Even though life appears to be favorable in terms of the second law of thermodynamics (as explained in part one of this post), there have still been very important questions left unanswered regarding the origin of life including what mechanisms or platforms it could have used to get itself going initially. This can be summarized by a “which came first, the chicken or the egg” dilemma, where biologists have wondered whether metabolism came first or if instead it was self-replicating molecules like RNA that came first.
On the one hand, some have argued that since metabolism is dependent on proteins and enzymes and the cell membrane itself, that it would require either RNA or DNA to code for those proteins needed for metabolism, thus implying that RNA or DNA would have to originate before metabolism could begin. On the other hand, even the generation and replication of RNA or DNA requires a catalytic substrate of some kind and this is usually accomplished with proteins along with metabolic driving forces to accomplish those polymerization reactions, and this would seem to imply that metabolism along with some enzymes would be needed to drive the polymerization of RNA or DNA. So biologists we’re left with quite a conundrum. This was partially resolved when several decades ago, it was realized that RNA has the ability to not only act as a means of storing genetic information just like DNA, but it also has the additional ability of catalyzing chemical reactions just like an enzyme protein can. Thus, it is feasible that RNA could act as both an information storage molecule as well as an enzyme. While this helps to solve the problem if RNA began to self-replicate itself and evolve over time, the problem still remains of how the first molecules of RNA formed, because it seems that some kind of non-RNA metabolic catalyst would be needed to drive this initial polymerization. Which brings us back to needing some kind of catalytic metabolism to drive these initial reactions.
These RNA polymerization reactions may have spontaneously formed on their own (or evolved from earlier self-replicating molecules that predated RNA), but the current models of how the early pre-biotic earth would have been around four billion years ago seem to suggest that there would have been too many destructive chemical reactions that would have suppressed the accumulation of any RNA and would have likely suppressed other self-replicating molecules as well. What seems to be needed then is some kind of a catalyst that could create them quickly enough such that they would be able to accumulate in spite of any destructive reactions present, and/or some kind of physical barrier (like a cell wall) that protects the RNA or other self-replicating polymers so that they don’t interact with those destructive processes.
One possible solution to this puzzle that has been developing over the last several years involves alkaline hydrothermal vents. We actually didn’t know that these kinds of vents existed until the year 2000 when they were discovered on a National Science Foundation expedition in the mid-Atlantic. Then a few years later they were studied more closely to see what kinds of chemistries were involved with these kinds of vents. Unlike the more well-known “black smoker” vents (which were discovered in the 1970’s), these alkaline hydrothermal vents have several properties that would have been hospitable to the emergence of life back during the Hadeon eon (between 4.6 and 4 billion years ago).
The ocean water during the Hadeon eon would have been much more acidic due to the higher concentrations of carbon dioxide (thus forming carbonic acid), and this acidic ocean water would have mixed with the hydrogen-rich alkaline water found within the vents, and this would have formed a natural proton gradient within the naturally formed pores of these rocks. Also, electron transfer would have likely occurred when the hydrogen and methane-rich vent fluid contacted the carbon dioxide-rich ocean water, thus generating an electrical gradient. This is already very intriguing because all living cells ultimately derive their metabolic driving forces from proton gradients or more generally from the flow of some kind of positive charge carrier and/or electrons. Since the rock found in these vents undergoes a process called surpentization, which spreads the rock apart into various small channels and pockets, many different kinds of pores form in the rocks, and some of them would have been very thin-walled membranes separating the acidic ocean water from the alkaline hydrogen. This would have facilitated the required semi-permeable barrier that modern cells have which we expect the earliest proto-cells to also have, and it would have provided the necessary source of energy to power various chemical reactions.
Additionally, these vents would have also provided a source of minerals (namely green rust and molybdenum) which likely would have behaved as enzymes, catalyzing reactions as various chemicals came into contact with them. The green rust could have allowed the use of the proton gradient to generate molecules that contained phosphate, which could have stored the energy produced from the gradient — similar to how all living systems that we know of store their energy in ATP (Adenosine Tri-Phosphate). The molybdenum on the other hand would have assisted in electron transfer through those membranes.
So this theory provides a very plausible way for catalytic metabolism as well as proto-cellular membrane formation to have resulted from natural geological processes. These proto-cells would then likely have begun concentrating simple organic molecules formed from the reaction of CO2 and H2 with all the enzyme-like minerals that were present. These molecules could then react with one another to polymerize and form larger and more complex molecules including eventually nucleotides and amino acids. One promising clue that supports this theory is the fact that every living system on earth is known to share a common metabolic system, known as the citric acid cycle or Kreb’s cycle, where it operates in the forward direction for aerobic organisms and in the reverse direction for anaerobic organisms. Since this cycle consists of only 11 molecules, and since all biological components and molecules that we know of in any species have been made by some number or combination of these 11 fundamental building blocks, scientists are trying to test (among other things) whether or not they can mimic these alkaline hydrothermal vent conditions along with the acidic ocean water that would have been present in the Hadrean era and see if it will precipitate some or all of these molecules. If they can, it will show that this theory is more than plausible to account for the origin of life.
Once these basic organic molecules were generated, eventually proteins would have been able to form, some of which that could have made their way to the membrane surface of the pores and acted as pumps to direct the natural proton gradient to do useful work. Once those proteins evolved further, it would have been possible and advantageous for the membranes to become less permeable so that the gradient could be highly focused on the pump channels on the membrane of these proto-cells. The membrane could have begun to change into one made from lipids produced from the metabolic reactions, and we already know that lipids readily form micelles or small closed spherical structures once they aggregate in aqueous conditions. As this occurred, the proto-cells would no longer have been trapped in the porous rock, but would have eventually been able to slowly migrate away from the vents altogether, eventually forming the phospholipid bi-layer cell membranes that we see in modern cells. Once this got started, self-replicating molecules and the rest of the evolution of the cell would have underwent natural selection as per the Darwinian evolution that most of us are familiar with.
As per the earlier discussion regarding life serving as entropy engines and energy dissipation channels, this self-replication would have been favored thermodynamically as well because replicating those entropy engines and the energy dissipation channels means that they will only become more effective at doing so. Thus, we can tie this all together, where natural geological processes would have allowed for the required metabolism to form, thus powering organic molecular synthesis and polymerization, and all of these processes serving to increase entropy and maximize energy dissipation. All that was needed for this to initiate was a planet that had common minerals, water, and CO2, and the natural geological processes can do the rest of the work. These kinds of planets actually seem to be fairly common in our galaxy, with estimates ranging in the billions, thus potentially harboring life (or where it is just a matter of time before it initiates and evolves if it hasn’t already). While there is still a lot of work to be done to confirm the validity of these models and to try to find ways of testing them vigorously, we are getting relatively close to solving the puzzle of how life originated, why it is the way it is, and how we can better search for it in other parts of the universe.