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Rewinding the tape of life

Here’s another of those Dawkins questions.

Stuart Kauffman’s thought experiment: If evolution could be re-run 1000 times, would certain patterns predictably recur? Humanoids?

Only I already answered it a few weeks ago! I know that Dawkins is more sympathetic to the idea of evolutionary convergence than I am, but I don’t think you’d get recurrence of humanoids. If we could reset everything to the precise state the world was in during the Cambrian, half a billion years ago, I think it would be safe to say we’d get a planet full of molluscs and arthropods, but that’s about all we could say — tetrapods are not inevitable. If we could rewind to a few billion years ago, before the evolution of eukaryotes, we could talk about a planet of algae and bacteria, but other derived forms would be so contingent on chance that no prediction is possible.

The best examples of actually doing this experiment come from the Lenski lab and their work on bacteria, where generations could be frozen and restarted at will, and the answer is…no, it isn’t inevitable that a lineage will emerge that carries even an expected optimal simple biochemical pathway.

Comments

  1. says

    While you are undoubtedly right in general, it seems to me that there are some arguments that can rescue sci-fi, if not Star Trek. There are some common evolutionary tendencies, no? Cephalization is one of them — sensory organs tend to be near the front as the organism travels, obviously, because you want to understand territory as you enter it. And whatever you have for a brain therefore also tends to be near the front so that signals get there faster. If I’m not mistaken, camera eyes have evolved independently at least two or three times. There are three basic skeletons, hydrostatic, exo- and endo-. But if you want to be a big thing on land, you need an endo-skeleton, nothing else will work. We can probably come up with a few more such items, but anyway, if we boldly go where no-one has gone before, and we encounter large terrestrial creatures, they will likely have endoskeletons, and heads containing a brain and sporting camera eyes along with whatever other sensory organs they may have — which are likely to include chemical and sound detectors. Other than that, they probably would not look like Klingons, granted. And of course, if the endoskeleton never appeared in the first place, you would never get large terrestrial creature at all.

  2. David Marjanović says

    Cephalization is one of them — sensory organs tend to be near the front as the organism travels, obviously, because you want to understand territory as you enter it.

    They’re more in the middle of a cephalopod, though.

    If I’m not mistaken, camera eyes have evolved independently at least two or three times.

    Are you counting compound eyes? Because you should…

    But if you want to be a big thing on land, you need an endo-skeleton, nothing else will work.

    But there is no “want”.

    And of course, if the endoskeleton never appeared in the first place, you would never get large terrestrial creature at all.

    Exactly.

  3. says

    Yes, there are other kinds of eyes than camera eyes — but they’re the best, and evolution on earth does appear to get there pretty easily. And of course evolution doesn’t “want” anything but that’s not my point. If there is a phylum with an endoskeleton, that’s the one that’s going to end up dominating the land.

  4. says

    They’re the best? For what?

    I’d agree that they’re great for projecting an image on a sheet of cells, but other kinds of eyes seem to be better as, for instance, movement detectors. It depends on what you’re trying to do with your eyes.

    Also, it’s clear that there are physical limitations — Dawkins also has a question about using different wavelengths, and very long wavelengths would require much larger detectors, larger than might be physically possible for a small organism, in order to form an image. Sometimes your modality isn’t about image formation.

  5. says

    I also have to question this assertion: “If there is a phylum with an endoskeleton, that’s the one that’s going to end up dominating the land.” Except on earth, where the phylum with the exoskeleton, the arthropods, dominate terrestrial environments.

  6. David Marjanović says

    Yes, there are other kinds of eyes than camera eyes —

    My point was that each ommatidium of a compound eye is a camera eye.

  7. says

    But that would bring into question whether or not there is an element of randomness or chance in the world at all—essentially a debate between determinism and indeterminism. Just for the sake of asking, PZ, where do you stand on that issue? The preponderance of philosophical thought seems to lead to determinism (particularly compatabilist thinkers like Hume) and I can’t really see a good argument for indeterminism outside of the “spooky free will” sometimes derided by Blind Brain Theorists.

  8. moarscienceplz says

    The best examples of actually doing this experiment come from the Lenski lab and their work on bacteria, where generations could be frozen and restarted at will, and the answer is…no, it isn’t inevitable that a lineage will emerge that carries even an expected optimal simple biochemical pathway.

    I assume this refers to the citrate eating adaptation, which required two separate mutations. No details are provided here about the “rewind” experiment, but the original experiment involved only 12 flasks, only 1 of which developed the new adaptation, IIRC. So it was already observed that 11 out of 12 colonies did not produce the adaptation. Assuming that the “rewind” experiment was on a similar scale, failing to observe the mutations repeat is hardly a solid proof that it was a one-time, never-to-be-repeated fluke. Of course it is not “inevitable”. if I played 1 million hands of stud poker, the odds say I can expect about 14 straight flushes. Does that mean that it is “inevitable” that I will get one? No, of course not, but it would be surprising if it didn’t happen at least once in that million tries, and again in the next million tries.
    Flight evolved four separate times – insects, pterosaurs, birds, and bats (and it could be argued that things like sugar gliders are working on it again). So, if a niche is really worthwhile, it appears to me naive to state categorically that even though an adaptation happened once, it (or something similar) would never happen again.

  9. xavierninnis4191 says

    IMH-h.s. grad-O, the big if is how likely is it that eukaryotic cells will emerge? Were they a virtual one off? It seems to me that it’s only once they, eukaryotes, emerge, that one can begin to make even semi-plausible estimates/guesses.

  10. Buford Dillman says

    PZ, does this mean that you do not subscribe to material determinism? You say “If we could reset everything to the precise state the world was in during the Cambrian…”. Well, if by that you mean that every atom was in the exact same place with all the exact same properties (spin, direction of movement, etc.) as it actually was at a particular point in time half a billion years ago, then why would you think things wouldn’t develop in exactly the same way they actually have? I am neither a Physicist nor a Philosopher but with the limited understanding that I have of those fields I find myself agreeing with determinism based on the notion that the universe is governed by the unchangeable laws of physics acting upon matter. Reading this post it seems like you don’t agree with that. Could you elaborate on that a little?

  11. Rich Woods says

    @Buford #11:

    I am neither a Physicist nor a Philosopher but with the limited understanding that I have of those fields I find myself agreeing with determinism based on the notion that the universe is governed by the unchangeable laws of physics acting upon matter.

    At the quantum level, the laws of physics are not deterministic but stochastic. That leaves an awful lot of percentage chances to be reproduced after the reset.

  12. says

    Yes, the arthropods dominate in terms of species number. I was being imprecise. But the vertebrates are the only animals that can get big. They dominate in terms of being noticeable to animals on our scale, and they’re the only phylum that can get big enough to be intelligent and create a technological civilization. That was my point.

    As for the camera eye, it’s best for the purpose of fine resolution. Arthropod eyes are good motion detectors, but I think it’s a fair claim that camera eyes are superior for most plausible niches and the main point is that evolution has produced remarkably similar camera eyes more than once. The argument is that there may be structures and schemes of organization that are more likely than others to emerge in any rerun of evolution. Things might tend to look similar in many ways, it’s not just anything goes. I don’t think that’s an unreasonable claim.

  13. francesco says

    Even at the level of bipedal, warm blooded vertebrates with functioning hands and binocular vision, human level intelligence is not inevitable. The theropod dinosaurs had all that and came in every size between elephant and crow, they lived in diverse environments including trees. They were as parallel , to early hominids, as it is reasonable to expect , but no theropod humanoid ever evolved.

  14. Crimson Clupeidae says

    At the quantum level, the laws of physics are not deterministic but stochastic. That leaves an awful lot of percentage chances to be reproduced after the reset.

    These types of scenarios can be (sort of) replicated rather quickly with a computer simulation (simple rules that provide stochastic, but not deterministic results) and show that while the outcome of following some given rules will end within a range, that end won’t always be the same, or even all that similar.

  15. krisrhodes says

    I think the balance of evidence is you are wrong.

    Lenski? How long is that, 40 yrs?

    Aruing that the lenski evidence means that the odds against complex derived forms with billions to years to work with?

    That’s like the creationist claims that you can’t walk a mile by taking a step at a time. Completely off base and ridiculous. “I don’t know” is a much better answer than what you present here given knowledge of change over time we have right now.

  16. Ichthyic says

    I think the balance of evidence is you are wrong.

    who?

    Aruing that the lenski evidence means that the odds against complex derived forms with billions to years to work with?

    who did that?

  17. David Marjanović says

    The preponderance of philosophical thought seems to lead to determinism (particularly compatabilist thinkers like Hume)

    Why do you speak of Hume in the present tense?

    Whether there is true random in the world has long ceased to be a subject philosophers can idly speculate about as if it were impossible to test their ideas by observation of the real world. A hundred years ago it passed into the realm of physics, of science. The outcome so far is that if you want to explain the world in a deterministic way, you need to bend over backwards and twist into pretty impressive knots…

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