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Self-Replicating Molecules.
How did certain chemicals combine to produce the first self-replicating molecules?
Answers
We don't know. Writings on the subject are still full of the words 'possibly' and 'perhaps'.
18:56 Wed 13th Nov 2013
The problem with the panspermia theory is that it's all well and good saying that amino acids can exist on space rocks. But can they survive the searing heat of entry and impact? That's the challenge. I believe no-one knows the answer to that question yet. My gut feeling is that they can't, and until that point is seems rather more likely that life originated on Earth. We're still short of any evidence for life existing elsewhere at all, other than Drake Equation arguments, let alone travelling between the planets.
//But can they survive the searing heat of entry and impact? That's the challenge. I believe no-one knows the answer to that question yet. //
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If you want clarification, NASA might be a better option.
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/ not to mention the metaphysics of consciousness/ That's a big leap Khandro, from a couple of dozen amino acids (known to exist) to metaphysics (not known to exist). Why don't you start another thread on that, although it has been done to death a few times I'm sure there is still some fun left in it.
jomifl; I'll think about it. There is of course another possibility, what I might call the "Star Trek" option; 3.8 billion years ago, when the planet was sufficiently cooled, it was visited by beings from elsewhere using technology we cannot imagine. Finding nothing of interest, they left, but not before emptying their on-board latrines, leaving sufficient organic material to kick-start the whole process, in which case we are all descended from, well ..... you know what.
@Khandro
That report I linked to typifies everything that is wrong with science reporting, these days. The headline is that a particular type of outer space radiation preferentially destroys R amino acids but, if you read further down the page, it said there was only a 2.5% bias in favour of L amino acids. Make of that what you will.
@jim & naomi
you're right to raise the issue of the heat of re-entry but what is remarkable about the detection of amino acids in meteorites is that many such finds involved rocks which had made it to the ground.
However, the trapped amino acids only remained intact by virtue of being trapped in micro cavities over 1cm below the outermost layer and not exposed to high temperatures. However, this means a long period of erosion or a freeze-fracure event is required to release the trapped material - but these would not be guaranteed as it has to avoid being buried in sediment.
One other thing I have thought of. The current theory about the formation of our moon involves a great impactor hitting the early earth. If the familiar computer animation is to be believed, this collision generated sufficient heat to melt the entire surface of the earth, essentially wiping out any extant life or roasting any pre-biotic chemicals. However, if there had been any such material, then we can expect some of the ejecta from the collision to have carried that into space.
So, before we get over-excited about space rocks containing amino acids, we have to have ways of being sure that the composition of any such meteorite find is consistent with it being from the solar system formation era and not collision ejecta bringing our own stuff back to us.
My main reason for disliking the Panspermia concept is the very fact that it involves dispersal of material out of a star system, despite the main emphasis being placed on its arrival at other systems. (Almost sleight of hand!)
The release of organic matter from the source system has to be dependent on a massive-scale rocky body collision of this sort, imho. Supernovae are too violent to be the dispersal mechanism (indeed the gamma radiation burst could destroy life to a radius of dozens of light years, even harming neighbouring star systems*); expansion to red giant certaily sheds materials (from the star surface, rather than the planets) that can condense and become other stars but, at the same time, the expansion phase can be expected to burn up any potential life-bearing planets.
Massive gravitational disturbance is one possibility though - multiple solar systems orbiting a common centre of gravity, two of them have a close flyby and a planet is thrown out of orbit. Doesn't bear thinking about though.
* not sure if Drake equation takes such hazards into account.
That report I linked to typifies everything that is wrong with science reporting, these days. The headline is that a particular type of outer space radiation preferentially destroys R amino acids but, if you read further down the page, it said there was only a 2.5% bias in favour of L amino acids. Make of that what you will.
@jim & naomi
you're right to raise the issue of the heat of re-entry but what is remarkable about the detection of amino acids in meteorites is that many such finds involved rocks which had made it to the ground.
However, the trapped amino acids only remained intact by virtue of being trapped in micro cavities over 1cm below the outermost layer and not exposed to high temperatures. However, this means a long period of erosion or a freeze-fracure event is required to release the trapped material - but these would not be guaranteed as it has to avoid being buried in sediment.
One other thing I have thought of. The current theory about the formation of our moon involves a great impactor hitting the early earth. If the familiar computer animation is to be believed, this collision generated sufficient heat to melt the entire surface of the earth, essentially wiping out any extant life or roasting any pre-biotic chemicals. However, if there had been any such material, then we can expect some of the ejecta from the collision to have carried that into space.
So, before we get over-excited about space rocks containing amino acids, we have to have ways of being sure that the composition of any such meteorite find is consistent with it being from the solar system formation era and not collision ejecta bringing our own stuff back to us.
My main reason for disliking the Panspermia concept is the very fact that it involves dispersal of material out of a star system, despite the main emphasis being placed on its arrival at other systems. (Almost sleight of hand!)
The release of organic matter from the source system has to be dependent on a massive-scale rocky body collision of this sort, imho. Supernovae are too violent to be the dispersal mechanism (indeed the gamma radiation burst could destroy life to a radius of dozens of light years, even harming neighbouring star systems*); expansion to red giant certaily sheds materials (from the star surface, rather than the planets) that can condense and become other stars but, at the same time, the expansion phase can be expected to burn up any potential life-bearing planets.
Massive gravitational disturbance is one possibility though - multiple solar systems orbiting a common centre of gravity, two of them have a close flyby and a planet is thrown out of orbit. Doesn't bear thinking about though.
* not sure if Drake equation takes such hazards into account.
My own reason for being slightly against panspermia is that it doesn't really solve the problem of how life emerged, just means that "it happened somewhere else". That's not really a good enough objection, of course, and hopefully in time we'll be able to discover the answer to how life emerged. Wherever it happened, I am fairly sure that it's a process that can occur far more easily than the apparent complexity of life would suggest -- and indeed is almost inevitable if the conditions are right. The rich variety of life even on this planet suggests that those conditions don't need to be all that precise, either, with life of sorts capable of surviving in virtually any situation you could think of. From intense cold to searing heat; with or without light; needing oxygen or abhorring it, and so on.
In the absence of any fully formed ideas, I just wanted to jot down some bits and pieces to give other an idea of my line of thinking
i) it seems practically obligatory that the association of amino acids with mRNA triplet-codons (or a steady progression towards the triplet type) happened before RNA or DNA strands got involved. The latter appear to have no meaning or purpose without the intermediary. Likewise, the ribosome facilitates the mRNA/RNA coming together but this makes it seem more like a later refinement than as a seed for the development of other parts of the mechanism.
ii) In the context of SIQ's reference to the cellular 'bag' as a boost to reagent concentration, the cell nucleus comes across as being a design refinement of that as opposed to a 'captured organism' structure, as other organelles have been dubbed, in the past (mitochondria and chloroplasts, to be specific). Extrapolating from that, it's possible that DNA was, itself, a refinement of a system which hitherto only required RNA, mRNA and ribosomes to manufacture proteins.
iii) ADP/ATP is all-pervasive as the energy exchange mechanism in the cell. Anabolic reactions, such as the building of polypeptides require energy input, so it gets involved. If a cell membrane is present, large molecules, such as amino acids require active transport across it into the cell, again requiring ATP. Adenosine is adenine with a ribose sugar attached, of course. With no shortage of energy input, from the sun, it seems logical that a chemical energy transduction system would come about at some point. Not a lot different from the type of chemical reactions which require heat input to overcome the enthalpy hump. Again, this all needs to be in place before complex polymer formation stands a chance of progressing beyond short chains.
iv) Apparently, the bacteria at the foot of the food chain at those deep ocean hydrothermal vents use sulphur as an oxidant (for turning food into energy). This means they, or something exactly like them, could have existed on earth before photosynthetic life forms began to release oxygen into the atmosphere and oceans. If they were found to use ATP in the same way as all other organisms, that would be a significant finding.
v) the evolutionary switch from sulphur to oxygen as preferred oxidant cannot occur until after life has reached the sunlit shallows and oxygen release has begun. Therefore the earliest photosynthetic life must have been the sulphur-using kind.
i) it seems practically obligatory that the association of amino acids with mRNA triplet-codons (or a steady progression towards the triplet type) happened before RNA or DNA strands got involved. The latter appear to have no meaning or purpose without the intermediary. Likewise, the ribosome facilitates the mRNA/RNA coming together but this makes it seem more like a later refinement than as a seed for the development of other parts of the mechanism.
ii) In the context of SIQ's reference to the cellular 'bag' as a boost to reagent concentration, the cell nucleus comes across as being a design refinement of that as opposed to a 'captured organism' structure, as other organelles have been dubbed, in the past (mitochondria and chloroplasts, to be specific). Extrapolating from that, it's possible that DNA was, itself, a refinement of a system which hitherto only required RNA, mRNA and ribosomes to manufacture proteins.
iii) ADP/ATP is all-pervasive as the energy exchange mechanism in the cell. Anabolic reactions, such as the building of polypeptides require energy input, so it gets involved. If a cell membrane is present, large molecules, such as amino acids require active transport across it into the cell, again requiring ATP. Adenosine is adenine with a ribose sugar attached, of course. With no shortage of energy input, from the sun, it seems logical that a chemical energy transduction system would come about at some point. Not a lot different from the type of chemical reactions which require heat input to overcome the enthalpy hump. Again, this all needs to be in place before complex polymer formation stands a chance of progressing beyond short chains.
iv) Apparently, the bacteria at the foot of the food chain at those deep ocean hydrothermal vents use sulphur as an oxidant (for turning food into energy). This means they, or something exactly like them, could have existed on earth before photosynthetic life forms began to release oxygen into the atmosphere and oceans. If they were found to use ATP in the same way as all other organisms, that would be a significant finding.
v) the evolutionary switch from sulphur to oxygen as preferred oxidant cannot occur until after life has reached the sunlit shallows and oxygen release has begun. Therefore the earliest photosynthetic life must have been the sulphur-using kind.
Hypo; This is impressive (seriously), but I've just been looking at the wikipedia entry for scientific method; methodhttp://en.wikipedia.org/wiki/Scientific_method and isn't it a fact that as fascinating as these lines of enquiry are, they are unfortunately never going to be repeatable, and therefore impossible to verify.
Quote; "The chief characteristic which distinguishes the scientific method from other methods of acquiring knowledge is that scientists seek to let reality speak for itself, supporting a theory when its predictions are confirmed and challenging a theory when its predictions prove false."
Quote; "The chief characteristic which distinguishes the scientific method from other methods of acquiring knowledge is that scientists seek to let reality speak for itself, supporting a theory when its predictions are confirmed and challenging a theory when its predictions prove false."
Hypo, just a thought re. concentration of components necessary to allow 'life' to develop. littoral splash pools would be excellent concentrators of the soup and perodic changes in pH might enable the zipping and unzipping of proto life molecules. It is possible to envisage a chain of amino acids possibly loosely attached to a substrate in such a way that complementary amino acids attach themselve to it to make a mirror molecule. A change in the molecule's environment such as salinity, or pH could cause them to unzip so that the 'mirror' floats off to make more originals. I don't know anything about the sensitivity of hydrogen bonds to pH changes or perhaps other factors but it is perhaps a plausible mechanism. It is also possible that molecules were combining and splitting in great abundance so that it was only a matter of time before a viable molecular chain came about.
//it doesn't really solve the problem of how life emerged, just means that "it happened somewhere else".//
That grossly underestimates the impact that such a discovery would have upon this planet. It would demonstrate very clearly indeed that evolution is an undeniable fact – and imagine the ramifications of that upon religion.
That grossly underestimates the impact that such a discovery would have upon this planet. It would demonstrate very clearly indeed that evolution is an undeniable fact – and imagine the ramifications of that upon religion.
Wow, so many of us know our stuff that the thread is shredding into many intelligent ideas beyond the prime question.
I favour concentrating on earth-only creation (science based) because outer-space evidence of some of life's basic chemicals is only indicative that these molecules can spontaneously occur. It in no way contradicts earth-based origin in fact supports the "easy", lol, formation of life forming molecules here (as well as elsewhere if you like). Earth is still the best place we know which would allow bacteria to form and, possibly with amoeba, to evolve eventually into us. Why amoeba? Because a credible theory is that they invaded certain bacteria to form the ordered structures which are the powerhouses of sophisticated cell life - the mitochondria mentioned by hypo.
So pure outer-space origin of life invading earth is interesting but wow, get this! If I'm right Khandro(!) said it was only putting the question further away. My thinking exactly.
So outer space is just a diversion (for my thinking).
SIQ.
I favour concentrating on earth-only creation (science based) because outer-space evidence of some of life's basic chemicals is only indicative that these molecules can spontaneously occur. It in no way contradicts earth-based origin in fact supports the "easy", lol, formation of life forming molecules here (as well as elsewhere if you like). Earth is still the best place we know which would allow bacteria to form and, possibly with amoeba, to evolve eventually into us. Why amoeba? Because a credible theory is that they invaded certain bacteria to form the ordered structures which are the powerhouses of sophisticated cell life - the mitochondria mentioned by hypo.
So pure outer-space origin of life invading earth is interesting but wow, get this! If I'm right Khandro(!) said it was only putting the question further away. My thinking exactly.
So outer space is just a diversion (for my thinking).
SIQ.
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