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What if it is extraterrestrial in origin but also exists on earth (originally coming from elsewhere)? How would they know the difference then?

antonvs 9y ago
If the alien organism originally evolved independently of life on Earth, then analyzing its genetic material - its equivalent of DNA - would tell us a lot. We haven't yet found any organisms on Earth that have evolved independently, so an organism found in space that has a different genetic structure from Earth life would be a good bet for being alien in origin. An alternative, albeit unlikely, possibility is that Earth life was originally seeded from somewhere else - panspermia, https://en.wikipedia.org/wiki/Panspermia https://en.wikipedia.org/wiki/Panspermia. In that case, Earth life and (some) alien life would have a common origin, and we'd have to examine the differences in its DNA to find out more about its history. Even in that case, it ought to be possible to determine that the organism had branched off from Earth life long enough ago to make it a candidate for alien life.
- wil421 9y ago
  If comets supposedly brought us water why not life? I just read an article yesterday about an asteroid from another solar system[1]. Maybe there was a Prometheus-esque comet with some single celled life in it. [1]http://time.com/5033026/mysterious-object-confirmed-another-solar-system/ http://time.com/5033026/mysterious-object-confirmed-another-...
  - antonvs 9y ago
    Is is possible? Yes. Is it likely compared to the alternative? No. Comets are theorized to have brought us water from within the solar system, so the comparison doesn't really apply unless life evolved in our solar system but somewhere other than Earth. Here's some of what has to happen for life to cross naturally between planets in different solar systems: 0. Life evolves somewhere else 1. Somehow, some of that life ends up on an asteroid heading out of that solar system. The mechanism for this is pretty iffy. In the case of the ISS, we took those bacteria with when we launched, and they still haven't left Earth's gravity well. 2. That life survives the multi-millennia trip to another solar system, on an airless rock exposed to interstellar radiation for millennia. 3. The asteroid with life on it happens to hit a planet that can support that life. 4. The life isn't all destroyed in the blazing fireball caused by its descent through an atmosphere. Each of those events has a probability, most of which are very low. To get the probability of the overall chain of events, multiply all the probabilities together, giving a much lower probability. For life to evolve on Earth, all that's needed is step 0. Therefore, without even trying to assign actual numbers, we can see that the probability of life having originally evolved on Earth is far higher than it having come from somewhere else.
    - JoeAltmaier 9y ago
      Bad conclusion. There are billions of 'other planets' and only one Earth. You'd have to add up the marginal probabilities of all the other scenarios, to compare with just the Earth scenario.
      - AnimalMuppet 9y ago
        But of those billions of other planets, as they get further and further away, the probability of an asteroid from them finding Earth goes down. So does the probability of life on the asteroid surviving the trip. I think that the number of planets within a distance R is O(R^2), but the probability of an asteroid from such a planet reaching us is O(1/R^2). But if the probability of life on the asteroid surviving the trip decreases with a longer trip, the net effect is probably that the further-away planets contribute little to the probability of life reaching us in this way.
        JoeAltmaier 9y ago
        The panspermia idea is, that it started somewhere and somewhen, and has been planet hopping ever since. We only need one nearby life-bearing planet to seed earth; it got seeded from further away and so on. Similar to how ancient Romans wore silk but didn't have trade with China. It only took each trader to trade with their neighbor, and silk could travel thousands of miles.
        AnimalMuppet 9y ago
        OK, but at that point, the "billions of planets out there" argument doesn't work. You're left with the probability of it transferring from a nearby planet.
        JoeAltmaier 9y ago
        Right, and the inevitability of it being present in a nearby planet went way up. Because once it started somewhere (and there are billions of 'somewheres') it would spread like a virus and infect every possible receptive environment. The statistics is hard. No simple counting of planets is enough. All the vectors have to be accounted for.
- greggarious 9y ago
  >An alternative, albeit unlikely, possibility is that Earth life was originally seeded from somewhere else - panspermia, https://en.wikipedia.org/wiki/Panspermia https://en.wikipedia.org/wiki/Panspermia. I've seen this hypothesis before. I've also seen the oft repeated assumption that other intelligent life might be out there, more advanced than ours. Crazy though: what if we are the most advanced life in th universe?
  - dashoffset 9y ago
    https://en.wikipedia.org/wiki/Fermi_paradox#Intelligent_alien_species_lack_advanced_technology https://en.wikipedia.org/wiki/Fermi_paradox#Intelligent_alie...
  - PeterisP 9y ago
    If the probabilities are such that it's feasible that we're the first/most advanced life in the universe, then (given our planets comparably young age) it's exceedingly likely that we're the only life in the observable universe. It would be plausible for us to be alone, and it would be plausible for us to be one of the less advanced lifeforms among many, but it would be quite surprising if we'd to be the most advanced civilization among many; since many/most of them would have gotten a significant head start.
    - greggarious 9y ago
      Did they get a head start though? All these atoms bouncing around started with the big bang after all :) One theory I'm fond of is that we are not alone, but life is so rare that the distances between intelligent civilizations are too vast for us to ever observe them (or vice versa).
      - PeterisP 9y ago
        Many, many other planets had conditions suitable (as far as we understand what's suitable) for creating life when our planet didn't exist yet; out of all feasible planets we're younger than average, so others would have had a head start.

antonvs 9y ago

If the alien organism originally evolved independently of life on Earth, then analyzing its genetic material - its equivalent of DNA - would tell us a lot. We haven't yet found any organisms on Earth that have evolved independently, so an organism found in space that has a different genetic structure from Earth life would be a good bet for being alien in origin. An alternative, albeit unlikely, possibility is that Earth life was originally seeded from somewhere else - panspermia, https://en.wikipedia.org/wiki/Panspermia https://en.wikipedia.org/wiki/Panspermia. In that case, Earth life and (some) alien life would have a common origin, and we'd have to examine the differences in its DNA to find out more about its history. Even in that case, it ought to be possible to determine that the organism had branched off from Earth life long enough ago to make it a candidate for alien life.

wil421 9y ago
If comets supposedly brought us water why not life? I just read an article yesterday about an asteroid from another solar system[1]. Maybe there was a Prometheus-esque comet with some single celled life in it. [1]http://time.com/5033026/mysterious-object-confirmed-another-solar-system/ http://time.com/5033026/mysterious-object-confirmed-another-...
- antonvs 9y ago
  Is is possible? Yes. Is it likely compared to the alternative? No. Comets are theorized to have brought us water from within the solar system, so the comparison doesn't really apply unless life evolved in our solar system but somewhere other than Earth. Here's some of what has to happen for life to cross naturally between planets in different solar systems: 0. Life evolves somewhere else 1. Somehow, some of that life ends up on an asteroid heading out of that solar system. The mechanism for this is pretty iffy. In the case of the ISS, we took those bacteria with when we launched, and they still haven't left Earth's gravity well. 2. That life survives the multi-millennia trip to another solar system, on an airless rock exposed to interstellar radiation for millennia. 3. The asteroid with life on it happens to hit a planet that can support that life. 4. The life isn't all destroyed in the blazing fireball caused by its descent through an atmosphere. Each of those events has a probability, most of which are very low. To get the probability of the overall chain of events, multiply all the probabilities together, giving a much lower probability. For life to evolve on Earth, all that's needed is step 0. Therefore, without even trying to assign actual numbers, we can see that the probability of life having originally evolved on Earth is far higher than it having come from somewhere else.
  - JoeAltmaier 9y ago
    Bad conclusion. There are billions of 'other planets' and only one Earth. You'd have to add up the marginal probabilities of all the other scenarios, to compare with just the Earth scenario.
    - AnimalMuppet 9y ago
      But of those billions of other planets, as they get further and further away, the probability of an asteroid from them finding Earth goes down. So does the probability of life on the asteroid surviving the trip. I think that the number of planets within a distance R is O(R^2), but the probability of an asteroid from such a planet reaching us is O(1/R^2). But if the probability of life on the asteroid surviving the trip decreases with a longer trip, the net effect is probably that the further-away planets contribute little to the probability of life reaching us in this way.
      - JoeAltmaier 9y ago
        The panspermia idea is, that it started somewhere and somewhen, and has been planet hopping ever since. We only need one nearby life-bearing planet to seed earth; it got seeded from further away and so on. Similar to how ancient Romans wore silk but didn't have trade with China. It only took each trader to trade with their neighbor, and silk could travel thousands of miles.
        AnimalMuppet 9y ago
        OK, but at that point, the "billions of planets out there" argument doesn't work. You're left with the probability of it transferring from a nearby planet.
        JoeAltmaier 9y ago
        Right, and the inevitability of it being present in a nearby planet went way up. Because once it started somewhere (and there are billions of 'somewheres') it would spread like a virus and infect every possible receptive environment. The statistics is hard. No simple counting of planets is enough. All the vectors have to be accounted for.
greggarious 9y ago
>An alternative, albeit unlikely, possibility is that Earth life was originally seeded from somewhere else - panspermia, https://en.wikipedia.org/wiki/Panspermia https://en.wikipedia.org/wiki/Panspermia. I've seen this hypothesis before. I've also seen the oft repeated assumption that other intelligent life might be out there, more advanced than ours. Crazy though: what if we are the most advanced life in th universe?
- dashoffset 9y ago
  https://en.wikipedia.org/wiki/Fermi_paradox#Intelligent_alien_species_lack_advanced_technology https://en.wikipedia.org/wiki/Fermi_paradox#Intelligent_alie...
- PeterisP 9y ago
  If the probabilities are such that it's feasible that we're the first/most advanced life in the universe, then (given our planets comparably young age) it's exceedingly likely that we're the only life in the observable universe. It would be plausible for us to be alone, and it would be plausible for us to be one of the less advanced lifeforms among many, but it would be quite surprising if we'd to be the most advanced civilization among many; since many/most of them would have gotten a significant head start.
  - greggarious 9y ago
    Did they get a head start though? All these atoms bouncing around started with the big bang after all :) One theory I'm fond of is that we are not alone, but life is so rare that the distances between intelligent civilizations are too vast for us to ever observe them (or vice versa).
    - PeterisP 9y ago
      Many, many other planets had conditions suitable (as far as we understand what's suitable) for creating life when our planet didn't exist yet; out of all feasible planets we're younger than average, so others would have had a head start.