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Hydrothermal Vents Weren’t Home?

It’s probably not surprising, but you would be hard pressed to find an area that’s full of more intractable arguments than origin-of-life studies. There are so many theories, because it’s relatively easy to add new ones, and it’s difficult to impossible to put many of them to the real test. Meanwhile, the scientific stakes are potentially quite high, not to mention the philosophical ones. The universe (or large parts of it) appears to be swimming in water and simple organic building blocks. Under what conditions can these become self-replicating, complexity-amplifying systems like us, or an amoeba? How many such conditions are there, and how many outcomes are possible? And how likely are any of them? All open questions.

The “RNA world” hypothesis has many adherents, a scheme in which DNA evolved later than RNA and where catalytically functional RNA molecules came before proteins. This requires all of those earlier forms to have died out since then – no extant RNA-based life has been discovered – but there are some reasonable arguments about how this could have been an intermediate stage. But where and how did this happen? Fossil evidence shows that life got going rather early in our planet’s history – one recent paper may have pushed that back to almost 4.3 billion years ago, and the planet itself is only 4.6 billion years old. For that matter, the oceans are probably only about 4.4 billion years old themselves, and it seems likely that there were many more warm-to-hot aqueous environments than otherwise. The relative abundance of Archaea species and others around deep-sea hydrothermal vents have led many to speculate that these high-energy high-mineral-flux environments, or something like them, might be origin-of-life locations – indeed, those latest contenders for the oldest fossils may well have come from ancient hydrothermal vent sediments. It does seem plausible that iron sulfides could have come in useful for redox chemistry, and there’s plenty of that around some of these vents as well.

Chemical stability is a problem, though. Folded DNA and RNA species don’t last that long under hot aqueous conditions, and at an even more fundamental level, the nucleosides and nucleotides themselves aren’t so stable, either. Cytosine can be deaminated to uracil, and (similarly) cytidine to uridine, and this has been suggested as a problem for the various hot-water hypotheses. But one thing that hasn’t been investigated is the effect of pressure. Many chemical reactions can be sped up or slowed down if their transition states have a different volume than the starting materials and products, so it’s possible that stability studies of biomolecules are giving the wrong answers if this isn’t taken into account – in fact, this has been invoked as a way to rescue the whole RNA-around-deep-vents hypothesis.

Unfortunately, this new paper shows that pressure makes things even worse. The authors, a team from New Zealand, find (by using a specialized NMR rig) that high pressure actually accelerates cytidine’s decomposition. As the authors put it, this provides “scant support” for the hydrothermal proposals:

An average half-life of ~10 days for individual cytosines on a primitive genomic strand of 5000 nucleotides, one quarter of which are cytosine, corresponds to a spontaneous mutation every couple of hours for each strand. In the absence of editing and repair mechanism. This is not conducive to preservation of genomic information in a Model T Ford of inefficient, slowly reproducing early life forms.

There are still some potential ways around this – it might be that the presence of various metal ions (either through chemical reaction or just ionic strength of the solutions) might change things, and the stability of cytidine when it’s present in actual RNA strands under these conditions hasn’t been determined yet (although the NZ team says that’s their next project). You could also imagine a mechanism where some essential molecules get synthesized at higher temperatures and then deposit or concentrate in less inhospitable areas downstream. But these cytidine/cytosine decomposition kinetics are what make rescue hypotheses like this necessary, because a straight origin of life in hot, high-pressure conditions now looks very unlikely by itself.

31 comments on “Hydrothermal Vents Weren’t Home?”

  1. In Vivo Veritas says:

    Definitive proof of God! In ChemBioChem of all places……

    1. Lawrence Wolfe says:

      Surely thou jesteth!

    2. Tom Cruise says:

      Nah more likely the Scientologists were right all along

      1. Derek Lowe says:

        Now there’s a phrase you sure don’t hear very often.

    3. Uncle Al says:

      Satan created God to make the imposition stick without lifting the heavy end or suffering spreadsheets.

      “blew life into clay” OK – where is the aluminum and silicon?

  2. Curious Wavefunction says:

    Haven’t read the paper, but if it presupposes a “replication first” view of the origin of life then that’s not the only one out there. There’s the “metabolism first” view too, nicely described by Dyson for example in his “Origin of Life”, in which simple chemicals like acetate can kickstart primitive versions of biochemical cycles like the Krebs cycle. There’s also Kauffman’s collectively autocatalytic sets which catalyze each’s other’s chemistry. In addition you can have protocells and membranes which can shield nucleotides from decomposition. Metabolism and imperfect replication without stable nucleic acids could very much have gotten life off the ground.

    1. David Borhani says:

      Agree 100%. See, for example: Lane N & Martin WF (2012) “The Origin of Membrane Bioenergetics” Cell 151:1406

    2. Anon says:

      “but if it presupposes a “replication first” view of the origin of life then that’s not the only one out there. … simple chemicals like acetate can kickstart primitive versions of biochemical cycles like the Krebs cycle.”

      Aren’t such cycles a form of replication just like DNA and RNA? Or do you mean cycles that can’t self-amplify, like the water cycle, is that a form of life?

      1. Curious Wavefunction says:

        They are, they just don’t involve nucleic acids so that RNA stability is not an issue. Ultimately it really boils down to error rate vs fidelity: if the error rate is too great for fitness then replication won’t occur, but if it’s reasonable then you can get a pretty fit pool of replicators that can enable a self-sustaining set of reactions even with errors .

        1. Chris Phoenix says:

          To quantify error rate: Since most errors will be negative, to a first approximation the error rate has to be one error per offspring, or the lineage will devolve.

          This strongly implies that evolution requires a digital information substrate. Analog replication can’t be error-free. DNA of course is digital.

          Begin speculation: Language is digital, but only if it is rich enough to clearly express distinctions between adjacent concepts. This richness is too intricate to easily arise by chance from proto-languages (grunts and pantomimes). This could explain why, out of thousands of highly intelligent species, only one has achieved language – and is a candidate for one of the more optimistic answers to the Drake Equation. End speculation.

  3. Kent G. Budge says:

    ” no extant RNA-based life has been discovered”

    Slight caveat: There are certainly many RNA-based viruses. But I agree that classifying viruses as living is problematic.

  4. Barry says:

    the most visible vestige of an hypothetical RNA world may be the ribosome. It is not a protein machine with an RNA prosthesis for reading codons. It is an RNA machine with protein staples to enforce/stabilize its folding

  5. Dr. Manhattan says:

    I thought this was all settled in the Star Trek TNG episode “The Chase”….

  6. KN says:

    Derek, you seem to be sci-fi fan, have you read the Starfish series by Peter Watts? It’s kinda on topic.

  7. Shane says:

    I have not read the paper but I am a Geochemist with an interest in this area . There are achaea that can live in high temperatures, but on the edge of these environments the pH/eH, pressure temperature changes allow for some interesting chemistry at the atomic level (that we can’t reproduce or measure as our tweezers are too small).

  8. Qedlin Saltum says:

    Hydrothermal vents and RNA based origin of life are the latest desperate and intellectually insulting propositions of the naturalists. RNA is an intermediary biomolecule that requires precise fabrication requiring proteins and amino acids to perform cellular functions, is highly reactive, easily mutated. There is no contamination control at hydrothermal levels to prevent destruction and the supposed energy sources to induce the biochemical assembly are just as likely to destroy as produce. Then there is the salinity, the ph, the currents, none of which are conducive to OoL. All the lab experiment demonstrations of minuscule assembly are intelligent design highly managed affairs that are bio-geochemically irrelevant.

    1. David Edwards says:

      The short answer to this: nonsense.

      The longer and more detailed answer to this:”let’s try out these reactions and see if they work” does not equal “design”, and certainly not the sort of supernatural magic design the creationist/ID crowd keep pontificating about, which involves, according to the creationists’ own assertions, perfect foreknowledge of the outcome, something scientists have never had.

      Indeed, all of human “design” is ultimately traceable to “let’s try this out and see if it works”, followed by “discard the failures, and build upon the successes”. Now where else do we see that sequence of processes in action, I ask myself?

  9. DrOcto says:

    Personally I’ve always leaned towards a membrane (or micelle?) first theory. The starting point was contained in these bubbles (initially randomly) producing more membrane components from surrounding available small organic components, that were able to diffuse into these micro-reactors. When the bubble became too large it split from natural causes.

    Processes that were naturally faster were favoured, introducing complexity, giving one bubble type a key advantage. Rinse and repeat.

    What thse processes initially were is a loss to me, but it is my belief that we shouldn’t only be focusing on the RNA/DNA/protien central dogma, but on biochemical processes that produce oily membranes and fatty acids. Unless someone can demonstrate that these came later.

    This is of course without any basis at all, but hey that is no different to any of the other theories in this area.

    1. Derek Lowe says:

      There’s a lot to be said for that, considering that partitioning/localization/membrane concentration gradients are such a key feature of living cells, though.

    2. Jonas Ankarloo says:

      Well…maybe different to the ID h̶y̶p̶o̶t̶h̶e̶s̶i̶s̶ crap, above.

  10. Some Dude says:

    Many people in the Origins of Life community find a “warm little pond” more plausible than deep ocean environments. The main reasons for this are higher stability of primitive membranes in low salt environments and the possibility for the concentration of organic substrates through cycles of evaporation. It’s hard to conceive of an oceanic environment where one gets the necessary concentrations for non-enzymatic prebiotic reactions, and if one limits oneself to surface chemistry, there is a big transition problem to membrane based life.

    1. Chris Phoenix says:

      What about a hybrid between membrane and clay? Clay does interesting self-assembly things to biomolecules – both *NA and amino acids. And, some biomolecules like to embed in amphiphile membranes.

      So, clay with biomolecules hanging off it could attract and keep a micelle that was basically empty – and then you’d have a semi-protected environment between the clay and the membrane, but with plenty of opportunity for diffusion – no need to invoke early transport proteins – the move inside the membrane could come later.

      1. Shane says:

        Similar to diatoms and their silicon surface chemistry, now that makes sense to me. Some silica surface chemistry (like most of our chromatography columns but in a surface nano-cage), the right mix of organic atoms forming a crystal seed and you have some basic organic crystal reproduction then along comes a micelle ……….

  11. Odon says:

    There is no contamination control at hydrothermal levels to prevent destruction and the supposed energy sources to induce the biochemical assembly are just as likely to destroy as produce.

    1. Jonas Ankarloo says:

      Hey spambot!

      Bye spambot!

  12. Arnob Endry says:

    view of the origin of life then that’s not the only one out there. There’s the “metabolism first” view too, nicely described by Dyson for example in his “Origin of Life”, in which simple chemicals like acetate can kickstart primitive versions of biochemical cycles like the Krebs cycle. There’s also Kauffman’s collectively autocatalytic sets which catalyze each’s other’s chemistry.

    1. Scott says:

      You are a spambot, copy/pasting another commenter’s words.

      Go do something anatomically improbable.

  13. tangent says:

    Dumb question, what are the current species living there doing about instability of their cytosine? Do they have a ton of redundancy and repair that we don’t think would be feasible to bootstrap?

  14. David Edwards says:

    On the subject of competing hypotheses …

    My personal view is that what people see currently as competing hypotheses for abiogenesis, will in a mature field, be seen instead as contributors to a larger picture, with one system providing useful reagents for another system, and so on down the line. Indeed, I gather that this integrative approach, is a long term next project for Jack Szostak. Take the individual bits of the puzzle, and see which ways of assembling them produce meaningful results.

  15. Scott says:

    Let’s see here, abstract says 200MPa, while surface pressure is 0.2MPa. 1000x the pressure.

    At 44psi/100ft depth (roughly 3x pressure per 100ft), we’re talking about thermal vents at 33,300ft depth, roughly speaking the bottom of the Marianas Trench! I think that’s too deep for origin of life, but then again, I lean towards tidal pools at the water/air interface for the likely origin point.

  16. One caveat about water pressure on a 4.3 billion year old Earth. The achievable heights of mountains, and conversely depths of the oceans is a function of crustal thickness, in an early Earth the crust would not be the 30-50 (or 5-10 under water)km thick it is today, and as a result the maximum water depths would be similarly restricted. If you take the average depth of today’s oceans you would have 8400 feet of water covering the whole of the planet, but a much larger portion of that would be in mantle rocks (greater mixing) and also in the primordial atmosphere.
    Anywho, the max water pressure would be around 50MPa, but considering that the vents would be likely to be concentrated around the shallowest portions of the oceans (lighter mantle raises the covering crust), likely the relevant pressure would be halving that again, if not getting very close to the surface where the thicker atmosphere would result in maybe .5-1MPA of pressure to play with.
    BTW, I love the backs of envelopes, they make the best calculation space.

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