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Thread: Complex molecules could hold the secret to identifying alien life

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    Complex molecules could hold the secret to identifying alien life

    Phys.org

    A new system capable of identifying complex molecular signatures could aid in the search for alien life in the universe and could even lead to the creation of new forms of life in the laboratory, scientists say.

    University of Glasgow researchers have developed a new method called Assembly Theory which can be used to quantify how assembled or complex a molecule is in the laboratory using techniques like mass spectrometry. The more complex the object, the more unlikely that it could arise by chance, and the more likely it was made by the process of evolution.

    ...

    Professor Cronin, Regius Professor of Chemistry at the University of Glasgow, said: "Our system is the first falsifiable hypothesis for life detection. It's based on the idea that only living systems can produce complex molecules that could not form randomly in any abundance. This allows us to sidestep the problem of defining life—instead we focus on the complexity of the chemistry."
    This article about the work from Chemical & Engineering News (sorry, behind a firewall) made this interesting point:
    Cronin’s team used the method to assign MA numbers to 2.5 million molecules. Then they used a subset to experimentally verify the expected correlation between the MA number and the number of peaks appearing in the second stage of tandem mass spectrometry analysis—an experimental indicator of complexity.

    The team then tested inorganic materials, living and dead biological samples, and some blinded samples sent by NASA. These samples included a bit of meteorite, which is not of biological origin, and fossil-containing lake sediment from 30,000 and 14 million years ago.

    The materials had a wide range of MA scores, but all the biologically derived samples had MA numbers above a certain threshold. This finding implies that such a threshold could be used to classify a molecule as derived from something living or not.
    Here is a link to the paper

    Abstract
    The search for alien life is hard because we do not know what signatures are unique to life. We show why complex molecules found in high abundance are universal biosignatures and demonstrate the first intrinsic experimentally tractable measure of molecular complexity, called the molecular assembly index (MA). To do this we calculate the complexity of several million molecules and validate that their complexity can be experimentally determined by mass spectrometry. This approach allows us to identify molecular biosignatures from a set of diverse samples from around the world, outer space, and the laboratory, demonstrating it is possible to build a life detection experiment based on MA that could be deployed to extraterrestrial locations, and used as a complexity scale to quantify constraints needed to direct prebiotically plausible processes in the laboratory. Such an approach is vital for finding life elsewhere in the universe or creating de-novo life in the lab.
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    This is similar in principle to what Chris McKay proposed as a method to determine whether Titan has life.

    "a key feature of life is that it is selective in the basic molecules it uses [55,56]. Molecules with similar chemical properties can be present in biology in vastly different concentrations. Life on Titan would also have to select specific molecules from the possible variations. Thus, the prediction... should hold on Titan that a biological distribution of molecules would be a series of relatively sharp spikes while an abiotic distribution would be smooth."

    Titan as the abode of life

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    Quote Originally Posted by Colin Robinson View Post
    This is similar in principle to what Chris McKay proposed as a method to determine whether Titan has life.

    "... Thus, the prediction... should hold on Titan that a biological distribution of molecules would be a series of relatively sharp spikes while an abiotic distribution would be smooth."
    .. which is exactly the opposite of what the Marshall paper demonstrates:
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    Quote Originally Posted by Selfsim View Post
    .. which is exactly the opposite of what the Marshall paper demonstrates:
    ???

    As I understand it, Marshall is saying that a body of biological-produced carbon compounds has a greater range of complexities — it has a greater proportion of very complex molecules than would be produced abiologically.

    That is not "exactly the opposite" of McKay's point that "Molecules with similar chemical properties can be present in biology in vastly different concentrations."

    Compared to non-living systems, living systems are highly specific in terms of what carbon compounds they produce, and they produce some very complex compounds as well as simpler ones...
    Last edited by Colin Robinson; 2021-Jun-14 at 12:37 PM.

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    Quote Originally Posted by Colin Robinson View Post
    <snip>

    As I understand it, Marshall is saying that a body of biological-produced carbon compounds has a greater range of complexities — it has a greater proportion of very complex molecules than would be produced abiologically.
    Either I don't understand what you are saying, or that is not my interpretation. I don't think they are making a particular point about the range of complexities for biologically produced compounds, though I suppose that could be inferred if non-biological processes only produce compounds up to a certain level of complexity (their molecular assembly index (MA)) and biological processes produce both less complex and more complex compounds.

    They are looking for signatures of biotic processes. Methane, for example, would not be a good signature, because it can be produced by biotic and abiotic processes. They have found that compounds with an MA above a certain value are apparently only produced by biotic processes. So there presence would be a sign of life. They have developed a mass spectroscopy method for measuring MA and they have experimentally demonstrated the relationship that compounds with MA above a threshold value were only from biological sources.

    While the idea of looking for a biotic signature might be similar to McKay's idea, it seems to be a very different method. But I also don't understand what property McKay is measuring when he proposes this spiky distribution. Homochirality?

    What I most appreciate about this paper is that they have developed a measure that could be easily implemented by a probe (mass spectroscopy) and they have experimentally demonstrated MA's effectiveness in showing biological origins of test molecules. McKay's paper comes across to me as almost completely speculative and non-experimental.
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    Quote Originally Posted by Colin Robinson
    As I understand it, Marshall is saying that a body of biological-produced carbon compounds has a greater range of complexities — it has a greater proportion of very complex molecules than would be produced abiologically.

    That is not "exactly the opposite" of McKay's point that "Molecules with similar chemical properties can be present in biology in vastly different concentrations."

    Compared to non-living systems, living systems are highly specific in terms of what carbon compounds they produce, and they produce some very complex compounds as well as simpler ones…
    McKay’s claim that ‘Nonbiological processes produce smooth distributions of organic material’ is special pleading. (See attachment #1 for his superficial 'sketch').
    He provides no quantitative evidence for this, and in fact, the SAM/Curiosity analysis of Mars’ organics, disproves it. See SAM/Cumberland spectrum in attachment #2 below, link here).

    Marshall etal point out that the molecular assembly index ‘dimension’ accounts for the specificity of biotic over abiotic molecules, because biochemical systems ‘have information decoding and encoding processes that drive networks of complex reactions to impose the numerous, highly specific constraints needed to ensure reliable synthesis’ .. (all of which, is well founded on known, modern prebiotic molecular complexity theory).

    Marshall etal have not posited anything ‘similar in principle to what Chris McKay proposed as a method to determine whether Titan has life’ (ie: your claim there). Theirs is based on real evidence and theory, whereas McKay just made something up to fit his Titan story.
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    Quote Originally Posted by Swift View Post
    Either I don't understand what you are saying, or that is not my interpretation. I don't think they are making a particular point about the range of complexities for biologically produced compounds, though I suppose that could be inferred if non-biological processes only produce compounds up to a certain level of complexity (their molecular assembly index (MA)) and biological processes produce both less complex and more complex compounds.
    Yes, and that's what is shown in Fig 4 graph b of their paper. That is the graph Selfsim cited, in the post I was responding to.

    While the idea of looking for a biotic signature might be similar to McKay's idea, it seems to be a very different method. But I also don't understand what property McKay is measuring when he proposes this spiky distribution. Homochirality?
    Yes, he calls chirality the "premier example" of the selectivity he is talking about.

    "The premier example of biochemical selectivity is chirality. This property is most well known in the amino acids used in proteins. Life on Earth uses only the L version of amino acids in proteins, not the mirror image, the D version. If life on Titan also uses molecules with chiral centers then detection of homochirality is a powerful indication of life... Chirality can be determined by chromographic and optical methods..."

    What I most appreciate about this paper is that they have developed a measure that could be easily implemented by a probe (mass spectroscopy) and they have experimentally demonstrated MA's effectiveness in showing biological origins of test molecules. McKay's paper comes across to me as almost completely speculative and non-experimental.
    It's true McKay's paper doesn't present a lot of new experimental results.

    It does present a strategy for distinguishing non-living carbon chemistry from a living chemistry different from Earth life, yet comparable to Earth life. This is what I see as the common factor in McKay's paper and Marshall's.

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    Quote Originally Posted by Colin Robinson View Post
    Yes, he calls chirality the "premier example" of the selectivity he is talking about.

    "The premier example of biochemical selectivity is chirality. This property is most well known in the amino acids used in proteins. Life on Earth uses only the L version of amino acids in proteins, not the mirror image, the D version. If life on Titan also uses molecules with chiral centers then detection of homochirality is a powerful indication of life... Chirality can be determined by chromographic and optical methods..."
    Whereas Marshall etal point out in the intro:
    It is possible to distinguish between living and non-living systems on Earth due to processes such as photosynthesis, carbon and nitrogen fixation, replication, chiral enrichment, and morphogenesis. The artefacts of these processes have been proposed as possible biosignatures. There are proposals to search for such artefacts in minerals, and via isotopic and atmospheric analysis. The problem with looking for such processes in a universal manner is the lack of a rigorous definition outside the context of known terrestrial biochemistry, and therefore these cannot be deployed to design experiments. This has led to several ambiguous results from ‘metabolic’ experiments done by the Viking Lander1 on Mars, and investigations of potential meteorite ‘microfossils’
    Chirality preferences also occur amongst inorganic molecules.

    The problem McKay has always had is with his approach to the problem.
    He assumes 'If life is present ...'. The whole point is that the truth of that proposition is unknown from the outset, (regardless of his assumption).

    Marshall etal have acknowledged the problem with directed searches for life, citing the Viking life experiment weaknesses, then moving forwards based on those facts.

    McKay's approach however, just leads back down the same old rabbit-hole experience of Viking.

    Quote Originally Posted by Colin Robinson
    It's true McKay's paper doesn't present a lot of new experimental results.

    It does present a strategy for distinguishing non-living carbon chemistry from a living chemistry different from Earth life, yet comparable to Earth life.
    .. (assuming the existence of what one is seeking to find, is actually in a given sample .. which is by no means 'a given') ..

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    Quote Originally Posted by Selfsim View Post
    Chirality preferences also occur amongst inorganic molecules.
    What do you mean by that statement, and do you have a source?

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    Quote Originally Posted by Colin Robinson View Post
    What do you mean by that statement, and do you have a source?
    Mineral forms such as in quartz (SiO4), AlPO4 and GaPO4, with tetrahedral networks, display chirality. HgS (cinnabar), found near volcanic vents/hot springs on Earth, is another. The environments where such mineral states are likely to occur, (other than on Earth), have been observed just in our local solar system and tend to register as potential targets by the exo-life hunting mindset.
    (Minerals play significant roles in some abiogenesis hypotheses too).


    A detailed discussion about what chirality means, and the ins and outs of how it would be detected in remote de novo environments, would be outside the scope of this thread (I’m thus unwilling). However, the point in raising it here, was made clear in the Marshall quote already given:
    The problem with looking for such processes in a universal manner is the lack of a rigorous definition outside the context of known terrestrial biochemistry, and therefore these cannot be deployed to design experiments.

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    Quote Originally Posted by Selfsim View Post
    Chirality preferences also occur amongst inorganic molecules.
    Quote Originally Posted by Colin Robinson View Post
    What do you mean by that statement, and do you have a source?
    Quote Originally Posted by Selfsim View Post
    [FONT=&]Mineral forms such as in quartz (SiO4), AlPO4 and GaPO4, with tetrahedral networks, display chirality.
    It's one thing to say that chirality occurs in inorganic molecules, another thing to say that chirality preferences occur, surely?

    A detailed discussion about what chirality means, and the ins and outs of how it would be detected in remote de novo environments, would be outside the scope of this thread (I’m thus unwilling).
    OK, but you were arguing against the idea that life chemistry selects particular chiral forms in a way that non-life chemistry typically doesn't.

    I have to say that you haven't convinced me.
    Last edited by Colin Robinson; 2021-Jun-18 at 04:54 AM.

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    Quote Originally Posted by Colin Robinson View Post
    ... OK, but you were arguing against the idea that life chemistry selects particular chiral forms in a way that non-life chemistry typically doesn't.
    ??

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