I thought hard about whether I should write this section or not, given the speculative nature of studying the origin of life. Then I thought it would be interesting for readers and it would also allow me to sort out my thoughts a bit on the subject. Of course, I will not be discussing the whole concept of abiogenesis, just some aspects which tie into deep sea geochemistry and are of interest. Just keep in mind that while everything I say in here is based on facts, the connections I make are my own opinion and are in no way proven. Do not take them as true; think about them, question my logic and make up your own informed minds.
Note: I won’t talk about the formation of the Solar Sytsem or of the evolution of the atmosphere – I’m keeping this strictly within the deep sea.
It’s always said that life on Earth is carbon-based. This is true, but also somewhat redundant. A more interesting statement would be to say that life on Earth is based on nitrogen – also true, and with more implication for any thinking about abiogenesis. It’s not hard to get plain organic molecules, but how do you generate nitrogen-containing organic compounds? Any scheme for the origin of life must find such a geochemically plausible pathway.
The connection with the deep sea is that that’s where these first biologically active molecules (if we may call them that) probably originated. Specifically, at hydrothermal vents or subduction zones, where you have so many reactive chemicals dissolving into superheated water, providing plenty of opportunities for reactions. This automatically means that before life on Earth could originate, it had to be geologically active.
Elemental nitrogen (N2) is very stable and useless for biochemistry. What we need are reduced nitrogen ions. Lightning and other high-energy processes can oxidise N2 to NO2- and NO3- (nitrite and nitrate ions). These can then be taken up by the ocean (if the lightning strikes on the ocean water, this is guaranteed to happen) and get transported down to the bottom of the ocean and into the lithosphere (at subduction zones and vents, sea water does get into the crust).
Hydrothermal vents are highly reducing environments. The nitrogen and its oxidised relatives all get reduced there to NH4+ (ammonium ion) and NH3 (ammonia) – both molecules that are heavily featured in biology. Experiments have shown that this happens without biological interference, catalysed by iron and nickel, two elements found in very high concentrations in such places because they’re part of the minerals making up the rock of the oceanic lithosphere (olivines, pyroxenes). This is a simple mechanism, shown to work both in nature and in the lab, and provides a plausible source of biologically important molecules (as well as other potentially useful side-products like iron oxides.)
But we still don’t have them as organic compounds. Obviously, we need to bring carbon into the story now. Another reaction that occurs in hydrothermal vents is the reduction of carbon dioxide (CO2) to methane (CH4), going through carbon monoxide (CO), again catalysed by iron and nickel. Both the CO and the CH4 are important. Around hydrothermal vents are layers of silicate minerals called zeolites. They can absorb CO and NH3, and experiments have shown that the absorbed molecules react to form HCN – a nitrogen-containing organic compound.
Importantly, the same experiments have shown that amino acids and adenine (the A in ATGC) can also form this way, but their realisticness can be debated.
Let’s stay with the HCN, a molecule that will undoubtebly show up in any discussion on abiogenesis. Keep in mind that the pathway I outlined above is not the only way to form HCN chemically (UV and lightning can generate it when CH4 and N2 are present, for example). But I support it for the simple reason that it’s realistic, in that I can imagine it happening on the early Earth, with high concentrations of HCN (and the amino acids) all concentrated in one place on the zeolites.
And high concentration is very important because it’s the only way HCN will react in organic chemistry. And from HCN, it’s easy to form at least the purine bases (adenine and guanine), and those same formation pathways can also form numerous amino acids.
Now you may be wondering how we can get CO2 (as it’s the precursor to CO and CH4) into hydrothermal vents. The answer is, again, the circulation of seawater in the oceanic lithosphere. CO2 is constantly getting converted to methane in hydrothermal systems. This occurs simply by reacting with H2, itself a product of serpentinisation.
Serpentinisation is when olivines and pyroxene in peridotite (the minerals in the rock making up the oceanic lithosphere) react with cold water (this reaction does not occur in the lithosphere but near the surface). What basically happens is that the iron in the minerals gets oxidised while the hydrogen from the water gets reduced to H2 – how much H2 is produced depends on several factors and it’s irrelevant to get into the details. Suffice it to say that the reaction happens and is quite common, and in modern hydrothermal vents where it occurs, archaeans that live off of hydrogen are very abundant.
Of course, a full analysis of prebiotic chemistry must include discussions of pH and redox conditions – which I will not go into because it would go beyond my current scope, expertise and possibly the realm of realism. And of course, the issue of pressure: we’re talking of the deep sea here, and while higher pressure favours these chemical reactions, they do not favour chemical stability – except in the case of large structures, such as RNA.
With that in mind, let’s press on into more hypothetical territory, and into a small introduction of the two main theories for the origin of life itself: the RNA world and the membrane world (okay, I’ll be honest and say I don’t know what the second one is actually called).
There are geochemically viable pathways for creating the components of nucleotides (nitrogen bases, ribose and phosphate). However, combining them to form actual nucleotides, and combining nucleotides to form RNA is still a matter of debate. It’s possible that there were some kinds of self-replicating mineral structures (dubbed protoRNA) that would serve as the basis for life.
But such an RNA world view is not entirely satisfactory. It implies that there were small prebiotic molecules found in low concentrations. But proteins, including RNA, need high concentrations of their components to be able to synthesise – i.e. if you have the nucleotides swimming around, they will not combine to form RNA. And this is where the membrane theory comes in.
The basic gist of it is that there must have been some structure that concentrated the various constituents together in a favorable environment – a membrane, if you will. This membrane should be partially permeable to allow chemicals to get in. Very suggestive findings have shown that esters and carboxylic acids – the molecules that make up lipids – are found in hydrothermal vents. The great thing about lipids is that they attract each other and therefore can naturally self-assemble into a membrane, which can be imagined as enclosing the nucleotide monomers. And from there, you can imagine protocells forming very gradually as natural selection starts acting.
And let’s not forget the point about pressure I mentioned: high pressures favour the formation and stabilisation of RNA and other large biomolecules, so there’s always such details that need to be considerd.
And this is the most important point about this topic: anyone can come up with a more or less plausible scheme for the origin of biological molecules and even confirm it with experiments and observations. But no matter how much evidence you accumulate, you will never be able to prove a scenario, only falsify it. There are many ways of producing biochemical building blocks and we will never know how it happened in the early Earth. That is a (sad, pessimistic?) fact.
Our only hope is to go for exobiology, which is a field fraught with its own problems, the main one being that we haven’t found any shred of evidence for exobiology, making it more akin to a pseudoscience.