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Thread: aliens will look like humans

  1. #31
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    Quote Originally Posted by DaveC426913 View Post
    Why? What is the basis of the argument?
    An active moving lifestyle calls for a complex and quick-acting sensory & control system, which can subsequently develop toward more and more intelligence. An inactive, non-moving or not-moving-much lifestyle does not.

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    Quote Originally Posted by Delvo View Post
    An active moving lifestyle calls for a complex and quick-acting sensory & control system, which can subsequently develop toward more and more intelligence. An inactive, non-moving or not-moving-much lifestyle does not.
    But that wasn't the basis of your argument. You were talking about bilateral symmetry:
    I think there is a valid argument that intelligent aliens would be bilaterally symmetrical...
    You followed up by making a connection between non-bilateral symmetry and sessility here on Earth, and then generalized it to exolife:

    ...Living things which are not bilaterally symmetrical (ed. here on Earth) generally move about more slowly (or are sessile)...
    But that generalization has no apparent basis.
    Last edited by DaveC426913; 2015-Sep-28 at 03:25 AM.

  3. #33
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    Quote Originally Posted by DaveC426913 View Post
    Why? What is the basis of the argument? Are you suggesting correlation implies causation,
    In this case, yes. I think there are substantial reasons why radial symmetry is found in sessile or drifting organisms, while bilateral symmetry is found in living things which move about more actively.

    If you are something like a sea anemone, which normally stays sitting in one place, radial symmetry is an evolutionary advantage because you can grab food with equal ease no matter what direction it is coming from. If you're an active swimmer like a fish, the same logic does not apply. You can swivel about to face your food, and if necessary swim towards it.

    Mind you, I claim no personal credit for this suggestion. If you google words like "bilateral symmetry, advantages" and "radial symmetry, advantages", you will get an idea of who else thinks this and why.

    and is extrapolatable to exolife?
    Yes, I think the same evolutionary logic would apply in any environment where there are mobile organisms and less mobile ones.

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    Quote Originally Posted by DaveC426913 View Post
    But that wasn't the basis of your argument. You were talking about bilateral symmetry:


    You followed up by making a connection between non-bilateral symmetry and sessility here on Earth, and then generalized it to exolife:


    But that generalization has no apparent basis.
    Please do not blame Delvo for things written by me. We are not the same person.

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    I'm finding this entire conversation to be completely hilarious!

    Not one person has yet tweaked to the fact that all of the 'hypothetical' life-forms being discussed, are either merely replicas of known terrestrial biota, or are replicas of already known fantasy figures (Centaurs).

    Does no one recognise that this is precisely all that is being spoken about, and not anything else? Doesn't anyone find this as being kind of odd?

    Is there a rule that says hypothetically evolved life-forms must be a replication of what is already known?

    If so, what is the difference between looking up an atlas of already known and catalogued creatures, and reading this thread, (or the OP article)?

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    Quote Originally Posted by Selfsim View Post
    Is there a rule that says hypothetically evolved life-forms must be a replication of what is already known?
    No, there isn't. My understanding though is that the thread is discussing a theory that for example, life forms that evolve independently of life on earth might, for example, adopt fins as a way of moving in water, because it is an obvious solution to the problem of how to move in water. I also find some of the examples funny, but I don't think that is the point. Would creatures on a rocky planet adopt legs as a means to move, or would they evolve wheels?
    As above, so below

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    Quote Originally Posted by Selfsim View Post
    Not one person has yet tweaked to the fact that all of the 'hypothetical' life-forms being discussed, are either merely replicas of known terrestrial biota, or are replicas of already known fantasy figures (Centaurs).
    Read again. That's precisely what I argued against in my posts.
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  8. #38
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    Quote Originally Posted by Selfsim View Post
    Does no one recognise that this is precisely all that is being spoken about, and not anything else? Doesn't anyone find this as being kind of odd?
    No, not odd at all.

    The lifeforms talked about in thread are here mostly because we need examples of life to make our points about biology, and Earth life is the only kind we know of. So yes, most of the things used for purposes of this discussion are going to resemble past or present Earth life to some degree. That does not mean we all believe that that's what alien life would actually be like.

    ADDED: Why would suggesting creatures with similar physiologies in a discussion about convergent evolution strike you as "hilarious"?
    Last edited by Noclevername; 2015-Sep-28 at 05:04 PM.
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  9. #39
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    Quote Originally Posted by Jens View Post
    No, there isn't. My understanding though is that the thread is discussing a theory that for example, life forms that evolve independently of life on earth might, for example, adopt fins as a way of moving in water, because it is an obvious solution to the problem of how to move in water. I also find some of the examples funny, but I don't think that is the point. Would creatures on a rocky planet adopt legs as a means to move, or would they evolve wheels?
    Well, lets take a look at the context of this ...
    All lifeforms on Earth are related by sharing a common set of polymers (polynucleotides, polypeptides, and polysaccharides). These polymers, and only these polymers, are known to give rise to the 4 basic functions we call 'life', (ie: replication, heritability, catalysis and energy utilization (metabolism)). This is explained by all terrestrial life having inherited this particular set of functions, and its structures, from its single common ancestor.

    Of the 390 naturally occurring amino acids, terrestrial life is composed of only a small subset of only 22 of these. The DNA comprising living organisms is comprised of only four nucleosides, (deoxyadenosine, deoxythymidine, deoxycytidine, and deoxyguanosine), out of the at least, 102 naturally occurring ones. (Not including the artificially synthesized subset). There is thus a vast scope for other theoretical subsets and combinations at this level, beyond just the terrestrial subset.

    Since there are 1.4 x 1070 informationally equivalent 'genetic' codes, from just amongst the same codons and amino acids as the terrestrial standard genetic code subset, there is clearly an astronomical scale scope beyond this again, for completely unrecognisable combination sets ('codes'), arising from amongst the theoretical superset. (No combinations outside of the standard terrestrial subset have yet been found on Earth, of course .. which also makes it an easily forgotten fact).

    There are also thousands of, (theoretically at least), thermodynamically equivalent glycolysis and core metabolic pathways. Any of these are theoretically as equally possible as eachother, as is the case opposite molecular chirality.

    In general, similar DNA and biochemistry give similar morphology and function. However, there is no known biological reason, besides common descent, to suppose that similar morphologies must have similar biochemistry. Where a line of descent in common with Earth's is not possible because it is beyond Earth, there is thus no known reason to assume that similar biochemistry will occur (from amongst this theoretically vast astronomical scale combinatorial landscape). Therefore in general, there is also no known reason that similar morphologies and functions to Earth's, must arise beyond Earth. In fact, in the absence of common descent or any other biochemical reasons to suppose that molecular sequences and morphological characters should be similar to Earth's, it is theoretically more likely, by far, that the two will be radically different.

    I'm sorry .. I just don't see how such enormous gaps in knowledge about the impacts of such theoretical variability, can be just skimmed over completely without empirical or theoretical explanation, and yet somehow, mysteriously gravitate towards the complete opposite of the theoretical prediction, and end up as recognisable terrestrial biota-'like' macro features! Ignoring this absence of explanation, makes most of what has been said in this thread, an overwhelmingly bewildering proposition, I'm afraid.

  10. #40
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    Quote Originally Posted by Noclevername View Post
    Read again. That's precisely what I argued against in my posts.
    Ok.

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    Quote Originally Posted by Noclevername View Post
    No, not odd at all.

    The lifeforms talked about in thread are here mostly because we need examples of life to make our points about biology, and Earth life is the only kind we know of. So yes, most of the things used for purposes of this discussion are going to resemble past or present Earth life to some degree. That does not mean we all believe that that's what alien life would actually be like.

    ADDED: Why would suggesting creatures with similar physiologies in a discussion about convergent evolution strike you as "hilarious"?
    All terrestrial life's similar functions, morphologies, biochemistry etc is explained only by way of a common descent. See my post #39.

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    Quote Originally Posted by Selfsim View Post
    Quote Originally Posted by Noclevername View Post
    No, not odd at all.

    The lifeforms talked about in thread are here mostly because we need examples of life to make our points about biology, and Earth life is the only kind we know of. So yes, most of the things used for purposes of this discussion are going to resemble past or present Earth life to some degree. That does not mean we all believe that that's what alien life would actually be like.

    ADDED: Why would suggesting creatures with similar physiologies in a discussion about convergent evolution strike you as "hilarious"?
    All terrestrial life's similar functions, morphologies, biochemistry etc is explained only by way of a common descent. See my post #39.
    And how does that relate to the post you just quoted?
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  13. #43
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    Quote Originally Posted by Noclevername View Post
    And how does that relate to the post you just quoted?
    You don't see any correlation between convergent evolution and common descent, eh?

    The topic is about the implications of convergent evolution on exo-life morphologies. As per the OP post:
    Quote Originally Posted by Gomar
    "The author said extra-terrestrials that resemble human beings should have evolved on some of the many Earth-like planets that have been discovered"
    I'm not going to answer for those who are talking OT.

  14. #44
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    Quote Originally Posted by Selfsim View Post
    You don't see any correlation between convergent evolution and common descent, eh?
    The term "convergent evolution" only applies when species A has features in common with species B which it does not have with other species C and D, even though species A has more recent ancestors in common with species C and D, than with species B. In short, only in cases where shared morphology does not correlate with the degree of common descent.
    Last edited by Colin Robinson; 2015-Sep-29 at 03:17 AM.

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    Quote Originally Posted by Selfsim View Post
    Well, lets take a look at the context of this ...
    All lifeforms on Earth are related by sharing a common set of polymers (polynucleotides, polypeptides, and polysaccharides). These polymers, and only these polymers, are known to give rise to the 4 basic functions we call 'life', (ie: replication, heritability, catalysis and energy utilization (metabolism)). This is explained by all terrestrial life having inherited this particular set of functions, and its structures, from its single common ancestor.

    Of the 390 naturally occurring amino acids, terrestrial life is composed of only a small subset of only 22 of these. The DNA comprising living organisms is comprised of only four nucleosides, (deoxyadenosine, deoxythymidine, deoxycytidine, and deoxyguanosine), out of the at least, 102 naturally occurring ones. (Not including the artificially synthesized subset). There is thus a vast scope for other theoretical subsets and combinations at this level, beyond just the terrestrial subset.

    Since there are 1.4 x 1070 informationally equivalent 'genetic' codes, from just amongst the same codons and amino acids as the terrestrial standard genetic code subset, there is clearly an astronomical scale scope beyond this again, for completely unrecognisable combination sets ('codes'), arising from amongst the theoretical superset. (No combinations outside of the standard terrestrial subset have yet been found on Earth, of course .. which also makes it an easily forgotten fact).

    There are also thousands of, (theoretically at least), thermodynamically equivalent glycolysis and core metabolic pathways. Any of these are theoretically as equally possible as eachother, as is the case opposite molecular chirality.
    If two organisms both perform thermodynamically equivalent glycolysis, i.e. both are getting energy from oxidation of simple sugars, then whatever metabolic pathways they use, there are evolutionary issues which both face, like ensuring an oxygen supply and a sugar supply... Considering just the sugars, there is a range of possible strategies for getting this, but the range is not unlimited — you can produce your own sugars from carbon dioxide and water, as plants do, in which case you need another energy source such as light, or you can go in search of nutrients as a motile animal does, or you can sit in the water and wait for nutrients to come to you, as is done by less active animals such anemones... I don't see why (for instance) use of different metabolic pathways would change the principle that bilateral symmetry is more advantageous for active swimmers, while radial symmetry is advantageous for the less active creatures: the sessile forms and the drifters.

  16. #46
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    Quote Originally Posted by Colin Robinson View Post
    In this case, yes. I think there are substantial reasons why radial symmetry is found in sessile or drifting organisms, while bilateral symmetry is found in living things which move about more actively.
    You are still making the error in thinking that Earth creatures with radial symmetry and are also sessile is an inextricable connection.

    Quote Originally Posted by Colin Robinson View Post
    If you are something like a sea anemone, which normally stays sitting in one place, radial symmetry is an evolutionary advantage because you can grab food with equal ease no matter what direction it is coming from. If you're an active swimmer like a fish, the same logic does not apply. You can swivel about to face your food, and if necessary swim towards it.
    Octopi are generally radially symmetrical (certainly their primary locomotion is radial, rather than bilateral), yet they are extremely agile, extremely mobile and by far the most intelligent invertebrates on the planet.

  17. #47
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    Quote Originally Posted by DaveC426913 View Post
    You are still making the error in thinking that Earth creatures with radial symmetry and are also sessile is an inextricable connection.
    Don't remember using the word "inextricable"...

    Octopi are generally radially symmetrical
    According to Wikipedia (page: Symmetry in Biology)

    "The octopus, however, has bilateral symmetry, despite its eight arms."

    According to New World Encyclopedia (page: Octopus)

    "Like all cephalopods, octopuses have bilateral symmetry,"

    (certainly their primary locomotion is radial, rather than bilateral),
    No sure what you mean by this. When an octopus crawls across this seabed, its eyes are usually above its feet, and the part of its body with the digestive tract is behind its eyes and brain. Where is the radial symmetry in that?

  18. #48
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    Quote Originally Posted by Colin Robinson View Post
    The term "convergent evolution" only applies when species A has features in common with species B which it does not have with other species C and D, even though species A has more recent ancestors in common with species C and D, than with species B. In short, only in cases where shared morphology does not correlate with the degree of common descent.
    ...
    Quote Originally Posted by Colin Robinson
    If two organisms both perform thermodynamically equivalent glycolysis, i.e. both are getting energy from oxidation of simple sugars, then whatever metabolic pathways they use, there are evolutionary issues which both face, like ensuring an oxygen supply and a sugar supply... Considering just the sugars, there is a range of possible strategies for getting this, but the range is not unlimited — you can produce your own sugars from carbon dioxide and water, as plants do, in which case you need another energy source such as light, or you can go in search of nutrients as a motile animal does, or you can sit in the water and wait for nutrients to come to you, as is done by less active animals such anemones... I don't see why (for instance) use of different metabolic pathways would change the principle that bilateral symmetry is more advantageous for active swimmers, while radial symmetry is advantageous for the less active creatures: the sessile forms and the drifters.
    You miss the entire point I was making. Noclevername, at least, expressed the hidden assumption behind the conversation you continue above .. ie:
    Quote Originally Posted by Noclevername
    The lifeforms talked about in thread are here mostly because we need examples of life to make our points about biology, and Earth life is the only kind we know of.
    We are already well-versed in how Earth-life functions.

    When you're ready to take the conversation back into the context of the OP & linked article however, (ie: into an 'exo' context), I'll assert that none of the unstated assumptions, can be taken as 'a given'. The most obvious one, which is the entire lynch-pin for all of your deductive arguments is that every living thing on Earth inherited the 4 basic functions we call 'life' from an original species, (ie: replication, heritability, catalysis and energy utilization (metabolism). Everything you state ... with no visible exceptions, represents the pursuit of sustaining these 4 basic functions. There is no basis for assuming that the same original species will show up elsewhere. In fact, from the vastness of the generalised theoretical combinatorial models taken from the biochemistry resulting in Earth-life, it is highly unlikely that such functions will recur, in the likely case of there being no possible ancestor in common with Earth's life.

    So, now going back to Noclevername's closing statement, in his reply to me .. ie:
    Quote Originally Posted by Noclevername
    That does not mean we all believe that that's what alien life would actually be like.
    .. all I'm getting from your argument, is the complete opposite of his above assertion.

  19. #49
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    Quote Originally Posted by Colin Robinson View Post
    Don't remember using the word "inextricable"...
    Because you keep talking about the two things as if one causes the other - and strongly enough to generalize to an alien life form.


    Quote Originally Posted by Colin Robinson View Post
    According to Wikipedia (page: Symmetry in Biology)

    "The octopus, however, has bilateral symmetry, despite its eight arms."

    According to New World Encyclopedia (page: Octopus)
    "Like all cephalopods, octopuses have bilateral symmetry,"
    No sure what you mean by this. When an octopus crawls across this seabed, its eyes are usually above its feet, and the part of its body with the digestive tract is behind its eyes and brain. Where is the radial symmetry in that?
    Remember why we're talking about this at all. Your suggestion (paraphrased by me) is that bilateral symmetry confers a strong advantage over other forms (strong enough that it is the "likely" form). I argue that our Earth-bound example of a radial body plan demonstrates that an alien's radial body form could do quite well.

    i.e. There is no inherent disadvantage of a non-bilateral symmetry so strong that it makes for a generally poor body form for an intelligent species.

  20. #50
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    Quote Originally Posted by Jens View Post
    To me this hinges on what you mean by similar. Somebody mentioned dolphins and sharks, saying that they are different. It's certainly true that a dolphin moves its flippers up and down whereas a shark moves its fins from side to side, but they both developed essentially the same means of propulsion.
    In this particular case, it's a straightforward result of both being Chordates. The long flexible spine is a defining feature, which was used for undulating locomotion through the water from the start. The dolphin's up-down motion is actually pretty novel and unusual for most of the group. But the long flexible spine simply naturally lends itself to locomotion by undulation.

    That said, the general body shape, fin positions, and proportions are driven by hydrodynamics, which explains the basic similarity between sharks and dolphins, despite the wide variety of Chordate body forms.

    The humanoid body form is not so restricted by hydrodynamic considerations. We can see in primates a pretty wide variety of proportions. So even if an alien were to have a basically humanoid body form, I wouldn't expect as much similarity as there is between a dolphin and a shark.

    My own expectation is that gravity level is something which would have a pervasive effect on the evolution of a hypothetical alien biosphere.

    With a higher gravity level, I don't think tetrapods would really be optimal. Something with more limbs could be more successful in crawling out of the water. Or maybe something with fewer limbs that inchworms its way over land (undulating vertically) with its belly/tail, while pulling itself along with just two hand-like claws.

    With a lower gravity level, something more arthropod-like or mollusk-like may be ideal than something with an endoskeleton. Or flight may be more pervasive.

    So, even if we take as given bilateral symmetry and two hands as manipulators, I don't think "humanoid" is a safe bet.

  21. #51
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    Quote Originally Posted by DaveC426913 View Post
    Because you keep talking about the two things as if one causes the other - and strongly enough to generalize to an alien life form.

    Remember why we're talking about this at all. Your suggestion (paraphrased by me) is that bilateral symmetry confers a strong advantage over other forms (strong enough that it is the "likely" form). I argue that our Earth-bound example of a radial body plan demonstrates that an alien's radial body form could do quite well.
    You gave one claimed example of a comparatively intelligent and active animal with a radial body plan — the octopus, which in fact is not radially symmetrical.

    i.e. There is no inherent disadvantage of a non-bilateral symmetry so strong that it makes for a generally poor body form for an intelligent species.
    I think the evidence says otherwise. Radial symmetry is found in various classes of animals (e.g. cnidarians, echinoderms), but is invariably associated with a sessile, drifting or slow-moving way of life. It is not unusual for a species which at one stage of its life cycle is sessile and radially symmetrical, to go thru another stage in which it is free-swimming and bilaterally symmetrical. E.g. The sessile, radial coral polyp develops from a free-swimming, bilaterally symmetrical planula.

    For a technical discussion about why bilateral symmetry is advantageous for active species see
    The manoeuvrability hypothesis to explain the maintenance of bilateral symmetry in animal evolution
    Last edited by Colin Robinson; 2015-Sep-30 at 12:44 AM.

  22. #52
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    Convergent evolution is largely about form; unless it's too speculative, we can presume fluid mechanics works the same in any Newtonian fluid environment: the external forms of sharks, tuna, dolphins, and nuclear submarines are dictated by the same physics. A fast-moving marine predator must have a body form of a certain type, optimized to minimize fluid dynamic drag. Similarly, a flying creature, that is one that uses wings, must have a certain form. Pterosaurs, birds, meganisoptera, and bats evolved similar solutions because they're forced by fluid mechanics.
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  23. #53
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    Quote Originally Posted by Selfsim View Post
    All terrestrial life's similar functions, morphologies, biochemistry etc is explained only by way of a common descent. See my post #39.
    See to me, that is an interesting issue rather than a truism. In post 50 IsaacKuo talks about the bio-mechanical aspects. I would agree that if a life form was for example living in plasma (if that is possible), it might be completely different from how we understand life, but that if a creature is living in a water environment, the propulsion might very well be something similar to what fish use, simply because mechanically it is an efficient way of propulsion. For creatures that live on land, limbs seem to be an efficient way to move, and for example a creature could have treads, but presumably that would mean having one organism move around the other, because otherwise it would be difficult to transfer energy to that part of the body. And similarly, it seems logical that photons would be used as a means of understanding the environment because the information contained in photons can provide a lot of information about the environment. X-rays would be difficult to use because it's hard to reflect them, for example. So while I understand the fact that some of the similarities of life on earth is because of common inheritance, I think the question is whether there are also similarities that are the result of sensible ways of addressing certain engineering problems.
    As above, so below

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    Quote Originally Posted by swampyankee View Post
    Convergent evolution is largely about form; unless it's too speculative, we can presume fluid mechanics works the same in any Newtonian fluid environment: the external forms of sharks, tuna, dolphins, and nuclear submarines are dictated by the same physics. A fast-moving marine predator must have a body form of a certain type, optimized to minimize fluid dynamic drag. Similarly, a flying creature, that is one that uses wings, must have a certain form. Pterosaurs, birds, meganisoptera, and bats evolved similar solutions because they're forced by fluid mechanics.
    These statements are erroneous as they have been stated completely backwards.

    Physics doesn't 'dictate' anything. The Laws of Physics are distinguished by observation, and are then formulated to be useful in making predictions within a stated context. Similarly, nothing 'must have' any particular form, nor is anything 'forced by fluid mechanics'. The Laws of Physics are nothing more than explanatory models. Explanatory models don't 'force' or 'dictate' anything.

    Earth's biology has an inherited trait, which renders it capable of functional adaption. The explanation for how this trait is sustained and evolves by inheritance, is modelled by the process known as 'Natural Selection'.

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    Quote Originally Posted by Selfsim View Post
    These statements are erroneous as they have been stated completely backwards.

    Physics doesn't 'dictate' anything. The Laws of Physics are distinguished by observation, and are then formulated to be useful in making predictions within a stated context. Similarly, nothing 'must have' any particular form, nor is anything 'forced by fluid mechanics'. The Laws of Physics are nothing more than explanatory models. Explanatory models don't 'force' or 'dictate' anything.
    Whatever your theory of science tells you about the laws of physics, the fact remains that streamlining works. It works for sharks, it works for tuna, it works for dolphins, and it works for submarines. Do you really think streamlining wouldn't work for fast-moving aquatic creatures and machines in the oceans of another planet?
    Last edited by Colin Robinson; 2015-Sep-30 at 03:09 AM.

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    Quote Originally Posted by Colin Robinson View Post
    Whatever your theory of science tells you about the laws of physics, the fact remains that streamlining works. It works for sharks, it works for tuna, it works for dolphins, and it works for submarines. Do you really think streamlining wouldn't work for fast-moving aquatic creatures and machines in the oceans of another planet?
    It makes no difference whatsoever, wherever there is no known explanation for the abundant empical evidence at hand, other than a Common ancestor that was capable of: replication and heritability and catalysis and energy utilization (metabolism), or no evidence of a grand Designer of said machines.

    PS: And oh yes .. it has everything to with what science has to say about the laws of physics .. and nothing to do with me.
    Last edited by Selfsim; 2015-Sep-30 at 06:41 AM. Reason: PS

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    Quote Originally Posted by Colin Robinson View Post
    You gave one claimed example of a comparatively intelligent and active animal with a radial body plan — the octopus, which in fact is not radially symmetrical... Radial symmetry is found in various classes of animals (e.g. cnidarians, echinoderms), but is invariably associated with a sessile, drifting or slow-moving way of life. It is not unusual for a species which at one stage of its life cycle is sessile and radially symmetrical, to go thru another stage in which it is free-swimming and bilaterally symmetrical. E.g. The sessile, radial coral polyp develops from a free-swimming, bilaterally symmetrical planula.
    It gets even worse than that for the case for body plan/symmetry being unconnected to lifestyle. Without cephalopods, the best example would be echinoderms, the world's most mobile radial critters. But not only are they still distinctly less mobile than most or all bilateral critters, they also are more bilateral than radial in some ways themselves! First, they probably came from a more mobile, bilateral ancestor, and still have bilateral larval forms that are more mobile than the adults. On top of that, two major lineages in the group have been developing back toward more active directional movement again, and have done so by developing new ways to convert from radial back to bilateral. Irregular urchins such as sand dollars are flat, rounded pentagons, but not equilateral, with an enlarged front corner, the other four corners all shifted back and closer together, the anus shifted back toward the short side opposite from the enlarged corner, and a few more bilateral features in some species. And sea cucumbers, which are compact in the radial directions but elongated in the axial direction to create a body with five surfaces running end to end, grow tube feet only on the three of them that touch the ground but not the two that don't, making a clear distinction between top and bottom (which, combined with the distinct front & back ends, yields symmetry on the right & left but not with any other plane).

    It's even easy to see in machines, which certainly can't be said to be constrained by biochemistry. Vehicles: bilateral. Computers to be used by the public approaching from any direction in the middle of a wide open space: radial groups. Computers to be used by the public under various space constraints: lined up parallel to walls or traffic flow (like organisms in the form of layers/mats/films). This isn't biochemistry or ontogeny or phylogeny. It's barely even physics. It's plain simple geometry.

    Quote Originally Posted by Selfsim View Post
    These statements are erroneous as they have been stated completely backwards.

    Physics doesn't 'dictate' anything... [/I]The Laws of Physics are nothing more than explanatory models. Explanatory models don't 'force' or 'dictate' anything.
    Bothering to point that out when you know someone was obviously referring to the observed phenomena which those models describe is simply avoidance of dealing with the subject of those phenomena themselves. You have yet to provide an actual counter to the claims you seem to think you're arguing against. Your theory seems to be that chondrichthean fish, artiodactyls (hoofed mammals), afrotherians (relatives of elephants & hyraxes & aardvarks), squamates (relatives of lizards & snakes), submarines, and torpedoes & missiles all inherited a biochemically-dictated type of shape from a common ancestor, which ancestry didn't affect the non-aquatic ones until they returned to the water (cetaceans, manatees, ichthyosaurs). How does that work? For that matter, what other example has ever been observed, in any other context, of biochemistry controlling macroscopic shapes in any way at all? Why would the same laws of macroscopic objects' motion that we see applying to Earth's life forms and to objects built of metal & such not also apply to life forms made from rather slight deviations from Earth's biochemicals (such as the example of proteins that use different amino acids but are still proteins)?

  28. #58
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    Quote Originally Posted by Jens View Post
    See to me, that is an interesting issue rather than a truism.
    The scientific process ignores postulated truth in 'truisms' .. there's simply no scientific value in 'truisms'. What you see as an 'issue', is in fact, a scientific theory, which is supported by the bulk of empirical evidence, with no exceptions yet discovered. It is in fact, a fundamental prerequisite for Darwinian Evolution theory. Therefore, in the generalised case, there is also no known reason that similar morphologies and functions to Earth's, must arise beyond Earth, where there is no ancestor in common with Earth's life.
    (See the quote and reference provided below, for more info).
    Quote Originally Posted by Jens
    In post 50 IsaacKuo talks about the bio-mechanical aspects. I would agree that if a life form was for example living in plasma (if that is possible), it might be completely different from how we understand life, but that if a creature is living in a water environment, the propulsion might very well be something similar to what fish use, simply because mechanically it is an efficient way of propulsion. For creatures that live on land, limbs seem to be an efficient way to move, and for example a creature could have treads, but presumably that would mean having one organism move around the other, because otherwise it would be difficult to transfer energy to that part of the body. And similarly, it seems logical that photons would be used as a means of understanding the environment because the information contained in photons can provide a lot of information about the environment. X-rays would be difficult to use because it's hard to reflect them, for example. So while I understand the fact that some of the similarities of life on earth is because of common inheritance, I think the question is whether there are also similarities that are the result of sensible ways of addressing certain engineering problems.
    ... So:
    Quote Originally Posted by Theobold
    If every living species descended from an original species that had these four obligate functions, {replication, heritability, catalysis, and energy utilization (metabolism)}, then all living species today should necessarily have these functions (a somewhat trivial conclusion). Most importantly, however, all modern species should have inherited the structures that perform these functions. Thus, a basic prediction of the genealogical relatedness of all life, combined with the constraint of gradualism, is that organisms should be very similar in the particular mechanisms and structures that execute these four basic life processes
    ... and they are.

    Ref: The fundamental unity of life
    .





  29. #59
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    Quote Originally Posted by Selfsim View Post
    The scientific process ignores postulated truth in 'truisms' .. there's simply no scientific value in 'truisms'. What you see as an 'issue', is in fact, a scientific theory, which is supported by the bulk of empirical evidence, with no exceptions yet discovered. It is in fact, a fundamental prerequisite for Darwinian Evolution theory. Therefore, in the generalised case, there is also no known reason that similar morphologies and functions to Earth's, must arise beyond Earth, where there is no ancestor in common with Earth's life.
    (See the quote and reference provided below, for more info).
    ... So:
    ... and they are.

    Ref: The fundamental unity of life
    .




    I completely agree if you use the term "must'" what I find interesting is the question, is it possible that creatures with some resemblance to life forms on earth •might• arise on other worlds not because of common ancestry but as a result of similar engineering choices emerging independently as solutions to common problems. I would wonder about the likelihood of this happening, but never the necessity.
    As above, so below

  30. #60
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    Quote Originally Posted by Jens View Post
    I completely agree if you use the term "must'" what I find interesting is the question, is it possible that creatures with some resemblance to life forms on earth •might• arise on other worlds not because of common ancestry but as a result of similar engineering choices emerging independently as solutions to common problems. I would wonder about the likelihood of this happening, but never the necessity.
    Precisely. I could see something bipedal, perhaps even vaguely humanoid, if quadrupeds evolved on another world, because nature tends to re-purpose more than create outright, but I don't see them looking 'like humans' except in the vaguest sense. It's possible, it happened once after all, but 'must' and 'will'? That's a little anthrocentric.

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