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Like every starry-eyed geek, I had been waiting to see what NASA's big announcement yesterday was going to be. Given the background of the one researcher who had been listed, I was hoping it was going to be about abiogensis: the origin of cellular life. We still have very little idea exactly how you go from an enriched soup of organic elements and random amino acids to a working, self-replicating organism. If we're reading the geological history of the Earth correctly, it seems to have happened very quickly (in geological terms, i.e., a couple million years) after conditions had settled down enough to allow life. (Which suggests it might even have happened several times, only to be wiped out by the next massive collision as the early Solar System cleaned itself up. The "heavy bombardment" period was fun times!) But as far as we can tell, all life today is descended from a single genesis event, unless you had multiple ones with the exact same biochemistry, or ones close enough that they could merge somehow. Fairly unlikely, most would say.
Truth is, though, we have little data from which to determine the probabilities of any of this. If someone had come up with a new model for how it might have happened, that would be really cool. Or, better yet, a laboratory demonstration for some step in the process! There haven't been any big breakthroughs in that field since the Miller-Urey experiment -- and probably won't be. We're talking about reactions happening once or twice over literally millions of years in literally an ocean's worth of material. Maybe some magical quantum computer will eventually let us simulate that, but I'm not holding my breath.
So when Gizmodo claimed that a new form of life which used arsenic instead of phosphorous had been discovered, that was really, really exciting. There is a minor branch of exobiology looking for descendants of other genesis events here on Earth. The thinking normally is that even if there had been other trees of life, they would be extinct as our current one won out long ago. If any would be surviving, it would be in places that would be hostile to our form of life but not to theirs. We keep finding more and more extreme extremophiles, so it's not completely unreasonable to think that in some hellish corner of the Earth there might be a colony of what would be in every practical sense alien life. Which could tell us all kinds of things about really alien life someplace else. (It might even have come from someplace else. We might have come from someplace else. Earth and Mars trade meteorites regularly, and would have even more frequently back in the day. Which would be neat, but not very significant. Xenospermia doesn't explain how life started, after all, anymore than blaming it on a god does.)
And As+/P- life wouldn't be a complete surprise -- arsenic and phosphorous are very similar, chemically. Cells can't tell the difference between arsenate (AsO4) and phosphate (PO4), which would be fine except that when you use arsenate to turn sugars into usable energy, they form unstable compounds that quickly hydrolyze, preventing the ATP from being formed. Arsenic is poisonous, right? Now you know why. We already knew of extremophiles that could handle far more arsenic than normal. But they just handled the waste products better, preventing ATP production from breaking down completely in the presence of arsenic. Bacteria that were built from arsenic from the ground up would be crazy cool.
Unfortunately, of course, that's not what the announcement was. By taking extremophiles that could already handle hypersaline, hyperalkaline, high arsenic conditions, researchers found a strain that could not only tolerate arsenic but use it, incorporating it into their cellular structures -- including their DNA. (Except it's technically not quite DNA any more, just a very very close analog.) They don't need arsenic, they don't even prefer arsenic. They grow much faster and better with phosphorous. They're just a weird, interesting branch of normal life that can kind of survive if forced to use arsenic. Like if you were trapped in an airport and had to survive on Cinnabons and Dan Brown novels. You could do it, but it would suck and you probably wouldn't live as long.
Overall, this is certainly very cool, just not as cool as I was hoping. It still has exobiological implications, though. At its most basic, it expands the range of environments in which one might find life. More importantly, it proves that our specific biochemistry isn't the only one that works. Granted, this other one is almost indistinguishably close, but until now there was no evidence that any others were possible. You'll have to find a epistomologist for a more rigorous opinion, but it seems like many given two is more likely than two given one. So here's hoping.
Truth is, though, we have little data from which to determine the probabilities of any of this. If someone had come up with a new model for how it might have happened, that would be really cool. Or, better yet, a laboratory demonstration for some step in the process! There haven't been any big breakthroughs in that field since the Miller-Urey experiment -- and probably won't be. We're talking about reactions happening once or twice over literally millions of years in literally an ocean's worth of material. Maybe some magical quantum computer will eventually let us simulate that, but I'm not holding my breath.
So when Gizmodo claimed that a new form of life which used arsenic instead of phosphorous had been discovered, that was really, really exciting. There is a minor branch of exobiology looking for descendants of other genesis events here on Earth. The thinking normally is that even if there had been other trees of life, they would be extinct as our current one won out long ago. If any would be surviving, it would be in places that would be hostile to our form of life but not to theirs. We keep finding more and more extreme extremophiles, so it's not completely unreasonable to think that in some hellish corner of the Earth there might be a colony of what would be in every practical sense alien life. Which could tell us all kinds of things about really alien life someplace else. (It might even have come from someplace else. We might have come from someplace else. Earth and Mars trade meteorites regularly, and would have even more frequently back in the day. Which would be neat, but not very significant. Xenospermia doesn't explain how life started, after all, anymore than blaming it on a god does.)
And As+/P- life wouldn't be a complete surprise -- arsenic and phosphorous are very similar, chemically. Cells can't tell the difference between arsenate (AsO4) and phosphate (PO4), which would be fine except that when you use arsenate to turn sugars into usable energy, they form unstable compounds that quickly hydrolyze, preventing the ATP from being formed. Arsenic is poisonous, right? Now you know why. We already knew of extremophiles that could handle far more arsenic than normal. But they just handled the waste products better, preventing ATP production from breaking down completely in the presence of arsenic. Bacteria that were built from arsenic from the ground up would be crazy cool.
Unfortunately, of course, that's not what the announcement was. By taking extremophiles that could already handle hypersaline, hyperalkaline, high arsenic conditions, researchers found a strain that could not only tolerate arsenic but use it, incorporating it into their cellular structures -- including their DNA. (Except it's technically not quite DNA any more, just a very very close analog.) They don't need arsenic, they don't even prefer arsenic. They grow much faster and better with phosphorous. They're just a weird, interesting branch of normal life that can kind of survive if forced to use arsenic. Like if you were trapped in an airport and had to survive on Cinnabons and Dan Brown novels. You could do it, but it would suck and you probably wouldn't live as long.
Overall, this is certainly very cool, just not as cool as I was hoping. It still has exobiological implications, though. At its most basic, it expands the range of environments in which one might find life. More importantly, it proves that our specific biochemistry isn't the only one that works. Granted, this other one is almost indistinguishably close, but until now there was no evidence that any others were possible. You'll have to find a epistomologist for a more rigorous opinion, but it seems like many given two is more likely than two given one. So here's hoping.