4 ms·

Personally I think the idea that life/prebiotic soup came to earth on a comet is suspect, for the reasons explained in these blog posts and the links therein:

• nitrogen 12y ago
  It's possible that there's no reason life chose L or D isomers, but choosing one and sticking to it was advantageous (I use the word "choose" loosely here).
  • bduerst 12y ago
    Abiogenesis is one of those things where there is way more speculation than evidence. Some speculation says that it is because the molecular machinery maintenance to handle both would be twice as taxing for organisms. Of course, having the reverse isomer molecules would be advantageous against predators (who couldn't digest it) and a boon to natural selection. Like I said, plenty of speculation to be had.
    • SixSigma 12y ago
      Not being digested doesn't stop you getting killed.
      • deleted 12y ago
        [deleted]
      • bduerst 12y ago
        By predators it does. Unless you're chomping on some raw grass right now -> then I stand corrected.
        • SixSigma 12y ago
          I just ate some bread
          • deleted 12y ago
            [deleted]
          • bduerst 12y ago
            Sounds digestible and non-sequitur.
            • delecti 12y ago
              I believe what they're getting at is that it's possible to do things to other species which leaves them digestible when they weren't initially. For example, human cooking, bird gizzards, cows chewing their cud, or rabbits eating their feces to give a second pass of digestion.
              • bduerst 12y ago
                You talk about cooking meat, but we still have enzymes in our stomachs (e.g. pepsinogen) that only digest L-peptide chains. As a predator, you do not digest D-peptide chains. - It is analagous to eating raw grass. - It is not analgous to eating processed/cooked grains, hence it is non-sequitur. - It is a matter of biochemistry, not culinary arts. I gave the original speculation as an example of there being too much speculation around abiogenesis and evolution. This discussion proves it.
        • SixSigma 12y ago
          A few points I also eat plenty of insoluble dietary fibre. Prey animals would need to signal their non digestability to their predators. Animals without predators would overwhelm the biome. Disease is a predator.
      • aaronem 12y ago
        It renders predation on you maladaptive, because the predator doesn't get back the energy expended in hunting, so over time it does actually make you less likely to get killed.
        • delecti 12y ago
          And then something evolves that can eat you, because there's an unfilled niche.
          • bduerst 12y ago
            This is a common misconception. Evolution is directionless and unguided. It's not like a startup, under the guidance of humans, pivoting for a niche market.
            • delecti 12y ago
              You're correct, and I didn't mean to make it seem as though it was an intentional process. But historically, when one aspect of the ecosystem goes unchecked, something usually comes along to take advantage of plentiful biomass, one notable exception being humans.
              • eru 12y ago
                We have lots of rats and roaches. Not directly feeding off our bodies, but on our extended phenotype.
                • Joeri 12y ago
                  Don't forget domesticated cats. It's still a wild animal, but one that has adapted itself to live off humans in quite clever ways (essentially by mimicking human children).
                  • eru 12y ago
                    Great point!
    • acomar 12y ago
      > Of course, having the reverse isomer molecules would be advantageous against predators (who couldn't digest it) and a boon to natural selection. This, of course, doesn't really mean much. Something being advantageous doesn't imply that it must show up as a product of natural selection nor does it imply that such an adaptation must survive into perpetuity.
      • darkmighty 12y ago
        This is the kind of change you really don't expect to have an evolutionary pathway. Random genetic changes to genes won't suddenly start a full system of reverted isonomers out of the blue, and there's way to slowly get there (it's negative all the way through than a little positive afterwards).
      • bduerst 12y ago
        I gave it as an example of there being too much speculation. There is absolutely no evidence for it and it is not meant to be taken as as serious as some of you are taking it. If anything, it proves there is too much speculation -_-
    • refurb 12y ago
      Of course, having the reverse isomer molecules would be advantageous against predators (who couldn't digest it) and a boon to natural selection. Why would the reserve isomer not be digestible? The human body can metabolism racemic amino acids.
    • simonh 12y ago
      > advantageous against predators Only until your species divergently evolves and one of the descendent species evolves into predators. Then it's game on again! Then again, if reverse isomers = indigestible, what is your species going to eat? I suppose this strategy might work for plants.
  • olefoo 12y ago
    It could be a 'frozen accident', where either one works but once a positive feedback loop starts one version crowds out the other.
• madaxe_again 12y ago
  Asimov wrote a great essay on this - see A5 in this. http://www.arvindguptatoys.com/arvindgupta/asimov-electron.pdf http://www.arvindguptatoys.com/arvindgupta/asimov-electron.p...
• pjungwir 12y ago
  I confess ignorance about this theory, but I see it a lot and I don't understand. It just seems bizarre to think that life began on a comet. Sure, we want to know the origin of life. What does pushing it off the Earth solve? I guess my question is, What are the positive reasons for thinking life arrived from a comet? I can't think of any. As a layman, the theory is so strange I'm not even sure why it needs to be refuted. Can someone explain why this idea is popular?
  • eru 12y ago
    One reason for its appeal is that the earliest fossils / signs we have of life on earth are pretty close to the ground not longer being molten lava. An origin of life in comets would give more time.
    • malka 12y ago
      Well, the first life that existed was maybe some kind of rna-like molecule with very little to no protection. Not even a cell. I do not think any of these could leave any traces, even a fossil.
      • eru 12y ago
        Yes. And that puts the origin of life even closer to the molten lava stage.
  • gizmo686 12y ago
    I see it more along the lines of being a hypothesis. We do not have nearly enough evidence to put it off the table, even though it is not the most likely explanation.