Exobiology - how might humanoids evolve on various planet types

Discussion in 'Astronomy, Exobiology, & Cosmology' started by pr0xyt0xin, Jun 16, 2016.

  1. pr0xyt0xin Registered Member

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    Here are some simple curiosities I've had inspired by science fiction. Apologies in advance if they sound totally uneducated. Feel free to pick and choose to comment on the ones that are interesting/logical to you.

    From several sources I've read that bipedal carbon-based humanoid aliens with sensory organs at the top of their bodies would be a fairly realistic thing on other "class M" planets. ;P But that doesn't mean they'd be identical to humans.

    - What would humanoid lungs look like/act like on a planet with a more prominent greenhouse effect? Would they manage waste gases better or utilize other gases in different ways? Would blood color change?
    - What would humanoid skin look like/act on a planet with a thinner atmosphere/ozone layer? Would it be darker or thicker/harder?
    - What would humanoid bones and musculature look/act like on a planet with stronger gravity? Weaker gravity? Would they grow shorter or taller?
    - What would happen to humanoid sensory organs/circulatory/nervous systems on any of the aforementioned planets?

    Obviously each would be the only variable in each hypothetical scenario (the planet would for all other practical purposes be "earth-like," in the Goldilocks zone, etc.)
     
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  3. DaveC426913 Valued Senior Member

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    Are you talking about humans transplanted to other planets, and how they would evolve from there - or are you talking about how creatures that evolved on the other planet would evolve from "scratch"?
    I'm going to assume the latter.

    There's no reason to think that exo-critters would evolve to a humanoid shape on other planets. The path to complexity is an extremely circuitous one, and the possble paths are uncountable.

    It has been argued by distinguished biologists that there's no reason it need even have happened here on Earth the way it did. Bilateral symmetry is but one of many possible configurations nature tried out.

    Stephen J. Gould, and expert in Pre-Cambrian life, describes the explosion of configurations that occurred (The Pre-Cambrian Explosion) 500 million years ago, with configurations of 5- 6- 8- 10- and even 20-fold symmetry. Almost all of them died off** except one - the one the evolved into us.

    **from causes unrelated to evolution, such as massive climate change.

    So, as Gould says, rewind the clock back 500 million years, and set it going again, and any of those configurations might have just as easily have gotten the upper hand, leading to complex lifeforms utterly unlike what we see today.

    It's unlikely bllateral symmetry would occur twice, even here on Earth, under the same conditions - it's very unlikely it would occur on another planet.
     
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  5. pr0xyt0xin Registered Member

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    Oh no I dont mean grow new life from scratch. I think maybe you confused the Star Trek reference. haha!

    I think I was more specifically referring to humanoids who were transplanted to other structures of planets. Which, I dont know how they explain it in fiction, but it's usually some religious phenomena or w/e. Gods transplanting them or something.
     
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  7. DaveC426913 Valued Senior Member

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    Ah. I see.
    So, how would contemporary humans, transplanted to another planet, evolve?
     
  8. Dinosaur Rational Skeptic Valued Senior Member

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    From DaveC426913 Post #2
    In the absence of some supporting arguments, the above bold phrase seems like a mere opinion. stated as objective fact. I do not think there have ever been land dwelling vertebrates that were not bilaterally symmetric. I see no reason to suppose that the design is not strongly preferred by evolution for land dwellers.

    For large vertebrates, bilaterally symmetric quadrupeds seems like the most likely initial evolutionary design. That design includes mice as well as elephants & dinosaurs. In spite of the large variety of land animals, all that I know of are bilaterally symmetric, which strong indicates that it is a design likely to be the dominant (if not the only) one elsewhere in the universe

    I do not know of any 6-limbed vertebrates, which strongly suggests that it is not a likely evolutionary design.

    Note that on Earth, the upright primate design evolved from a quadruped design, which existed for a long time prior to the existence of primates.

    6 limbs requires extra resources for little, if any, advantage. I am assuming that a centaur design would evolve from a 6-legged creature.

    A 6-limbed vertebrate would not be able to run faster than a quadruped, but would require a lot of extra resources in the brain & nervous system as well as in maintenance & repair functions.
    Note that insects & other non-vertebrates do not control their limbs independently to the extent that quadrupeds do.

    On Earth, bilateral symmetry is the dominant design for larger creatures who do not live in water. I do not know of any land dwellers with other symmetries, but do not claim to be a zoologist & there might be some.
     
  9. DaveC426913 Valued Senior Member

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    The support would be in the form of Stephen J. Gould's several books. He was the leading expert on the Precambrian Explosion.

    They're fascinating.

    No. You are assuming a level playing field, and an equal shot at perpetuity.
    One branch (the vertebrates), dominated the large macro-fauna niche. There were many contenders.

    No. It's because all those examples have a common ancestor from Precambrian times.

    If the common ancestor had 20-fold symmetry, so would its descendants.


    If 20 artists had widely varying styles, and 19 of them were killed in an avalanche, you'd see the world eventually dominated by one art style. It would be folly to conclude that this one style had evolved many times - or that is was the most likely or successful. You're looking at a single artist's style that grew to dominate the art scene by happenstance.

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    Last edited: Jun 17, 2016
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  10. pr0xyt0xin Registered Member

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    I think I see Dave's point, at this point I'd certainly be interested in checking out Gould's works. They sound fascinating. But yeah, I guess it was my understanding that bilateral symmetry makes a good amount of structural sense in a plethora of environments, and that life had a tendency to go in that direction. Could be a falsehood though.

    That being said, its a little off topic at this point. HA! The end goal was to satiate my curiosity regarding how earth-like humans would adapt to various environments. How would the human body change if subjected to those scenarios? (at least relatively gradually, I would assume)
     
  11. KilljoyKlown Whatever Valued Senior Member

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    I to have read a lot of science fiction and have thought about what other technological civilizations might look like. Let's try this thought experiment with how life on earth might have evolved if everything was the same except the gravity. Let's set the gravity at 2G's. How would that have affected evolution? Life in the oceans might not change all that much, but getting around on land would require some serious differences from the current evolution of life on the land. At the very least bone and muscle would have to be stronger and maybe being a biped would never happen because falls would be a lot more serious. In any event that's only one variable and I can't say that humans would be able to evolve under that condition.

    I do think life will exist anywhere in the universe that it can and of all worlds that have life only a very small percentage of them would be able to evolve a technological species.
     
  12. pr0xyt0xin Registered Member

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    Just on the note of "falls being more serious," sorta lends itself to an assumption that humanoids taken to a double gravity planet might just evolve to be shorter and thicker (bones, etc.). Their hearts would be closer to their brains for ease of bloodflow. Plant life would also likely not grow as tall, thus negating any need for life to reach it. Much more happening at and below the surface I would imagine. Quadrupeds (4 legs or more) would probably rule on this planet. Also, if I had to guess, slower creatures might flourish (tortoises/sloths). More herbivores? Just guessing on that one, because of the amount of exertion it would take for the type of apex predators we have on our planet (big cats/reptiles) to move/hunt in double gravity.

    On the flip side, with half gravity (i feel there's far more theories/fiction on the topic of low gravity because Mars is semi-inhabitable) we might see a lot of the reverse. Stands to reason that trees and plants might grow taller, similarly, less effort for things to move might allow for faster/taller creatures with less sturdy skeletons. Logically, muscles may also be comparatively weakened (needing to lift/push/pull lesser overall weight). Seems like depending on the various ecosystems on a planet like this, bodies wouldn't metabolize calories as quickly and the creatures would be capable of going longer periods of time without feeding.

    Anyway, just leaning toward the realm of "what is plausible or more likely." Obviously there's no certainty in any of this.
     
    Last edited: Jun 17, 2016
  13. KilljoyKlown Whatever Valued Senior Member

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    Sense humanoids all started as quadrupeds and evolved into bipeds, it might be a problem evolving to bipeds on a 2G world. We really need to become a space faring species if we want to find out what's what on this subject.
     
  14. DaveC426913 Valued Senior Member

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  15. Ophiolite Valued Senior Member

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    Incorrect. Simon Conway Morris, Chair of Evolutionary Palaeobiology at Cambridge is also an expert on the Burgess Shale and is critical of Gould's work. He provides many reasons for thinking that humanoid shapes would evolve on other planets. (See, I think, Life's Solution: Inevitable Humans in a Lonely Universe.)
     
  16. KilljoyKlown Whatever Valued Senior Member

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    Good to know you are still active. I would tend to agree with that statement. Not only do I believe life will exist where ever it can. I believe intelligence will evolve on any life bearing world that develops a biosphere that can support it. For that intelligent life to become technological you need a physical form that can make and use tools and is willing to live in societies that can communicate and learn from each other. The humanoid form is ideal for that purpose.
     
  17. DaveC426913 Valued Senior Member

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    What does a humanoid shape have to do with tool use, societal structures, communication or learning?
     
  18. Ophiolite Valued Senior Member

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    I was not necessarily agreeing with Conway Morris' view of humanoid evolution. I was correcting Dave's observation that "There's no reason to think that exo-critters would evolve to a humanoid shape on other planets". One might think that the view of Conway Morris and yourself is needlessly anthropocentric.
     
  19. DaveC426913 Valued Senior Member

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    Stephen J. Gould had some criticisms of Conway Morris as well. Eloquent and succinct as always, he said nuh-uh!
     
  20. Ophiolite Valued Senior Member

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    Sorry! Succinct. His Magnum Opus, The Structure of Evolutionary Theory, is a convoluted concatenation of nested clauses that rise to crescendos so distant from their origin that the work is rife with missing links. While I value Gould, his writing style revealed in "Structure" proves he actually had an editor when he wrote Wonderful Life, but the poor chap must have died when he was most needed.
     
  21. Dinosaur Rational Skeptic Valued Senior Member

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    From pr0xyt0xin Post #9
    With lower gravity creatures can jump higher, but cannot run faster. I would expect creatures to run slower.

    Running requires friction with the surface you are running on.​

    How fast can you run on ice?

    Lower gravity would result in less friction & slower running speed.

    I am not sure about the broad jump. Slower running speed & less friction would tend to decrease the broad jump distance. Weaker gravity would tend to increase it.
     
  22. Yazata Valued Senior Member

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    My guess that the reason why terrestrial vertebrates and land arthropods are bilaterally symmetrical is that their ancestors that first left the early oceans were. The reason why all the vertebrates are four-limbed (even if the forelimbs have subsequently evolved into arms or wings) is that their original terrestrial ancestors were quadipeds. What we see are original ancestral types elaborating and filling new niches, while always remaining variations on the original theme.

    Arthropods and vertebrates are both very successful evolutionary lineages in terrestrial environments, each of them with radically different anatomies. That suggests to me that there isn't any one-size-fits-all anatomical solution to life on land and that a variety of unrealized possibilities might exist.


     
    Last edited: Jun 26, 2016
  23. Yazata Valued Senior Member

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    The title of the book might give away its motivation.

    It's easy to invent reasons for various details of human anatomy after the fact. Two eyes to provide stereoscopic vision for example.

    But I can easily imagine an alien biologist doing the same thing.

    Our three-legged tripod-shaped biologist might argue that a bipedal lifeform is inherently unlikely, because it would be unstable in the direction perpendicular to the legs and would have a tendency to topple over without constant readjustments of its balance.

    It might argue that while having two eyes facing forward would provide stereoscopic vision in that direction, it would leave the organism blind from behind. So it would conclude that three eyes, located 120 degrees apart around the circumference of the head would be much more likely, because it would allow two of the eyes to come to bear on all parts of a 360 degree field of view.

    It might argue that bilateral symmetry and an anterior/posterior distinction would leave organisms only facing in one direction, obviously less efficient that their own trilateral symmetry that allows them to face in all directions at once.
     
    Last edited: Jun 26, 2016

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