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By Occams Razor, however, it’s probably better to assume that it formed spontaneously on Earth: "Just turning on the spark in a basic pre-biotic experiment will

by 3rd3 12y ago

By Occams Razor, however, it’s probably better to assume that it formed spontaneously on Earth: "Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids." [1] [1]: https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment

thret 12y ago
This is conjecture. We don't really know how many billions of years it takes to spontaneously evolve life. Panspermia may well be a much simpler vector.
alanh 12y ago
Hmm, I wonder if Occam's razor is really applicable. Is it simpler for life to form on multiple planets, or one and spread? I would guess your interpretation is more likely, but I suppose we don't know
- 3rd3 12y ago
  It’s applicable if one hypothesis is simpler than the other. Panspermia simply defers the origin of life elsewhere and now you have to explain how seeds survived travelling through the hostile environment of space and impacting on Earth, which complicates it quite a bit.
  - tjradcliffe 12y ago
    Ockham's razor is either useless, or a special case of Bayes' Rule. People argue endlessly about it because they fail to recognize they are assuming different priors. If the Urey experiment is reasonably representative of conditions on primordial Earth, no further work is required to get amino acids. So for some people P(terrestrial primordial amino acids) is ~1 If the Murchison meteor and other evidence is correct, amino acids are common in the universe. Data from the Philae lander may tell us more about this. So for some people P(extra-terrestrial primordial amino acids) is ~1 Nor are these exclusive groups. It appears amino acids aren't all that hard to make, so they may have both fallen from the sky and been created on Earth. But... amino acids are not life. There is a further step that is required for the actual pan-spermia hypothesis: the synthesis of more complex molecules, like RNA and DNA. If you think P(RNA|amino acids) ~ 1 then pan-spermia becomes at best irrelevant: not impossible, but not very interesting. If you think P(R/DNA|amino acids) ~ ε then pan-spermia becomes very important, because it is far more likely than not that life on Earth is due to RNA or DNA that descend from an original synthesis that happened elsewhere. Evidence suggests RNA synthesis given amino acids is not very probable. I don't believe it has ever been observed to happen spontaneously the way amino acids are spontaneously synthesized in the Urey experiment. So pan-spermia cannot be dismissed on the basis that it is uninteresting. DNA propogating itself across the universe is also not very probable, so the two hypotheses remain in competition, and it isn't obvious which is more plausible. Pan-spermia requires things like "DNA can survive re-entry." So the OP is correct: discovering that DNA can survive re-entry makes "Life originated elsewhere" more plausible. You'll notice I never used the word "simplicity" in any of this. There is no simplicity: only probability and plausibility. Evidence is more or less probable. Propositions are more or less plausible. Bayes' rule is the only way to update our beliefs consistently, such that no matter what order we get the evidence we come to the conclusion.
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jonahrd 12y ago
From the same article: "The Murchison meteorite that fell near Murchison, Victoria, Australia in 1969 was found to contain over 90 different amino acids, nineteen of which are found in Earth life."
ajcarpy2005 12y ago
There's Occam's Razor but then there's also things like Gravity. Gravity happens on all planets/masses. It could be that life formed on Earth and also forms other places. It could be that the life on Earth has been interbred on a genetic level (think virus insertion of bacteria-based epigenetics) Perhaps for life to really thrive we need very specific planetary conditions (such as Earth-like but for different life with different chemistries they may thrive under different planetary conditions) but the more basic modules of life could be quite plentiful in comparison to what we think of as full-fledged organisms. There is some evidence of bacteria on other planets and in space even. (although scientists are being careful not to rule out contamination from Earth)
duaneb 12y ago
Occam's razor is only useful if it removes complexity. Since amino acids can form just as easily on comets, for instance, how is it any more plausible to form on earth? All our water comes from comets; it would be ludicrous to think it's not also a source of organic material. Indeed, Philae confirmed that organic molecules were abundant on the comet on which it landed. Now whether RNA came from a comet is another story, and one that will probably be very difficult to answer, given the lack of archeological evidence. And finally, what does it matter where our RNA came from? Our roots are in these tiny, self-assembling structures. That's the miracle. It's not like we're surrounded by other solar systems that may be candidates for precursors to our life: either life sat around in a very primitive form for probably billions of years before coming to rest at earth, or it formed somewhere in the vicinity of our solar system.
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by 3rd3 12y ago

thret 12y ago

This is conjecture. We don't really know how many billions of years it takes to spontaneously evolve life. Panspermia may well be a much simpler vector.

alanh 12y ago

Hmm, I wonder if Occam's razor is really applicable. Is it simpler for life to form on multiple planets, or one and spread? I would guess your interpretation is more likely, but I suppose we don't know

3rd3 12y ago
It’s applicable if one hypothesis is simpler than the other. Panspermia simply defers the origin of life elsewhere and now you have to explain how seeds survived travelling through the hostile environment of space and impacting on Earth, which complicates it quite a bit.
- tjradcliffe 12y ago
  Ockham's razor is either useless, or a special case of Bayes' Rule. People argue endlessly about it because they fail to recognize they are assuming different priors. If the Urey experiment is reasonably representative of conditions on primordial Earth, no further work is required to get amino acids. So for some people P(terrestrial primordial amino acids) is ~1 If the Murchison meteor and other evidence is correct, amino acids are common in the universe. Data from the Philae lander may tell us more about this. So for some people P(extra-terrestrial primordial amino acids) is ~1 Nor are these exclusive groups. It appears amino acids aren't all that hard to make, so they may have both fallen from the sky and been created on Earth. But... amino acids are not life. There is a further step that is required for the actual pan-spermia hypothesis: the synthesis of more complex molecules, like RNA and DNA. If you think P(RNA|amino acids) ~ 1 then pan-spermia becomes at best irrelevant: not impossible, but not very interesting. If you think P(R/DNA|amino acids) ~ ε then pan-spermia becomes very important, because it is far more likely than not that life on Earth is due to RNA or DNA that descend from an original synthesis that happened elsewhere. Evidence suggests RNA synthesis given amino acids is not very probable. I don't believe it has ever been observed to happen spontaneously the way amino acids are spontaneously synthesized in the Urey experiment. So pan-spermia cannot be dismissed on the basis that it is uninteresting. DNA propogating itself across the universe is also not very probable, so the two hypotheses remain in competition, and it isn't obvious which is more plausible. Pan-spermia requires things like "DNA can survive re-entry." So the OP is correct: discovering that DNA can survive re-entry makes "Life originated elsewhere" more plausible. You'll notice I never used the word "simplicity" in any of this. There is no simplicity: only probability and plausibility. Evidence is more or less probable. Propositions are more or less plausible. Bayes' rule is the only way to update our beliefs consistently, such that no matter what order we get the evidence we come to the conclusion.
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jonahrd 12y ago

From the same article: "The Murchison meteorite that fell near Murchison, Victoria, Australia in 1969 was found to contain over 90 different amino acids, nineteen of which are found in Earth life."

ajcarpy2005 12y ago

There's Occam's Razor but then there's also things like Gravity. Gravity happens on all planets/masses. It could be that life formed on Earth and also forms other places. It could be that the life on Earth has been interbred on a genetic level (think virus insertion of bacteria-based epigenetics) Perhaps for life to really thrive we need very specific planetary conditions (such as Earth-like but for different life with different chemistries they may thrive under different planetary conditions) but the more basic modules of life could be quite plentiful in comparison to what we think of as full-fledged organisms. There is some evidence of bacteria on other planets and in space even. (although scientists are being careful not to rule out contamination from Earth)

duaneb 12y ago

Occam's razor is only useful if it removes complexity. Since amino acids can form just as easily on comets, for instance, how is it any more plausible to form on earth? All our water comes from comets; it would be ludicrous to think it's not also a source of organic material. Indeed, Philae confirmed that organic molecules were abundant on the comet on which it landed. Now whether RNA came from a comet is another story, and one that will probably be very difficult to answer, given the lack of archeological evidence. And finally, what does it matter where our RNA came from? Our roots are in these tiny, self-assembling structures. That's the miracle. It's not like we're surrounded by other solar systems that may be candidates for precursors to our life: either life sat around in a very primitive form for probably billions of years before coming to rest at earth, or it formed somewhere in the vicinity of our solar system.

deleted 12y ago

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