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What's the probability of a similar evolution on a different planet?

  1. Feb 24, 2014 #1
    Although I have taken Discrete Mathematics I am really bad with probability and would appreciate your insight in this.

    I had a discussion with a friend about aliens. So the question that arose is what is the probability that aliens on a different planet somewhere look similar to an animal species on earth. For example: What's the probability that there are space bears on planet X that are very similar to earth's bears?

    According to my friend this is similar to finding the probability that 2 people(~2 evolutions) roll the dice and both of them get the same numbers each time. In that case the question could really then be "What's the probability that if both people throw the dice 3 times, they will get the same numbers each time?". Is this approach correct?
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  3. Feb 24, 2014 #2

    Simon Bridge

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    Say Alice and Bob roll the dice and they notice that the numbers match up.
    That would be the same odds as one person rolling three doubles in a row - (1/36)(1/36)(1/36)=1/46656
    People win lotteries with much longer odds all the time - so by your friends own argument, and taking into account the size of the Universe, the situation you are thinking of is certain!

    But really your friend has just plucked a number out of the air.

    The approach is not correct - because the die rolls are independent and we don't know that the existence of us does not mean the existence of another like us is more likely. Maybe it's more like cockroaches: if you see one, there must be lots!

    To treat the matter more seriously:
    You have to define "similar", and then look at the data we have to see what affects the course of evolution.

    We only have empirical evidence of one evolving system of life - this one.
    We don't know for sure that this system is typical - but we have no particular evidence that the course of evolution here is not typical either. We are still gathering data on that one - but we can make do with what we have and see what that indicates so far.

    In Earth's history, there have been a series of major extinction events which have had a large impact on the course of evolution. Considering the biodiversity limiting effect (look up "Cambrian explosion" for example) it is easy to imagine that these events could have selected quite a different set of body-plans, say, therefore, setting evolution on a wildly different course.

    However - also see "parallel evolution".
    It is also reasonable that similar ecological niches can be filled by similar creatures.

    This is where the definition of "similar" comes in.
    Would an Australian Marsupial Wolf be counted as "similar" to the Timber Wolf for the purposes of this debate? You can look them up to see what I'm talking about. Perhaps, if Australia had been left alone for longer there may have been marsupial versions of homonids? Bottom line: we don't know.

    Even with radically different body-plans surviving an extinction event, biodiversity often returns to the pre-extinction levels. It may well have been, had events come out differently, that there still would be upright bipeds occupying the position of "top intelligence" on earth.

    The possibility of parallel evolution with the limits of physics suggest that intelligent, self aware, life forms, who similar enough to modern Earth Humans that we would say "gosh they are so amazingly like us aren't they?" are more likely that we would initially expect. It certainly means that not just anything can happen. It's probably much more likely than, say, giant intelligent amoebas.

    Still - I'd put it as less likely than two people rolling a die three times and getting the same sequence of numbers.

    Something else important to remember is the size of the Universe.
    Even a very small chance event can happen given a very large number of opportunities to happen.
    The chance of a star forming/ending up in a particular volume of space would have been pretty small - but there is a LOT of space so it is certain that stars will appear somewhere.

    One of the things we have learned in science over the years is that the way to bet is that we are not unique or special. i.e. the Earth is not the center of the Universe, the galaxy, or even the Solar system. Even the Earth and Moon are best described as orbiting each other (about their common center of gravity) rather than the Moon orbiting the Earth. We, ourselves, are another kind of animal. And so on. Basically so many of the ways that humans have counted themselves unique or special or "chosen" through history have turned out to be wrong in some important way - the rest remaining untested - that the smart money is on the existence of intelligent life "out there" that is like us in some important way.

    It's also on us not finding it any time soon.
    Last edited: Feb 24, 2014
  4. Feb 26, 2014 #3


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    The same factors that drive evolution on earth -- who survives and who becomes extinct, makes this very different from a random process. It makes similarity much more likely than random changes in genes would.
  5. Feb 26, 2014 #4

    Simon Bridge

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    Well said. It's one thing to say that things happen by chance and something else to say the outcome is totally random.

    On Earth, broad similarity is the norm for organisms sharing a way of making a living.
    The oft noted massive diversity is in the ways of making a living - but even those are restricted by physics.

    "The chances that life just occurred are about as unlikely as a typhoon blowing through a junkyard and constructing a Boeing 747."
    -- Chandra Wickramasinghe (Astronomer as expert witness in court, in Arkansas, in 1981)​
    A somewhat better analogy would be starting with a million junkyards, painstakingly testing the wreckage left in each one after the tornado to find the most flight worthy, making a million exact copies of that junkyard, unleashing another million tornadoes, running another series of exhaustive tests, and so on, until you produce some kind of machine - no matter how crude and un-Boeing-747-like - capable of flying at least a few yards.
    -- Micheal Le Page in New Scientist

    ... after more refinement, any resulting contraptions (there would be many) would look quite similar - there being few body-plans that would be able to fly at all let alone well.

    The "evolution can't work because of random" argument is one of the more pervasive misunderstandings. Lots has been written on it. The New Scientist article is not bad.
    However, there are a lot of very bad articles on this subject as well.

    A good place to start is a primer on probability and statistics. It is very difficult to navigate the mess of straw-men and false dichotomies etc without some grounding in probability. Arm yourself.
    I usually suggest John Paulos' "Innumeracy" to start with - there's probably more recent equivalents - rather than some dry text book.
  6. Feb 27, 2014 #5
    .. It's approximately slim. Increase in complexity = Increase in sensitivity. Directional possibilities still outruns fragile sensitivity and can greatly affect the temporal phases(evolution) of any complex structure. Even if we have a 2 identical separated system that can harbor it's own carbon based life. The slightest variation of condition in a system can change the structure dramatically. The only way that it can be somewhat similar is -- if we can have 2 identical system that somehow temporarily meet at one time in it's course of evolution and have the same variation, rates and conditions. E.g In simple carbon structure such as diamond. We might say that it is identical to any other diamond but may vary in size color shape depending on the condition-- of which can be lacking to the other system even if it is identical for most of its part. And as complexity extends the dials are more complicated and sensitive. It can adapt, break and repair and try to be stable which led to variation of changes.
  7. Feb 27, 2014 #6

    Simon Bridge

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    Seeing as how they only need to be similar to any old animal species on Earth - the odds look a bit mor elikely for the similarity. The more interesting question would be "how similar?"

    You can get similarity between quite disparate species - re: Octopus eye-structure compared to human or the kind of similarity you get from parallel or convergent evolution - which is striking from a distance.
    an accessible discussion: http://evolution.about.com/od/Evolution-Glossary/a/Analogous-Structures.htm

    But I don't think you'd get the Vargr from Traveller, or the telepathic hi-tech houscats as in The Cat from Outer Space - which may be more in the context of the question (SF) than the evolution vs creationism debate that we usually see.

    I still think Pythikos (OP) needs to define "similar" and provide the context of the discussion.
    Without that we cannot really answer the question properly.
    Last edited: Feb 27, 2014
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