I never really got the Fermi paradox. There are so many unknowns about “what it takes to reach intelligent life” that I don’t think we really can give any estimate—and not seeing any trace of intelligent alien life just should encourage us to consider the “rare earth hypothesis” (that there are some very specific settings regarding Earth, like having a big moon, which are required for intelligent life to evolve, and are very rare in the universe). There are so many things we can forget to consider about Earth (like, the fact it’s far from the center of the galaxy, so not exposed frequently to nova) making it special, and so many unknown about the details of evolution of rational life, that it doesn’t strike me as impossible that there is in average 1 intelligent life form emerging between 10 billions of years and 15 billions of years after the big bang in a sphere of 5 billions light-years wide, leading to us being alone in our past cone.
Life arose on Earth sometime in the first few hundred million years after the young planet had cooled to the point that it could support water-based organisms on its surface. The early emergence of life on Earth has been taken as evidence that the probability of abiogenesis is high, if starting from young-Earth-like conditions. We revisit this argument quantitatively in a Bayesian statistical framework. By constructing a simple model of the probability of abiogenesis, we calculate a Bayesian estimate of its posterior probability, given the data that life emerged fairly early in Earth’s history and that, billions of years later, curious creatures noted this fact and considered its implications. We find that, given only this very limited empirical information, the choice of Bayesian prior for the abiogenesis probability parameter has a dominant influence on the computed posterior probability. Although terrestrial life’s early emergence provides evidence that life might be common in the Universe if early-Earth-like conditions are, the evidence is inconclusive and indeed is consistent with an arbitrarily low intrinsic probability of abiogenesis for plausible uninformative priors. Finding a single case of life arising independently of our lineage (on Earth, elsewhere in the Solar System, or on an extrasolar planet) would provide much stronger evidence that abiogenesis is not extremely rare in the Universe.
Interesting, but I was more thinking in the “simple life” ⇒ “intelligent life” step than about abiogenesis. As you said, we don’t have any strong evidence about the likelihood of abiogenesis, we only have two weak ones : the fact it happened early on Earth, and the Miller’s experiment which shows that under right conditions, build blocks of life can arise quickly.
But for the “simple life” ⇒ “intelligent life” step, we have even much less evidence. We know it took a long time on Earth, about 4 billions of years. But are we specially lucky (due to the conditions on Earth, and due to random events) ? We just don’t know.
There seems to be many evidence pointing to it not being so easy. Consider a parameter like isolation. If your star is too isolated, it won’t get enough heavy elements to sustain life. Heavy elements like oxygen, carbon (and worse for iron, calcium, …) were not created in significant quantities during the big bang. They were created later on in the heaviest (hottest) stars and in supernovae. So your star needs to be close enough to other stars (inside a dense enough galaxy) so it gets those elements. But if there are too many stars nearby, then some will go nova close to your planet before it had time to develop intelligent life, and will wipe all life (or at least, all complex life). So there seems to be a very narrow range of amount of nearby stars for intelligent to be able to develop itself.
The same goes for the mutation rate (dependant on natural radioactivity, strength of magnetic shield, …). If the mutation rate is too slow, evolution will take longer, and you won’t reach intelligent life before your main star goes giant red or nova. But if the mutation rate is too high, complex DNA will be unstable, and complex life won’t evolve.
The same goes for so many parameters. For each of them, we don’t know much about the acceptable range, nor how likely we are to find it. So… better admit we really don’t know about the odds on simple life to evolve into intelligent life, nor about the average time it would take. Not finding any intelligent life is, to me, strong evidence that it takes time and it requires rare conditions.
I think mutation rate would be subject to evolution as well. We can evolve to do more, or less DNA repair (and checks; up to acute radiation poisoning, cell self destruct is good enough).
Also, the effects of radioactivity are presumed linear. Extrapolating from high doses, the doses of up to 10x our natural background must have extremely minor effect on the mutation rate, vast majority of mutations arising due to other causes.
(The radiation releases in nuclear accidents are only a big deal because very huge number of people is exposed, resulting in massive dis-utility even if cancer rate increase is very minor)
I never really got the Fermi paradox. There are so many unknowns about “what it takes to reach intelligent life” that I don’t think we really can give any estimate—and not seeing any trace of intelligent alien life just should encourage us to consider the “rare earth hypothesis” (that there are some very specific settings regarding Earth, like having a big moon, which are required for intelligent life to evolve, and are very rare in the universe). There are so many things we can forget to consider about Earth (like, the fact it’s far from the center of the galaxy, so not exposed frequently to nova) making it special, and so many unknown about the details of evolution of rational life, that it doesn’t strike me as impossible that there is in average 1 intelligent life form emerging between 10 billions of years and 15 billions of years after the big bang in a sphere of 5 billions light-years wide, leading to us being alone in our past cone.
http://arxiv.org/abs/1107.3835
Interesting, but I was more thinking in the “simple life” ⇒ “intelligent life” step than about abiogenesis. As you said, we don’t have any strong evidence about the likelihood of abiogenesis, we only have two weak ones : the fact it happened early on Earth, and the Miller’s experiment which shows that under right conditions, build blocks of life can arise quickly.
But for the “simple life” ⇒ “intelligent life” step, we have even much less evidence. We know it took a long time on Earth, about 4 billions of years. But are we specially lucky (due to the conditions on Earth, and due to random events) ? We just don’t know.
There seems to be many evidence pointing to it not being so easy. Consider a parameter like isolation. If your star is too isolated, it won’t get enough heavy elements to sustain life. Heavy elements like oxygen, carbon (and worse for iron, calcium, …) were not created in significant quantities during the big bang. They were created later on in the heaviest (hottest) stars and in supernovae. So your star needs to be close enough to other stars (inside a dense enough galaxy) so it gets those elements. But if there are too many stars nearby, then some will go nova close to your planet before it had time to develop intelligent life, and will wipe all life (or at least, all complex life). So there seems to be a very narrow range of amount of nearby stars for intelligent to be able to develop itself.
The same goes for the mutation rate (dependant on natural radioactivity, strength of magnetic shield, …). If the mutation rate is too slow, evolution will take longer, and you won’t reach intelligent life before your main star goes giant red or nova. But if the mutation rate is too high, complex DNA will be unstable, and complex life won’t evolve.
The same goes for so many parameters. For each of them, we don’t know much about the acceptable range, nor how likely we are to find it. So… better admit we really don’t know about the odds on simple life to evolve into intelligent life, nor about the average time it would take. Not finding any intelligent life is, to me, strong evidence that it takes time and it requires rare conditions.
I think mutation rate would be subject to evolution as well. We can evolve to do more, or less DNA repair (and checks; up to acute radiation poisoning, cell self destruct is good enough).
Also, the effects of radioactivity are presumed linear. Extrapolating from high doses, the doses of up to 10x our natural background must have extremely minor effect on the mutation rate, vast majority of mutations arising due to other causes.
(The radiation releases in nuclear accidents are only a big deal because very huge number of people is exposed, resulting in massive dis-utility even if cancer rate increase is very minor)
I was just continuing your attack on the common assumptions for the values involved with the Drake equation.