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> For one thing, if you were an advancing species some 12 billion years ago (an era where there’s reason to think that biological life as we know it might have

> For one thing, if you were an advancing species some 12 billion years ago (an era where there’s reason to think that biological life as we know it might have already been possible) This was eyebrow-raising for me, as I thought the consensus was advanced civilization was only "recently" possible due to the lack of heavy elements in the first generations of stars. The article fleshes out this claim a bit at the end... > During the period when biochemistry could have first got underway—some 100 million years after the Big Bang—the heaviest elements were woefully scarce as the first generations of stars forged new atomic nuclei. > Bereft of those atoms, terrestrial-sized planets might have wound up as “carbon worlds,”4 chemically rich in some ways, but with a severe shortage of some of the heavier elements that today’s life fully relies on. Life could have gotten going, but with different restrictions and imperatives. ...but still seems to be making a giant unsupported leap from "life may have been possible without heavy elements" to "advanced civilization may have been possible without heavy elements"

adrian_b 5y ago
Life as we know it cannot start without at least iron, cobalt and nickel, besides the 5 lighter elements H, C, N, O and S. Iron, cobalt and nickel are the minimum set of elements required as catalysts and, as far as we know at this time, life cannot appear without them. Actually life can appear using Fe, Co & Ni, when both dihydrogen and carbon monoxide are available in the environment. When only carbon dioxide is available instead of carbon monoxide, molybdenum or tungsten becomes also necessary. So at least the elements around the iron peak are needed, which are produced in supernovas. Realistically, for the evolution of more complex life forms, also a few of the heavier elements that are produced in intense neutron fluxes are also required. So no, according to current knowledge life was impossible 12 billion years ago and I cannot see any workaround, because the catalysts are at least as essential for life as the light elements that compose the bulk of the living matter.
- joshuahedlund 5y ago
  Thank for those details. (And yes I get the arguments that "things could happen that are outside of our capacity to imagine" but while true at some levels it can only take you so far with fundamental finite properties like the number of elements in the periodic table and the compounds you can make with them)
- rbanffy 5y ago
  > according to current knowledge life was impossible 12 billion years ago At what point in time we can expect the first supernovas to seed planets with heavy elements?
- 1_person 5y ago
  My understanding is that the earliest stars would have begun to form only a few hundred thousand years after the event and begun to explode not terribly long after that on account of their mass and rapid reaction rate, seeding the early universe with some amount of nucleosynthesis product almost from the outset. Of course, I think it was rather a while longer before it cooled down enough for things other than raging thermonuclear fireballs to find hospitable.
- mrfusion 5y ago
  How does all life use tungsten? I’ve never heard of that.
  - adrian_b 5y ago
    I have said "molybdenum or tungsten". Most living beings use molybdenum, but tungsten can replace molybdenum and it is used for this purpose by some living beings (mostly by Archaea). It is pretty certain that the common ancestor of all existing living beings already had molybdenum enzymes, while tungsten began to be used later, as an adaptation to higher temperatures. Nevertheless, there was a hypothesis that maybe before the oxygenation of the oceans tungsten was used in preference to molybdenum, because oxygenated molybdenum compounds are more soluble than oxygenated tungsten compounds while in reduced environments their solubility properties are reversed. However this hypothesis is most likely wrong, because it is contradicted by the distribution and phylogeny of the molybdenum enzymes and tungsten enzymes, which are consistent with the universal use of molybdenum already much earlier than the evolution of the ability for water photolysis and with the gain of the ability to use tungsten as an alternative only in certain archaea and bacteria. The earliest life forms were probably dependent on having ammonia and carbon monoxide in their environment, as sources of carbon and nitrogen, in which case molybdenum is not strictly needed. The use of molybdenum enzymes enabled the use of the more abundant carbon dioxide and dinitrogen as sources of carbon and nitrogen. This achievement must have happened some time before LUCA (last universal common ancestor), which already had these enzymes.
- Gatsky 5y ago
  The workaround is that we are totally wrong about theoretical predictions stretching back billions of years.
bena 5y ago
A large problem with any of this is that we have a very, very, very limited dataset with regards to life. We know of one planet, in one solar system, in one galaxy that supports life. And it is incredibly hard to collect data from other planets in our own solar system much less data from other solar systems and other galaxies may be impossible to get any meaningful information with regards to the existence of life. But it's also the only data we have to go off of. So the best we can say is that this couldn't have happened in those conditions.
- dotancohen 5y ago
  I agree with you, but the counterpoint is that anything other that this might not fit any defensible definition of life.
- dheera 5y ago
  Agreed. I would think that silicon-based life is possible and would require an entirely different set of elements to sustain. I would argue that a better definition of life should be based on (a) entropy and (b) propagation. Living systems (a) actively counteract entropy-increasing forces within their system as long as they are alive, and (b) are able to propagate indefinitely given the necessary energy inputs. I've heard other definitions of life including dependence on water and I think those aspects should not be part of such a definition.
  - adrian_b 5y ago
    There are 2 constraints for the set of chemical elements required for life, one is the ability to build very complex molecules from them and the other is the existence of some exothermic chemical reactions between the simple substances that are produced abiotically, i.e. the existence of some chemical reactions that can provide energy. It is quite likely that there could be some alien life forms that would use much more varied silicon compounds than those used on Earth, but it is quite unlikely that there are environmental circumstances where silicon could replace carbon as the main structural element. Silicon without carbon can provide only a much less variety of molecules and all the naturally occurring compounds are too stable, i.e. you cannot extract energy from them by rearranging the atoms, like you can in carbon compounds. The most important condition for the appearance of life is an energy source. There are various abiotic processes that produce dihydrogen, carbon monoxide and carbon dioxide, and these gases can enter exothermic chemical reactions (e.g. dihydrogen + carbon monoxide => acetic acid), which can sustain life. There are no such energy sources from silicon compounds. When oxygen is in excess, silicon forms silicates, otherwise it forms carbides and nitrides. All these are inert. The only environment where silicon could play a role in the appearance of life would be a place with a very large abundance of hydrogen, i.e. a large planet like Jupiter, where it might happen that all the elements are present as compounds of hydrogen, so the silicon would be present as silanes, and there are differences in stability between various silanes that might be exploitable as an energy source if there exists any abiotic process that generates less stable silanes instead of the more stable silanes. This possibility remains open just because I have not seen yet any model of such an environment to determine what compounds may appear and what chemical reactions are possible, so it cannot be completely excluded that silicon could play a role there, unlike on medium-size planets or smaller bodies, where the hydrogen is less abundant and silicon does not combine with it, but with the more electronegative elements. I am not aware of any attempt to analyze what chemical reactions are possible in an environment with too much hydrogen, but I suspect that both the too oxidized environments with too much oxygen and the too reduced environments with too much hydrogen make impossible the occurrence of chemical reactions that could be used as a source of energy for life forms, so I think that life on Jupiter-like planets is unlikely.
deleted 5y ago
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adrian_b 5y ago

Life as we know it cannot start without at least iron, cobalt and nickel, besides the 5 lighter elements H, C, N, O and S. Iron, cobalt and nickel are the minimum set of elements required as catalysts and, as far as we know at this time, life cannot appear without them. Actually life can appear using Fe, Co & Ni, when both dihydrogen and carbon monoxide are available in the environment. When only carbon dioxide is available instead of carbon monoxide, molybdenum or tungsten becomes also necessary. So at least the elements around the iron peak are needed, which are produced in supernovas. Realistically, for the evolution of more complex life forms, also a few of the heavier elements that are produced in intense neutron fluxes are also required. So no, according to current knowledge life was impossible 12 billion years ago and I cannot see any workaround, because the catalysts are at least as essential for life as the light elements that compose the bulk of the living matter.

joshuahedlund 5y ago
Thank for those details. (And yes I get the arguments that "things could happen that are outside of our capacity to imagine" but while true at some levels it can only take you so far with fundamental finite properties like the number of elements in the periodic table and the compounds you can make with them)
rbanffy 5y ago
> according to current knowledge life was impossible 12 billion years ago At what point in time we can expect the first supernovas to seed planets with heavy elements?
1_person 5y ago
My understanding is that the earliest stars would have begun to form only a few hundred thousand years after the event and begun to explode not terribly long after that on account of their mass and rapid reaction rate, seeding the early universe with some amount of nucleosynthesis product almost from the outset. Of course, I think it was rather a while longer before it cooled down enough for things other than raging thermonuclear fireballs to find hospitable.
mrfusion 5y ago
How does all life use tungsten? I’ve never heard of that.
- adrian_b 5y ago
  I have said "molybdenum or tungsten". Most living beings use molybdenum, but tungsten can replace molybdenum and it is used for this purpose by some living beings (mostly by Archaea). It is pretty certain that the common ancestor of all existing living beings already had molybdenum enzymes, while tungsten began to be used later, as an adaptation to higher temperatures. Nevertheless, there was a hypothesis that maybe before the oxygenation of the oceans tungsten was used in preference to molybdenum, because oxygenated molybdenum compounds are more soluble than oxygenated tungsten compounds while in reduced environments their solubility properties are reversed. However this hypothesis is most likely wrong, because it is contradicted by the distribution and phylogeny of the molybdenum enzymes and tungsten enzymes, which are consistent with the universal use of molybdenum already much earlier than the evolution of the ability for water photolysis and with the gain of the ability to use tungsten as an alternative only in certain archaea and bacteria. The earliest life forms were probably dependent on having ammonia and carbon monoxide in their environment, as sources of carbon and nitrogen, in which case molybdenum is not strictly needed. The use of molybdenum enzymes enabled the use of the more abundant carbon dioxide and dinitrogen as sources of carbon and nitrogen. This achievement must have happened some time before LUCA (last universal common ancestor), which already had these enzymes.
Gatsky 5y ago
The workaround is that we are totally wrong about theoretical predictions stretching back billions of years.

bena 5y ago

A large problem with any of this is that we have a very, very, very limited dataset with regards to life. We know of one planet, in one solar system, in one galaxy that supports life. And it is incredibly hard to collect data from other planets in our own solar system much less data from other solar systems and other galaxies may be impossible to get any meaningful information with regards to the existence of life. But it's also the only data we have to go off of. So the best we can say is that this couldn't have happened in those conditions.

dotancohen 5y ago
I agree with you, but the counterpoint is that anything other that this might not fit any defensible definition of life.
dheera 5y ago
Agreed. I would think that silicon-based life is possible and would require an entirely different set of elements to sustain. I would argue that a better definition of life should be based on (a) entropy and (b) propagation. Living systems (a) actively counteract entropy-increasing forces within their system as long as they are alive, and (b) are able to propagate indefinitely given the necessary energy inputs. I've heard other definitions of life including dependence on water and I think those aspects should not be part of such a definition.
- adrian_b 5y ago
  There are 2 constraints for the set of chemical elements required for life, one is the ability to build very complex molecules from them and the other is the existence of some exothermic chemical reactions between the simple substances that are produced abiotically, i.e. the existence of some chemical reactions that can provide energy. It is quite likely that there could be some alien life forms that would use much more varied silicon compounds than those used on Earth, but it is quite unlikely that there are environmental circumstances where silicon could replace carbon as the main structural element. Silicon without carbon can provide only a much less variety of molecules and all the naturally occurring compounds are too stable, i.e. you cannot extract energy from them by rearranging the atoms, like you can in carbon compounds. The most important condition for the appearance of life is an energy source. There are various abiotic processes that produce dihydrogen, carbon monoxide and carbon dioxide, and these gases can enter exothermic chemical reactions (e.g. dihydrogen + carbon monoxide => acetic acid), which can sustain life. There are no such energy sources from silicon compounds. When oxygen is in excess, silicon forms silicates, otherwise it forms carbides and nitrides. All these are inert. The only environment where silicon could play a role in the appearance of life would be a place with a very large abundance of hydrogen, i.e. a large planet like Jupiter, where it might happen that all the elements are present as compounds of hydrogen, so the silicon would be present as silanes, and there are differences in stability between various silanes that might be exploitable as an energy source if there exists any abiotic process that generates less stable silanes instead of the more stable silanes. This possibility remains open just because I have not seen yet any model of such an environment to determine what compounds may appear and what chemical reactions are possible, so it cannot be completely excluded that silicon could play a role there, unlike on medium-size planets or smaller bodies, where the hydrogen is less abundant and silicon does not combine with it, but with the more electronegative elements. I am not aware of any attempt to analyze what chemical reactions are possible in an environment with too much hydrogen, but I suspect that both the too oxidized environments with too much oxygen and the too reduced environments with too much hydrogen make impossible the occurrence of chemical reactions that could be used as a source of energy for life forms, so I think that life on Jupiter-like planets is unlikely.

deleted 5y ago

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