orbit. The surface remains molten and is reshaped many times. One of these Mars-sized impacts ejects the Moon (4.51 bya) and gives us our spin and polar tilt.
    • Our Moon is uniquely large (in our solar system) in comparison to the planet it orbits, 1.2% of Earth’s mass. This causes tidal effects unique to Earth, which are important to geothermal activity. The cratered surface of the Moon shows what the period 4.51 to 3.9 bya was like.
    • 3.9 bya - the path that Earth travels has basically been cleared (other planets too, so they stop hurling stuff in our direction). The surface begins to cool, and Earth separates into a molten core, a mantle, and a rocky crust.
    • Oldest known rocks on Earth, 4.031 (± 0.003) bya
    • The Earth's super-heated atmosphere at this point is mostly carbon dioxide, with some water vapor, ammonia and methane. As the Earth cools, the water vapor gradually condenses and falls from the sky. The Earth becomes totally covered in a sea that is 4,000 meters deep.
    • Due to the unusual composition of the Earth, including metals like iron and nickel and even heavier elements like uranium, thorium, platinum, iridium, and osmium, the core remains hot from radioactive decay. This allows volcanism and plate tectonics which continue to this day. No other planet in the solar system has plate tectonics.
    • Plate tectonics causes the crust to become uneven, leading to deep open trenches and ever-moving continents which rise above the surface of the water. Dry land appears. Rifts appear undersea where the plates are pulling apart, causing undersea magma and superheated gas vents.
    • The earliest forms of life (prox. 3.8 bya) are PROBABLY extremophiles (archaeans that use hydrogen sulfide, methane and carbon dioxide and the extreme heat of undersea geothermal vents in the ocean trenches to grow bacteria-like non-cellular structures). These are "alive" in the sense that they accrete and reproduce, but they are not mobile, do not evolve (still exist unchanged today), and have no functions other than to exist. They're more a chemical process than a living being. If life exists elsewhere in the universe -- given all the unlikely requirements listed above -- this is the most likely extent of it.
    • Being in the deep ocean protects these bacteria from the destructive forces on the Earth’s surface like impacts and ultraviolet radiation
    • ”Below the photic zone—the sunlit, upper reaches of the ocean—many microbes have evolved chemosynthetic (instead of photosynthetic) processes that create organic matter by using oxygen in seawater to oxidize hydrogen sulfide, methane, and other chemicals present in vent and seep fluids.”
    • ”Major types of bacteria that live near these vents are mesophilic sulfur bacteria. These bacteria are able to achieve high biomass densities due to their unique physiological adaptations. For example, Beggiatoa spp. is able to carry an internal store of nitrate as an electron acceptor that helps with the harvesting of free sulfide in the upper sediment region of the vents.”
    • In this era there was no oxygen in the atmosphere (or sea), so ultraviolet radiation from the sun would’ve prevented any emergence of life on land.
    • 3.8 to 3.5 bya -- through a process of evolution which is described in the book but too complicated for me to understand -- in a relatively short span of time (300 million years) these self-reproducing extremophile bacteria evolved into stromatolites (such as have been found in the Warrawoona Series in Australia). These microbial mats are the first fossilized evidence of life, sandwiched between lime deposits. “They have been found on every continent in rocks half a billion years old and older. Today, they are found only in one type of environment on Earth, in quiet, briny tropical waters.” Everywhere else they would be eaten today.
    • Cyanobacteria, the so-called "blue-green algae mats" (although in today's usage "algae" refers to eurkaryotes, not prokaryotes like the first cyanobacteria), evolved next. These photosynthesize, so they must reside on the surface of the seas (the details here are contradictory in different sources online). They metabolize water, hydrogen sulfide and carbon dioxide and produce oxygen as waste. Between 2.7 bya when they took over the seas and today (they still exist) they fundamentally changed the chemistry of Earth by adding oxygen, a poisonous gas which causes all sorts of chemical compounds to break down (oxidize).
    • Between 3.2 and 1.4 bya another microfossil, called acritarches, are also found. Even after reading about then I'm confused what they were.
    • A few times in its history Earth froze over completely: once 2.5 bya and again between 800-600 mya. The first of these may have been triggered by the cyanobacteria outgassing oxygen, and subsequent loss of carbon dioxide in the atmosphere.
    • With the oceans covered over with up to 1500 meters of ice (for as long as 30 million years, 2.5 to 2.47 bya), the oceans and the atmosphere became decoupled for the first time in Earth's history.
    • With the oceans and atmosphere no longer exchanging gasses and minerals, the atmospheric carbon dioxide begins to climb again from volcanic activity. Meanwhile dissolved metals and oxygen in the oceans also climbed under the ice lid from undersea outgassing.
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    Summary of "Rare Earth", part 2:
    Quote Originally Posted by NoCoPilot

    • Eventually (~2.47 bya) the atmospheric CO2 triggers a greenhouse effect, raising Earth's temperature , which melts the ice covering the planet, which releases all the built-up oxygen and minerals from the sea.
    • At 2.45 bya, with the sudden rise in Earth's temperature, the release of iron and manganese and oxygen from the oceans, there was a phytoplankton bloom. Cyanobacteria releases more oxygen into the atmosphere, which kills prokaryotes not used to it but creates an ecological niche for new life forms (like eurkaryotes) who can use oxygen to metabolize. The old order -- which has endured virtually unchanged on Earth from 3.8 bya to 2.5 bya -- is swept away and a new diversity of oxygen-utilizing organisms evolve (2.5 bya to ~800 mya). Little direct fossil evidence exists however, because these soft-bodied organisms left no trace.
    • This sudden increase in oxygen (called the Great Oxygenation Event, 2.45 bya), first in the oceans (causing most of the dissolved iron to oxidize and drop out of solution as rust), and then, as the oceans reach oxygen saturation, in the atmosphere, where it provides the mechanism for the metabolism of terrestrial minerals (as oxides) leading to ways for plants and animals to consume their environment.
    • This newly-oxygenated atmosphere for the first time protects the surface of the Earth from solar ultraviolet radiation as well as cosmic rays. Without this protection anything assembled on land would be quickly split apart again.
    • Under this new protective canopy, and with the sudden abundance of metals and oxygen, and sudden return to warmth, the era between 2.45 bya and 1.2 bya sees an explosion of life. Simple self-replicating bacterial mats and virus-like proto-living arrangements of amino acids and proteins, evolve into prokaryotes (single cells with no nucleus and a single strand of DNA) and then to eurkaryotes (single cells with a nucleus containing multiple strands of DNA, and mitochondria to supply energy, and chloroblasts to photosynthesize, and other organelles). This 'creation of life' is the least understood stage, because nobody has yet figured out how to get from amino acids to RNA. But once the ball starts rolling the diversification expands exponentially.
    • It is assumed eurkaryotes evolved from prokaryotes "eating" other prokaryotes and finding that having cells of differing specialties living inside you can be advantageous.
    • The date of the first appearance of eurkaryotes and subsequent multi-celled life is still contentious (no fossils exist of course). The oldest estimates are 1.2 bya, youngest 550 mya, median somewhere around 750 to 800 mya.
    • Sexual reproduction -- reproduction by combining two cells rather than simply dividing -- is dated to around 1 bya. This sped up the pace of evolution by some order of magnitude.
    • Also during 2.5 to 1.2 bya, loads of comets and meteors may have been impacting the Earth, bringing water and amino acids from outer space. Something in this barrage MAY have triggered RNA, although nobody outside of science fiction can say what.
    • Between 2.7 and 1.25 bya stromatolites dominate the fossil record, but beginning about 1.25 bya -- which is both the end of the bombardment of Earth's surface, and the beginning of eurokaryotic and multi-celled life -- they begin to disappear. They apparently start getting eaten. Today they are rare.
    • Between 800-600 mya several more ice ages occur, as the atmosphere bounces back and forth between too much CO2 and too much oxygen. Each time life is winnowed, then the survivors are boosted with a fresh supply of raw materials, and eventually the number of oxygen-breathers reaches a stasis point, where they begin to control the planet and prevent these "Snowball Earth" cycles.
    • When the last ice age recedes about 600 mya, the result is the so-called Cambrian Explosion usually dated to 541 mya. All kinds of body styles evolve (according to Stephen Jay Gould's "Wonderful Life", many more than eventually survive. As many as 100 phylum were created. Today 28-35 survive). This explosion in diversity is due to the evolution of expression genes (according to Sean Carroll's "Endless Forms Most Beautiful"), where small changes in the timing of development can affect the number and size and placement of limbs, the overall size of organisms, and a myriad other variations. Life explodes in diversity -- and consequently habitats.
    • 580 to 550 mya - The first to appear are the ediacarans, leaf-like primitive multicellular organisms which could be either colonies of individual cells (like sponges) or simple animals (like jellyfish) or even plants (or something in between, or something totally unknown today). They left impressions in the ocean floor, probably because they fed on the algae mats at the bottom of the sea and got buried by sand-falls. Their numbers after 550 mya dropped precipitously; probably something evolved to feed on them.
    • 550 to 500 mya - "trace" fossils and SSFs dominate. Trace fossils left no body parts, but left long trackways or feeding patterns in ocean sediments. Unknown what they were exactly, but they were mobile (unlike ediacarans) and they fed on other organisms. SSFs are small shelly fossils, under 1/2", and so pulverized as to be unreconstructable. They might be the trace fossils source.
    • 530 to 500 mya - The age of trilobites, brachiopods, mollusks and echinoderms. The first bony parts to be fossilized. Trilobites range from microscopic to over three feet long. During this 30 million years the variety and quantity of life on earth is unprecedented, even today.
    • Not a single new phylum has emerged since the Cambrian Explosion. The last 550 MY has been a period of whittling down, of slow extinction.
    • In addition to the evolution of sexual reproduction, another driver for the Cambrian Explosion was undoubtedly the evolution of predation. Rather than having to make their own living, larger and more sophisticated organisms could take advantage of another creature's success by devouring them. This led to an arms race / size race / teeth / claws / shells.
    • Also, the Cambrian Explosion marks the first appearance of shells and skeletons. Available free calcium in the environment was one factor. The sudden appearance in the fossil record of bones and shells probably makes this period look like more of an anomaly than it was.
    • Another factor in the Cambrian Explosion was plate tectonics. The continents migrated from the poles to the equator.
    • "On Earth there have been about 15 [mass extinctions] during the last 500 million years, 5 of which eliminated more than half of all species then inhabiting our planet." Ordovician–Silurian extinction events (450-440 mya), Late Devonian extinction (375-360 mya), Permian–Triassic extinction event (252 mya), Triassic–Jurassic extinction event (201 mya), Cretaceous–Paleogene extinction event (65 mya), Holocene extinction (10,000 ya to present).
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  11. #10  
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    Quote Originally Posted by NoCoPilot View Post

    However, there are distinct discontinuities within the progression that I don't think it is an exaggeration to call "leaps."
    ...
    There may well be intermediate steps between having RNA and not having RNA. Between having a cell membrane and not having a cell membrane. Between photosynthesizing and not photosynthesizing. Between sponges and babies. But these intermediate steps have not survived the fossil record and we don't understand them.
    So you are saying that by "leap," you mean gaps in our knowledge. I'll agree with that.
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