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"a priori" means "from theoretical deduction rather than evidence or experience", so yes, that's entirely the point of that sentence. What the author is saying

"a priori" means "from theoretical deduction rather than evidence or experience", so yes, that's entirely the point of that sentence. What the author is saying is "theoretically, life could have started on Mars just as easily as it did here".

PaulDavisThe1st 6y ago
The theories we have about how life started on Earth tend to disfavor Mars as a similar starting point, based on what we currently know about Mars. I think the author was trying to say "I mean, look, Earth and Mars are not that different, and even though we don't have any current evidence for the appropriate chemical conditions on Mars, I don't see why it couldn't have been where life started instead of Earth, because ... well, just because they're sort of similar, kind of. Oh, and next to each other."
- pfdietz 6y ago
  Those theories don't have much in the way of evidence that would compel anyone to believe them, though. So citing them isn't much of an argument.
- brudgers 6y ago
  based on what we currently know In two hundred years, conclusions drawn from what we currently know will be as out of date as conclusions from two hundred years ago are today. Or to put it another way, things known to the best of our knowledge are both as good as we can do and probably wrong because one of the things we know is that historically most of what people know at any given time turns out to be wrong at any given time.
  - PaulDavisThe1st 6y ago
    Absolutely correct. But this article was written now, and makes a claim that is totally unsupported by anything we know now. I'm entirely down for our descendants in 200 years laughing at our scientific parochiality, just like we often get to do at our double-century antecedents. But that doesn't justify writing right now that Mars and Earth have a priori likelihoods of being the origin of life in the solar system.
- davedx 6y ago
  Citation needed. There's plenty of evidence Mars once had much more surface water than it does today. IIRC liquid water is one of the primary preconditions for the evolution of Earth-like life. What about Mars disfavoured it?
tuatoru 6y ago
"theoretically, life could have started on Mars just as easily as it did here" Yes, leaving aside the 20% lower luminosity of the sun[1] a few billion years ago (when Mars had a reasonable atmosphere) putting Mars outside the habitable zone, and Mars's lack of plate tectonics and a protective magnetic field: - Life certainly could have evolved on Mars. I expect that one day prion-like or viroid-ish molecules may well be found. But so what? It's one or two billion years from there to a bacterium, let alone a lost civilisation. And Mars did not have the time. [1] https://en.wikipedia.org/wiki/Faint_young_Sun_paradox https://en.wikipedia.org/wiki/Faint_young_Sun_paradox
- davedx 6y ago
  Your linked article actually says there is evidence Mars was warmer and wetter in the past. How then was it "outside the habitable zone"? And what proof do you have that life 4B years ago needed to be in what we define now as a habitable zone, needed a magnetic field, and needed active plate tectonics to arise? These are exactly the kinds of assumptions the article is challenging.
  - tuatoru 6y ago
    Life didn't need those things to arise; that's just a matter of the right chemistry and physics being available for a relatively short period of time. But it would seem to need them for sustained evolution to be possible. So as I said, I'd expect lifeforms at the complexity level of viroids, but not at the level of eukaryotes. Evolution needs time, lots of time in which conditions don't change too much. (Chemistry that works in dilute solution at 300K doesn't work well at 200K or 500K.) It also needs populations to be able to spread to new areas. Without a magnetic field, Mars's atmosphere got stripped off. No more liquid water, no more areographical distribution, so no more evolution. Plate tectonics is needed to ensure that nutrient chemicals don't get locked up permanently in sedimentary rock. As for the habitable zone, life as we know it needs liquid water on the surface. Mars may have briefly been habitable, but not for long enough. To be a challenge, the article needs to provide plausible alternative evolutionary pathways that get us eukaryotes without needing these things (time, liquid water, chemical recirculation, limited variability in physcal conditions). It doesn't do that. It's just spitballing. If early Mars was OK for life to begin, then so was early Earth. We don't need any interplanetary travel.
Barrin92 6y ago
What point is there in making a theoretical deduction about an empirical issue? Yes, life could have 'theoretically' started anywhere where life by definition is possible. Fine, but that's a tautology, and in contrast to what the author seems to believe actually not scientific and thus deserves to be rejected. If we're talking about scientific statements then show me life actually existing somewhere that isn't earth and then we can make some grounded theories
bryanrasmussen 6y ago
I suppose if your theory was that god created life that would definitely make Mars just as likely a starting place, or if the theory is that life actually came from some meteorite impact, but most theories for life's origins do not seem to favor Mars. I mean when you say theoretically and then make a big not necessarily obvious statement you should actually point out which theory you're using.

PaulDavisThe1st 6y ago

The theories we have about how life started on Earth tend to disfavor Mars as a similar starting point, based on what we currently know about Mars. I think the author was trying to say "I mean, look, Earth and Mars are not that different, and even though we don't have any current evidence for the appropriate chemical conditions on Mars, I don't see why it couldn't have been where life started instead of Earth, because ... well, just because they're sort of similar, kind of. Oh, and next to each other."

pfdietz 6y ago
Those theories don't have much in the way of evidence that would compel anyone to believe them, though. So citing them isn't much of an argument.
brudgers 6y ago
based on what we currently know In two hundred years, conclusions drawn from what we currently know will be as out of date as conclusions from two hundred years ago are today. Or to put it another way, things known to the best of our knowledge are both as good as we can do and probably wrong because one of the things we know is that historically most of what people know at any given time turns out to be wrong at any given time.
- PaulDavisThe1st 6y ago
  Absolutely correct. But this article was written now, and makes a claim that is totally unsupported by anything we know now. I'm entirely down for our descendants in 200 years laughing at our scientific parochiality, just like we often get to do at our double-century antecedents. But that doesn't justify writing right now that Mars and Earth have a priori likelihoods of being the origin of life in the solar system.
davedx 6y ago
Citation needed. There's plenty of evidence Mars once had much more surface water than it does today. IIRC liquid water is one of the primary preconditions for the evolution of Earth-like life. What about Mars disfavoured it?

tuatoru 6y ago

"theoretically, life could have started on Mars just as easily as it did here" Yes, leaving aside the 20% lower luminosity of the sun[1] a few billion years ago (when Mars had a reasonable atmosphere) putting Mars outside the habitable zone, and Mars's lack of plate tectonics and a protective magnetic field: - Life certainly could have evolved on Mars. I expect that one day prion-like or viroid-ish molecules may well be found. But so what? It's one or two billion years from there to a bacterium, let alone a lost civilisation. And Mars did not have the time. [1] https://en.wikipedia.org/wiki/Faint_young_Sun_paradox https://en.wikipedia.org/wiki/Faint_young_Sun_paradox

davedx 6y ago
Your linked article actually says there is evidence Mars was warmer and wetter in the past. How then was it "outside the habitable zone"? And what proof do you have that life 4B years ago needed to be in what we define now as a habitable zone, needed a magnetic field, and needed active plate tectonics to arise? These are exactly the kinds of assumptions the article is challenging.
- tuatoru 6y ago
  Life didn't need those things to arise; that's just a matter of the right chemistry and physics being available for a relatively short period of time. But it would seem to need them for sustained evolution to be possible. So as I said, I'd expect lifeforms at the complexity level of viroids, but not at the level of eukaryotes. Evolution needs time, lots of time in which conditions don't change too much. (Chemistry that works in dilute solution at 300K doesn't work well at 200K or 500K.) It also needs populations to be able to spread to new areas. Without a magnetic field, Mars's atmosphere got stripped off. No more liquid water, no more areographical distribution, so no more evolution. Plate tectonics is needed to ensure that nutrient chemicals don't get locked up permanently in sedimentary rock. As for the habitable zone, life as we know it needs liquid water on the surface. Mars may have briefly been habitable, but not for long enough. To be a challenge, the article needs to provide plausible alternative evolutionary pathways that get us eukaryotes without needing these things (time, liquid water, chemical recirculation, limited variability in physcal conditions). It doesn't do that. It's just spitballing. If early Mars was OK for life to begin, then so was early Earth. We don't need any interplanetary travel.

Barrin92 6y ago

What point is there in making a theoretical deduction about an empirical issue? Yes, life could have 'theoretically' started anywhere where life by definition is possible. Fine, but that's a tautology, and in contrast to what the author seems to believe actually not scientific and thus deserves to be rejected. If we're talking about scientific statements then show me life actually existing somewhere that isn't earth and then we can make some grounded theories

bryanrasmussen 6y ago

I suppose if your theory was that god created life that would definitely make Mars just as likely a starting place, or if the theory is that life actually came from some meteorite impact, but most theories for life's origins do not seem to favor Mars. I mean when you say theoretically and then make a big not necessarily obvious statement you should actually point out which theory you're using.