Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

No, this is not a question for biologists.

  1. Sep 8, 2004 #1
    I see four possibilities for how life got started on earth, listed in descending probability.

    1. Genesis of life occurred on earth.

    2. Genesis of life on another planet (probably outside of the solar system). Followed by transport of DNA to earth by natural means. This possibility becomes more likely if it turns out that the genesis of life is rare.

    3. Accidental seeding of life on earth by an extraterrestrial probe.

    4. Purposeful seeding of life on earth. I consider this possibility to be extremely remote if for no other reason than it was a royally botched job. (unless it doesn't matter to the seeder that the process would take 4 billion years).

    Before the advent of telescopes that will allow us to detect extraterrestrial life, does anybody care to make any predictions about what we will find about the distribution of life in the galaxy, given each of the 4 cases? I am not asking just about spacial distribution, but also distribution varying by type of planet (or even type of star).

    Yes, we have no data (other than negative SETI results), so this is not science. More of a logic puzzle where you will have to wait 4 or 5 decades to find out if you are right.
  2. jcsd
  3. Sep 8, 2004 #2


    User Avatar
    Gold Member

    I pick #1.

    Over the time scales that our planet has existed, there were plenty of oportunities for compex organic entities to develop. The first organic entities would dominate, since there would have been NO competing entities exploiting the same energy potientials. We should expect that the same DNA that we regard as "ours" may be traceable to one-celled animals that arose to exploit the environment in these early eras. Of course, YMMV.
  4. Sep 8, 2004 #3
    Poke, jab, jab, poke

    :smile: I agree, but you didn't answer the question :smile:
  5. Sep 8, 2004 #4
    Given that I believe scenario #1 is most likely, I would expect the distribution of life through out the universe to be random, and have no other pattern other to be heavier in places were earth type stars(and indeed any star type conducive to life)are most prominent.
  6. Sep 8, 2004 #5


    User Avatar
    Science Advisor
    Gold Member

    Given the evidence, #1 seems most probable. Evidence of development of the self-replicating DNA triplet code ("life"), dates back as far as 3.5 billion years ago. The crust of the earth did not solidify and oceans form until 3.8 billion years. That does not leave much time for 'seeding', at least from outside the solar system. It is, however, possible life may have first originated on mars and the earth was 'seeded' by planetesimal collisions involving mars. Those occured within a compatible time frame.
  7. Sep 8, 2004 #6


    User Avatar
    Science Advisor
    Gold Member

    It didn't take 4 billion years unless you mean seeding for inteligent life. Life formed almost instantly once the Earth cooled from its formation, and about 3 billion years for complex life to form.

    Given that there has never been an object from outside our solar system seen passing through our solar system, it would seem that something entering our solar system and hitting Earth would be too rare to be considered a possiblity. I think it's probable that over the life of the Earth that it does occasionally get hits from intergalactic dust, but I think that life would need something rock-sized or larger to survive the multi-billion intergalactic journey and the firey entry into our atmosphere.

    I vote for #1.
    Last edited: Sep 8, 2004
  8. Sep 9, 2004 #7


    User Avatar

    Staff: Mentor

    Well, we do have data about #1. Lab experiments suggest it is quite possible.
    If life formed on earth soon after it cooled to the point where it was possible (as evidence cited by others suggests), life may be very prevalent on planets where it is possible.
  9. Sep 9, 2004 #8
    Russ_waters summarized very well the opinion of scientists in general, as can also be seen in the other posts.

    I think the reason why we cannot detect aliens signals so far is that they are too far, and the amplitude is too small. But they are also wondering where we are :rolleyes: Well, I like to think it's true.
  10. Sep 9, 2004 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    #2, on Mars. Life may have *started* on Earth as soon as the crust cooled enough, but the period of late heavy bombardment - not to mention the earlier formation the Moon - meant that life couldn't survive on Earth until as many as several hundred million years after Mars became habitable. The frequent transfer of matter between the planets ensured that life from Mars 'seeded' Earth, before it could get (re-)started here. Similarly, if we find life on Io (maybe if only extinct), it should be similar in many respects to life on Earth and (extinct?) life on Mars; it may even be possible to construct a phylogenetic tree :surprised
    Did you know that several tonnes of non-solar system material 'fall' onto the Earth every year?

    The interstellar medium has a significant dust component, and as the solar system is moving wrt the local ISM, we are sweeping up this dust. True, there's precious little of it cf comet doo and ground up asteroid muck; however, not only does analysis of 'radar' echos from micrometeorites show this ISM component, but IIRC one star in particular has been identified as a possible source :tongue:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook