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I think what will really help us to get at a better estimate is to answer the question whether there exists life in our solar system that evolved independently

I think what will really help us to get at a better estimate is to answer the question whether there exists life in our solar system that evolved independently of that on Earth. If we should find traces of bacteria on Mars or another planet, that would immediately take the corresponding probability factor in the Drake equation to a number close to 1 (i.e. most solar systems with planets in the habitable zone produce simple organic life), which would then leave us only with the question of how likely it is that such bacterial life evolves into something more complex. That's an incredibly hard question to answer as well of course, but it's also something that we might be able to tackle within the next 50 years given a deeper understanding of how living cells work and how they came to be (and it seems we're making good progress there). So personally I'm quite confident that we will be able to get to a much better estimate of Drake's equation in the next 50 years.

throwaway_yy2Di 10y ago
I think what will really help us to get at a better estimate is to answer the question whether there exists life in our solar system that evolved independently of that on Earth. That's definitely the big question. But you left out the elephant in the room: is there life right here on Earth that's evolved independently? Abiogenesis doesn't simply halt after it's achieved its "goal". Perhaps we take the common-origin hypothesis for granted (that the "tree of life" has one root, and is in fact a tree), but it's actually nontrivial, strong evidence about the rate of abiogenesis, right under our noses. If it's true. There's no hard reason why another, orthogonal family of life couldn't coexist with DNA/RNA life. We have 10^7 extant species of DNA/RNA life; that's an existence proof that hostile, competing "types" of life can coexist. Suppose we found terrestrial "XNA life" with a biochemistry totally unrelated to anything else, with no possible common origin or lateral gene transfer. Then we would know life on Earth developed independently N >= 2 times, instead of N >= 1. That would change a lot; we could update P(life | Earth-like planet) from the subjective probability range [10^-Graham's number, 1) to something more like [10^-4, 1). What's really surprising is that this is completely untested! We genuinely don't know if there's "alien" life on Earth, because we don't know how to look for it. There's no black-box tool to approach unknown biochemistry: Some have postulated the existence of a 'shadow biosphere' on Earth, teeming with life that has gone undiscovered because scientists simply don't know where to look. It could contain life that relies on a fundamentally different biochemistry, using different forms of amino acids or even entirely novel ways of storing, replicating and executing inherited information that do not rely on DNA or proteins. [...] The trick is deciding what to look for and how to detect it. The usual way that researchers search for new organisms — by sequencing DNA or RNA — will not pick up life that does not depend on them. http://www.nature.com/news/life-changing-experiments-the-biological-higgs-1.10310 http://www.nature.com/news/life-changing-experiments-the-bio... ...We could even be oblivious to unfamiliar forms of life right under our noses... http://www.sciencemag.org/news/2015/02/shadow-biosphere-might-be-hiding-strange-life-right-under-our-noses http://www.sciencemag.org/news/2015/02/shadow-biosphere-migh...
- TheOtherHobbes 10y ago
  As soon as you get v1.0 life, it starts bioforming its environment. v2.0 life would have to evolve in the changed environment - which would be considerably less hospitable, because the most useful chemical processes and energy sources will be colonised by v1.0 life. v1.0 life will (more or less literally) get the low hanging fruit. v2.0 would have to evolve around the edges, which means the probability gradient is higher and therefore less likely.
  - ThePhysicist 10y ago
    But I think this actually happens with normal life as well: Individual species of bacteria try to conquer a biological niche by shifting conditions such that only they can survive. But by doing this they invariably create chemical by-products that can be harnessed by other life forms, thereby fueling their growth. It's an interesting question whether there are any biospheres on Earth that are colonized exclusively by a single species of life (I doubt it). So although it's a very valid point I'm not sure if it's a sufficient argument against the hypothesis of having two independently evolved and fundamentally different types of life on a given planet.
- ThePhysicist 10y ago
  Fascinating, thanks for pointing this out! Discovering a new kind of life on Earth that has coexisted with us without being noticed would be a huge event of course, and it would probably challenge what we regard as life in a general way. This would be good stuff for a science fiction story as well.
  - apenwarr 10y ago
    It does make a good sci fi story: https://www.amazon.com/dp/B00M64A8EG/ https://www.amazon.com/dp/B00M64A8EG/
    - ThePhysicist 10y ago
      Wow, thanks for the link :)
greggman 10y ago
what if life didn't evolve here but arrive on other space debris? Has that theory been ruled out?
- ende 10y ago
  The term you're looking for is panspermia. Nothing can necessarily be ruled out but it seems both 1) highly unlikely from a mechanistic perspective and 2) unnecessary to the explanation of life origin.
- asamarin 10y ago
  That's called panspermia, and no, it hasn't been ruled out. Main problem with it though is that it doesn't explain how life as we know it came into existence, it simply shifts its origin from Earth to somewhere else

throwaway_yy2Di 10y ago

I think what will really help us to get at a better estimate is to answer the question whether there exists life in our solar system that evolved independently of that on Earth. That's definitely the big question. But you left out the elephant in the room: is there life right here on Earth that's evolved independently? Abiogenesis doesn't simply halt after it's achieved its "goal". Perhaps we take the common-origin hypothesis for granted (that the "tree of life" has one root, and is in fact a tree), but it's actually nontrivial, strong evidence about the rate of abiogenesis, right under our noses. If it's true. There's no hard reason why another, orthogonal family of life couldn't coexist with DNA/RNA life. We have 10^7 extant species of DNA/RNA life; that's an existence proof that hostile, competing "types" of life can coexist. Suppose we found terrestrial "XNA life" with a biochemistry totally unrelated to anything else, with no possible common origin or lateral gene transfer. Then we would know life on Earth developed independently N >= 2 times, instead of N >= 1. That would change a lot; we could update P(life | Earth-like planet) from the subjective probability range [10^-Graham's number, 1) to something more like [10^-4, 1). What's really surprising is that this is completely untested! We genuinely don't know if there's "alien" life on Earth, because we don't know how to look for it. There's no black-box tool to approach unknown biochemistry: Some have postulated the existence of a 'shadow biosphere' on Earth, teeming with life that has gone undiscovered because scientists simply don't know where to look. It could contain life that relies on a fundamentally different biochemistry, using different forms of amino acids or even entirely novel ways of storing, replicating and executing inherited information that do not rely on DNA or proteins. [...] The trick is deciding what to look for and how to detect it. The usual way that researchers search for new organisms — by sequencing DNA or RNA — will not pick up life that does not depend on them. http://www.nature.com/news/life-changing-experiments-the-biological-higgs-1.10310 http://www.nature.com/news/life-changing-experiments-the-bio... ...We could even be oblivious to unfamiliar forms of life right under our noses... http://www.sciencemag.org/news/2015/02/shadow-biosphere-might-be-hiding-strange-life-right-under-our-noses http://www.sciencemag.org/news/2015/02/shadow-biosphere-migh...

TheOtherHobbes 10y ago
As soon as you get v1.0 life, it starts bioforming its environment. v2.0 life would have to evolve in the changed environment - which would be considerably less hospitable, because the most useful chemical processes and energy sources will be colonised by v1.0 life. v1.0 life will (more or less literally) get the low hanging fruit. v2.0 would have to evolve around the edges, which means the probability gradient is higher and therefore less likely.
- ThePhysicist 10y ago
  But I think this actually happens with normal life as well: Individual species of bacteria try to conquer a biological niche by shifting conditions such that only they can survive. But by doing this they invariably create chemical by-products that can be harnessed by other life forms, thereby fueling their growth. It's an interesting question whether there are any biospheres on Earth that are colonized exclusively by a single species of life (I doubt it). So although it's a very valid point I'm not sure if it's a sufficient argument against the hypothesis of having two independently evolved and fundamentally different types of life on a given planet.
ThePhysicist 10y ago
Fascinating, thanks for pointing this out! Discovering a new kind of life on Earth that has coexisted with us without being noticed would be a huge event of course, and it would probably challenge what we regard as life in a general way. This would be good stuff for a science fiction story as well.
- apenwarr 10y ago
  It does make a good sci fi story: https://www.amazon.com/dp/B00M64A8EG/ https://www.amazon.com/dp/B00M64A8EG/
  - ThePhysicist 10y ago
    Wow, thanks for the link :)

greggman 10y ago

what if life didn't evolve here but arrive on other space debris? Has that theory been ruled out?

ende 10y ago
The term you're looking for is panspermia. Nothing can necessarily be ruled out but it seems both 1) highly unlikely from a mechanistic perspective and 2) unnecessary to the explanation of life origin.
asamarin 10y ago
That's called panspermia, and no, it hasn't been ruled out. Main problem with it though is that it doesn't explain how life as we know it came into existence, it simply shifts its origin from Earth to somewhere else