Experimental physicist Rob Sheldon on the recent merely “plausible” origin of life find
|January 10, 2018||Posted by News under Chemistry, Intelligent Design, Origin Of Life|
I’m no chemist, my last class being Organic Chem in college, but I’ll take a stab at this paper. Like most of these Origin-of-Life papers, the chemistry is awesome, the interpretation, well, a bit pretentious.
OOL Theories: Among the favorite materialistic origin stories, there is “DNA-first”, “RNA-first” and “Metabolism-first”. I suppose we might add “protein-first” and “cellwall-first” to the list as well. Once a self-replicating structure of your favorite model was in place, then the magic of natural selection would automatically add wings to your pig, and Voila! OOL.
The Problem: that first self-replicating system is a real pain. It has to be simple enough to materialize, but complicated enough to perform. Zooming in on metabolism-first, the first cell needed a machine that took in food and produced energy, aka electrons. In cells today, this is the citric acid cycle, a chemistry “machine” that starts with a 4-carbon molecule, adds an acetate group to make a 6-carbon molecule, then uses oxygen to remove two carbon dioxide gas molecules to recreate the initial 4-carbon molecule and a bunch of electrons.
Now mind you, oxidizing organics isn’t difficult to do, I can do this abiotically with a propane torch, but nothing recycles. The cell does it gently with enzymes so as to harvest the electrons, create ATP, and otherwise recycle all the pieces minus the acetate groups (and the dissolved oxygen). But for OOL, one would prefer no enzymes.
So what this paper shows, is that they have identified a cycle that doesn’t use enzymes or citric acid, in which the consumables are glyoxylate (a 2-carbon chemical) replacing acetate and hydrogen peroxide (at 50% strength) replacing dissolved oxygen. So robust is this chemistry, that just by adding the two consumables in a different order: 1,2 gives a different cycle than 2,1 but it still comes back to the beginning again. (We’ll ignore the 50C temperature for the second cycle–they said it was just a shortcut to avoid waiting a month for the cycle to complete.)
a) These are all organic and biological chemicals, so calling this cycle “abiotic” merely means that it has not yet been observed in living organisms. This is a peculiar use of the word “abiotic” and intended to imply more significance than it deserves.
b) Having to add the chemicals sequentially pretty much destroys this as a likely metabolism reaction, because it is a “driven” system. The whole point of metabolism is that it provides energy for independent living, but if something has to keep adding the ingredients 1, 2, 1, 2, 1 , 2 then the energy may be available, but it is certainly not solely self-dependent, like a teenager with dad’s car keys.
c) Ignoring for the moment the availability of rather complex hydrocarbons (they list aspartate, 3-carboxy-fumarate, pyruvate) as precursors to the cycle, the availability of hydrogen peroxide should really be questioned. I have a bottle of it on my shelf at maybe 5% strength, and it turns to water all on its own, being highly unstable. Anything stronger than 5% will bleach your hair. 50% strength is used to clean all the organics off diatoms. 100% H2O2 is used as rocket fuel in those portable jet packs. So nasty is the stuff, that your cells have all sorts of enzymes whose job it is to remove hydrogen peroxide before it breaks something.
Nevertheless, they think that running your car engine on rocket fuel might actually work, and they cite 3 references to abiotic hydrogen peroxide:
1) Borda, M. J., Elsetinow, A. R., Schoonen, M. A. & Strongin, D. R. Pyrite-induced hydrogen peroxide formation as a driving force in the evolution of photosynthetic organisms on an early earth. Astrobiology 1, 283–288 (2001). [paywalled]
FeS2 with Fe(3) at defect sites + H20->H2O2 + Fe(2) + OH
This relies on iron pyrite being “sensitized” or energized before getting water on it. They say exposing it to sunlight (UV?) improves the reaction rate. The paper is published in Astrobiology magazine, so of course there is no quantitative measure here, everything is qualitative. But in practice, the amount of H2O2 is very limited, as well as self-limiting as the OH builds up. It is one of those “in principle” sources of unobtainium.
2) Encrenaz, T., Greathouse, T. K., Lefevre, F. & Atreya, S. K. Hydrogen peroxide on Mars: observations, interpretation and future plans. Planet Space Sci. 68, 3–17 (2012).
Once again, this was a modelled 40 parts per billion concentration in the Mars atmosphere as seen from Earth. By comparison, water was at 250,000 ppb, giving a ratio of 160ppm of H2O, driven by solar UV most likely.
3) Carlson, R. W. et al. Hydrogen peroxide on the surface of Europa. Science 283, 2062–2064 (1999).
This was at 0.13% concentration. Not bad. But the abstract tells the full story: “The inferred abundance is consistent with radiolytic production of hydrogen peroxide by intense energetic particle bombardment and demonstrates that Europa’s surface chemistry is dominated by radiolysis.”
So what have we learned. Hydrogen peroxide is a violent chemical that is either formed in small amounts by UV light, or larger amounts with nuclear-reactor levels of radiation. In neither case does it approach the levels needed for this reaction. Nor would life likely last long in intense UV or intense radiation environments.
So while the chemistry is interesting, it is like trying to run your car engine by periodically injecting nitroglycerine into the fuel port. It may turn over to make a “cycle”, but will self-destruct long before “natural selection” adds cruise control.
Then why even propose it as a OOL step?
The paper is unintentionally honest:
“However, attempts at using TCA [citric acid cycle] pathways as a template for prebiotic reactions have not yet yielded sustainable non-catalytic cycles…The reaction conditions are quite mild [50C + 50% H2O2] as compared to those required for nonenzyme-catalyzed TCA and reverse-TCA (rTCA) steps that use canonical intermediates. Examples in the literature include three non-contiguous steps in the rTCA: oxaloacetate to malate (75%), fumarate to succinate (95%) and succinate to αketoglutaric acid (2.5%), carried out in ultraviolet (UV)-irradiated solution of aqueous ZnS colloid … Recently, a non-enzymatic promotion of multiple reactions of the rTCA cycle in consecutive sequence mediated by Zn(2+), Cr(3+) and Fe(0) has been demonstrated in 1 M aq. HCl and 1 M aq. H2SO4 at 80–140 °C.”
The closest abiotic attempt at “metabolism first” required fuming sulfuric and hydrochloric acid in a pressure cooker. This paper uses only fuming hydrogen peroxide at 50C.
See also: New recipe for early life on Earth is marketed as merely “plausible”? The experiment is interesting but it says a lot about the (lack of) progress in a field when the best that can currently be said of a scenario is that it is plausible. On the other hand, the concept they are testing deserves a lot of respect, in these times, for not just being another what-if, of which we seem to go through dozens around here every year.