Twelve reasons we haven't found aliens

Discussion in 'Astronomy, Exobiology, & Cosmology' started by wegs, Feb 5, 2020.

  1. DaveC426913 Valued Senior Member

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    Some predators on Earth have them proximal. Take the octopus.

    The octopus has it eyes on the outside, with its mouth on the inside.
    It can't see its own mouth, but it can touch it.

    In a water environment, gravity is not necessarily a strongly defining factor.

    Octopi are self-propelled and voracious predators.
     
    Last edited: Feb 7, 2020
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  3. DaveC426913 Valued Senior Member

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    Unless you're claiming it makes only as much evolutionary sense as other plans - then showing counter-examples right here on Earth (nevermind an alien evolution) does refute the general notion that bilateral plan is superior or "makes the most sense".

    While true, you did don't use the word superior specifically, you are frequently using words that suggest bilateralism is the more likely to succeed, especially for predators.
     
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  5. James R Just this guy, you know? Staff Member

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    Are you sure that the octopus beak is on the same side of the creature as its anus?

    Both are on the outside. You can't have a mouth inside a creature, or it couldn't eat anything. The purpose of a mouth, or in this case a beak, is to capture food from the outside world and bring it inside the creature.

    It is for octopuses, which spend most of their time on the sea floor. Clearly, the octopus has well-defined top and bottom sides, and it maintains a preferred up-down orientation when it moves, most of the time (like we humans do).

    Indeed, and bilaterally symmetrical as well. How many eyes does an octopus have?

    You're not getting distracted by the tentacles, are you?
     
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  7. James R Just this guy, you know? Staff Member

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    I was fairly specific about the circumstances in which bilateralism becomes an evolutionary advantage.
     
  8. DaveC426913 Valued Senior Member

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    A simple Google for octopus anatomy will make it plain.


    A bell-shaped creature has an outside and an inside.

    If you are pretending I meant internal versus external, I think you're reaching. I think you know how it was meant to be taken.

    Sure. Radial critters can have a preferred top and bottom. It is not characteristic of bilateral critters.

    Two. And they are radially symmetrically distributed by 180 degrees.

    You didn't explain; you posited. And it's not granted; the arguments are weak, and have counter examples.

    Sure. Specific is by no means general.

    Bilateral symmetry is one possible configuration. But others seem to work, as far as they are given the chance. They did well for tens of millions of years before the great die off.
    There's no compelling reason to think that an alien planet - even if similar to Earth - will favor bilateral symmetry over other plans.
     
  9. James R Just this guy, you know? Staff Member

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    Really? Could you post a plain image or reliable text that clearly shows it? I'd appreciate it.

    Whatever. I don't see where you're trying to go with this. Is there a relevant point you want to make?

    I'd say that supports my contention that gravity is important, even in marine environments.

    Is it your assertion that octopusus are radially symmetrical?

    Have you actually looked at any pictures of octopuses?

    Remember what I originally wrote: "There are some benefits to being bilaterally symmetrical, moreso for active predators than for passive feeders."

    Granted, I didn't fully elaborate on what the benefits are.

    Are you denying that there are benefits? Do you believe that it's just an evolutionary accident that there are so many predator species with bilaterial symmetry?

    None of that was ever disputed by me.

    In general, or "moreso for active predators than for passive feeders"?

    If you have a counterargument to put, maybe now would be a good time for you to start presenting it.
     
  10. billvon Valued Senior Member

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    I agree. It is evolutionarily adequate - but certainly not the best (or only) choice. Evolution selects that which works, not that which is best. (Mammalian reproduction and eyes are great examples here.)
     
  11. James R Just this guy, you know? Staff Member

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    Interesting. What is the best choice for a body plan? And why?
     
  12. exchemist Valued Senior Member

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    Octopuses are bilaterally symmetrical, as are many molluscs. For dispersing waste, its anus is at a point inside the mantle that is close to the funnel and thus well away from the mouth, though obviously not at the opposite end of animal.

    Echinoderms are not bilaterally symmetrical in their adult stage, but interestingly, and perhaps significantly, the larval stage is bilaterally symmetrical.

    While it seems easy to see a logic in the advantages from a bilaterally symmetrical body plan (sensory organs and mouth at a "front" end when the animal moves, and locomotion related to the asymmetry in the environment due to gravity), I find it it is harder to see the countervailing advantages from other body plans.
     
  13. wegs Matter and Pixie Dust Valued Senior Member

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    It's probably fairest to say that any organism is shaped by and adapts to, its environment. This could mean that other life forms on other planets, would be much stronger than we are, from a physical/intelligence standpoint. (or much weaker, depending) And all life requires energy. Let's talk about what forms of energy are on other planets, and how they might affect the size, strength and sustainability of alien life.

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  14. DaveC426913 Valued Senior Member

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    Beak and anus are mid-form, somewhat proximal inside the bell area.


    You originally asserted that "It makes evolutionary sense for the sensory organs that are used to locate prey to be on the food intake end of the creature,

    An octopus' eyes are not on any end, but mid-form, and are on the outside of its bell-shaped body. Its mouth is also not on any end mid-form, but is on the inside of its bell-shape.


    Octopus are perfectly happy going either direction. When they want to walk across the floor, they tend to go tentacles first, head following. When they want to move fast, they jet away head first.
    They can also flip those if they want.


    Whether it's important or not, you were using it to justify bilateral symmetry being more likely. And that's a non sequitur. There's no obvious reason why the presence of gravity favors bilateral symmetry over other body plans.

    No. Their evolution is steeped in bilateral symmetry like most other Earth critters.

    But there is no reason why that shape isn't just as viable coming from a radial body plan.

    This seems needlessly sarcastic. Especially coming from the guy who had to ask me for a pic of an octopus to confirm my facts about its anatomy. Hmm?

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    You wrote number of things in addition to that.

    "It makes evolutionary sense..."
    "It seems more likely to me that top-of-the-food-chain predators are more likely to be bisymmetrical than otherwise."

    It certainly sounds like you're suggesting it'll be a favorite of evolution. If not, then the statement "There are some benefits to being radially symmetrical, moreso for active predators than for passive feeders." would hold just as true.

    There are benefits to everything. There are benefits to a radial body plan.

    Of course it's no accident! They all descended from the same bilateral ancestor!

    (That is a very odd question from someone who knows about evolution! I makes we wonder if we're talking past each other.)


    It is not a given that one body plan dominating the others is because it is significantly superior. Evolution is a wobbly path.

    Analogously: Humans have a flaw in the form of the Vagus nerve. (In case you're not familiar with it) We share it with other vertebrates, but it's particularly problematic for us, because our our vertical stature. It arose when fish had gills/heart and brain in a different arrangement. As humans evolved, the nerve is now circuitously routed under our arm. The issue is that it is "baked in" to our deep ancestry. It cannot be changed.

    So: once we had bilat symmetry, it can't be changed. But that's doesn't mean it would have any reason to dominate in an alien biosphere. It doesn't even means it would have dominated here on Earth, if things has gone a different way.



    Don't condescend. I have been counterarguing.

    Frankly, I don't think you're made your case in the first place. You keep saying it "makes sense" but how can you pretend it's not a biased argument about what "makes sense"? The fact that bilat dominates does not necessarily mean it's the preferred design. Some features so are so basic that they cannot be changed via evolution - we're stuck with some things.

    Not to mention that we have counter examples of alternate "makes sense" right here on Earth - and that's not even talking about an alien biosphere with it's own initial starting conditions.




    Finally, a sanity check on the direction of the discussion: You seem to be suggesting that bilateral symmetry would be strongly-favored in an alien biosphere. If that's not what you're suggesting, and you're merely suggesting that it has some advantages, might you just as easily say other body plans also have some advantages?
     
    Last edited: Feb 7, 2020
  15. Seattle Valued Senior Member

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    Octopus don't have tentacles by the way. They have arms.
     
  16. DaveC426913 Valued Senior Member

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    True. I shouldn't have deferred to JamesRs' use of the word.
     
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  17. billvon Valued Senior Member

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    There is no one best choice. Insects - with their basic six limb and four wing plan - are a much better body plan for flying creatures than birds and bats, which have to give up arms for wings. But their exoskeleton (which is a feature completely separate from their hexapod body plan) doesn't scale, so they remain small. Most annelids are radially symmetric and that works very well for them.
     
  18. foghorn Valued Senior Member

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    I guess your thinking about the sort of life which goes on to ''space travel''? And, from there pays us a visit?
    You may know of the idea of Panspermia, life on Earth could have originated elsewhere in space and came to Earth via space dust, meteoroids, asteroids, comets.
    https://en.wikipedia.org/wiki/Panspermia
    This bacterium (below) can survive vacuum and very high doses of radiation.
    https://en.wikipedia.org/wiki/Deinococcus_radiodurans

    But, apparently there are hundreds of cases of craft landing and having beings exit them on Earth.
     
  19. wegs Matter and Pixie Dust Valued Senior Member

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    Yes, I get the sense that the article is focusing on that, as well.

    I'm aware of the Panspermia idea. Not sure how I feel about the theory, honestly. I mean, is it basically saying that the building blocks of life on earth originated from other planets?
     
  20. DaveC426913 Valued Senior Member

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    The building blocks they speak of are amino acids, from which life's enzymes and proteins - including DNA - are formed.

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  21. wegs Matter and Pixie Dust Valued Senior Member

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    I realize what the building blocks are, but I’m skeptical that they originated on another planet.
     
  22. DaveC426913 Valued Senior Member

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  23. James R Just this guy, you know? Staff Member

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    Okay. But you said that bilateral symmetry is not the "best" body plan. So which ones are better?

    Or are you now willing to retract your claim and agree with me that the "fitness" of a body plan depends on the environment in which the organism has to survive and reproduce?
     

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