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Thread: Would common ancestry occur everywhere life evolves?

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    Would common ancestry occur everywhere life evolves?

    Here on Earth evidence strongly suggests all present life shares a common ancestor. I wonder whether this is mere coincidence, or should common ancestry be necessary for life to evolve beyond a certain complexity level?
    I.e.: is there an evolutionary advantage to sharing common ancestry?

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    First, we need to take note of a vital caveat. Our knowledge of biological systems and 99.99999% of speculation concerning alternative biological systems is based entirely upon the single example of life that we have. Note how different extra-solar planetary systems turned out to be from our expectations based upon our own system. Put another way, extrapolating from a sample size of one can be entertaining, but it is questionable science.

    If life arose more than once, then at one point there would be multiple lines/families, all members of which shared a common ancestor, but were unrelated to any and all other lines. Such may have been the situation on the early Earth. A soundly argued case has been made for this in the last decade, and if pressed, I can try to find the relevant citation. With multiple lines present we might well expect evolutionary pressures to favour one line over others so that in a short time only the fittest (in evolutionary terms) survived, leaving us three and a half billion years later contemplating plausible alternatives.

    One such alternative is that the origin of life is sufficiently difficult that it only arose a single time and hence all subsequent has it as the original common ancestor.

    Or there were multiple origins and the others were eliminated not by evolution, but by bad luck. Keep in mind early life seems to have appeared around the time of the Late Heavy Bombardment. Not a benign environment.

    To answer your question as you finally phrased it, I see more of an evolutionary advantage to multiple lines since that would provide more ways of tackling opportunities and would create more niches. The ability/inability of one line to feed upon an alternate line could work both ways.

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    I think you need to turn the problem on its head.

    Not having a common ancestry requires that life evolved twice on the same planet, at the same time.
    The two types would be competing all the time, which would make it very unlikely that both would prevail to some degree of complexity. They would have to evolve or at least procreate at exactly the same rate.

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    Life tends to modify the environment to suit itself. Thus it changes the environment from one in which it arose, and other forms presumably could, to one in which the current life has a competitive advantage that might preclude some other life from arising.

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    Quote Originally Posted by DaveC426913 View Post
    I think you need to turn the problem on its head.

    Not having a common ancestry requires that life evolved twice on the same planet, at the same time.
    The two types would be competing all the time, which would make it very unlikely that both would prevail to some degree of complexity. They would have to evolve or at least procreate at exactly the same rate.
    Good answer.

    One could imagine a world with two completely separate oceans (or landmasses) where life could start and then evolve independently until at some point they encountered one another. They would then affect each others further evolution (even if their biology was so different that they could not eat each other's food sources, they would compete for space, light, water, etc).

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    Quote Originally Posted by transreality View Post
    Life tends to modify the environment to suit itself. Thus it changes the environment from one in which it arose, and other forms presumably could, to one in which the current life has a competitive advantage that might preclude some other life from arising.
    I kind of doubt that. I think more often another organism evolves or arises to take advantage of the changes that one organism has brought about. A good example is the oxygen catastrophe.


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    Quote Originally Posted by DaveC426913 View Post
    Not having a common ancestry requires that life evolved twice on the same planet, at the same time.
    The two types would be competing all the time, which would make it very unlikely that both would prevail to some degree of complexity. They would have to evolve or at least procreate at exactly the same rate.
    It seems pretty unlikely, but I donít see why it would be impossible for life to arise twice on the same planet, say in two small seas separated by a large distance, and the two lineages to coexist.


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    Quote Originally Posted by Jens View Post
    It seems pretty unlikely, but I don’t see why it would be impossible for life to arise twice on the same planet, say in two small seas separated by a large distance, and the two lineages to coexist.
    I agree. But I assume that if they encountered each other quite early on (before they had diversified into a wide range of different organisms - plants, animals, etc for example) then one (the more "advanced"?) would quickly destroy the other. So it seems possible that abiogenesis could have happened multiple times but the new simple organisms would just be eaten by other, existing, organisms around them.

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    what if the first life that arose in 2 or more places, was compatible, I mean to interbreed, or at east share DNA....at the single cell stage.....maybe it arises quite often in the same way, at the nuts and bolts stage..
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    Quote Originally Posted by WaxRubiks View Post
    what if the first life that arose in 2 or more places, was compatible, I mean to interbreed, or at east share DNA....at the single cell stage.....maybe it arises quite often in the same way, at the nuts and bolts stage..
    No, even small differences make interbreeding or sharing DNA impossible, even when there is shared ancestry, so short of some kind of intelligent design I would say itís basically unthinkable.


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    Quote Originally Posted by Jens View Post
    Originally Posted by WaxRubiks
    what if the first life that arose in 2 or more places, was compatible, I mean to interbreed, or at east share DNA....at the single cell stage.....maybe it arises quite often in the same way, at the nuts and bolts stage..
    No, even small differences make interbreeding or sharing DNA impossible, even when there is shared ancestry, so short of some kind of intelligent design I would say itís basically unthinkable.
    I don't know... I'm thinking it.

    I would not go as far as saying unthinkable or impossible. It depends on how different they were and how many different ways there are to create a lifeform. For example, one idea for very early life (almost pre-life) is basically a "cell" with a lipid membrane and some RNA inside of it, that acts both as the holder of the genetic material and as the enzyme for chemical reactions to make more of it. There might not be that many ways of make such a "lifeform" (I put it in quote because some may not consider that actually alive). Maybe the two different versions only have some small differences in the their RNA sequence, but if they meet, they are compatible.

    I don't know which is harder to imagine, that there was a single event where life formed on Earth and that was it, or that when the conditions were right it happened in many places around the same time on Earth. I can make arguments against either of those scenarios (it is always easier to argue against a particular scenario, then to prove it).

    It all comes down to the questions how easy/hard is it to make life, and how many different ways are there to do it. We don't know the answer to either of those.
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    Quote Originally Posted by Swift View Post
    I don't know which is harder to imagine, that there was a single event where life formed on Earth and that was it,
    It is more palatable when you consider how a geologic time scale plays into it.

    Say, abiogenesis can happen multiple times, but the complexity - the delicacy - of its first stages means it might take an average of a half billion years of being left alone in its primordial goo.
    So, it arose the first time and that one had a half billion years to practice at survival.

    Now a second type might get started, but how far would it get? It won't have whole oceans to develop in, and it won't have a half billion years of unfettered access to resources.

    Any first foothold life gets - provided it expands fast enough - will strangle subsequent forms to death.

    From our 4.5Gy distance, we see only one instance of life arising, but that's not really the case is it?
    Last edited by DaveC426913; 2019-Feb-15 at 12:06 AM.

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    With multiple lines present we might well expect evolutionary pressures to favour one line over others so that in a short time only the fittest (in evolutionary terms) survived
    One such alternative is that the origin of life is sufficiently difficult that it only arose a single time
    My gut feeling gravitates toward the first quote as much as it reaches instantaneous escape velocity away from the second. I like to believe that if life were a gambler, it's consistent strategy would be to go all in on the power of abundance. So I'd say if life emerges, it does so in several variants. But indeed: it's just the gut talking.

    I think you need to turn the problem on its head. Not having a common ancestry requires that life evolved twice on the same planet, at the same time.
    The two types would be competing all the time, which would make it very unlikely that both would prevail to some degree of complexity. They would have to evolve or at least procreate at exactly the same rate.
    This observation I find useful. Though I'd think (or rather: my gut thinks) that life didn't commence twice, but numerous times. This reminds me of this interactive graphic I once found that explained complex relationships. The graphic featured a meadow, a flock of sheep, and a pack of wolves. By entering parameters such as the growth rate of the grass and the number of offspring in the sheep and wolves, the graphic then showed how those parameters affected this ecosystem through a number of generations. This though, reveals another aspect: if (some of) those different life forms apart from competing for nutrients also fed on each other, their mutual relationship would become complex to the point that even the least evolutionary fit could be the one making it through. Or is my imagination running wild, now?

    Life tends to modify the environment to suit itself. Thus it changes the environment from one in which it arose, and other forms presumably could, to one in which the current life has a competitive advantage that might preclude some other life from arising.
    I kind of doubt that. I think more often another organism evolves or arises to take advantage of the changes that one organism has brought about. A good example is the oxygen catastrophe.
    Couldn't it be both? The evolutionary fittest lifeform transforming the environment to its advantage until certain thresholds are passed, so that the longer it keeps to its strategy the more disadvantaged it becomes? Could climate change, in a way, be seen as example of such dynamic?

    No, even small differences make interbreeding or sharing DNA impossible, even when there is shared ancestry, so short of some kind of intelligent design I would say itís basically unthinkable.
    This comment sends me leapfrogging to the symbiogenesis theory, where an organism is engulfed by, and becomes part of, another with a partial DNA transportation from the guest to the host as result. Definitely going wild here, but might different strands of life once have played such tricks on each other, but this so long ago that nothing of the guest remains? Impossible to know, right?

    I don't know which is harder to imagine, that there was a single event where life formed on Earth and that was it, or that when the conditions were right it happened in many places around the same time on Earth. I can make arguments against either of those scenarios (it is always easier to argue against a particular scenario, then to prove it).
    In so far knowing is concerned: spot on.

    Any first foothold life gets - provided it expands fast enough - will strangle subsequent forms to death.
    Hmm. That, I'd say, is the logical conclusion. But how much about life's emergence is logical...?

    But I reckon the matter remains inconclusive. Thanks a million for sharing your thoughts, though. And I'll be on the lookout for any further insights!

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    Quote Originally Posted by gufghur View Post
    Couldn't it be both? The evolutionary fittest lifeform transforming the environment to its advantage until certain thresholds are passed, so that the longer it keeps to its strategy the more disadvantaged it becomes? Could climate change, in a way, be seen as example of such dynamic?
    Don't forget there are (at least) two evolutionary strategies.
    Specialists - like Koalas and Cheetahs - have a well-defined nichethat works very well during ecologically stable epochs. Their food source - and thus their survivial techniques - change little over long periods.
    Generalists - like humans and rats - are opportunists, and are very adaptable in terms of food sources and survival techniques. They do very well during times of ecological instability.

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