Solving the riddle of the origin of life would be a major step forward in the solution of the absent aliens problem. Is it hard to originate life, meaning there are hardly any planets in the Milky Way where it could happen? Then the problem is solved – there never were any aliens. Is it easy to originate life, meaning there are lots, meaning millions, of planets in the Milky Way that possess all the conditions for originating life. Then we have to look for reasons in evolution or the generation of intelligence or civilization collapse or elsewhere in later eras to solve the problem.
Life has two essential attributes. Approximate replication and energy consumption. Life needs energy to sustain a living organism and to build another one. There are two categories, or at least popular science writers like to say there are two categories of theories about the origin of life. One is called the Replicator theory, which has a specific flavor called the RNA world, and the other is called the metabolism theory. The first says that something formed spontaneously which then catalyzed similar things to be formed; this one was promoted in this blog. The second one says there was some energy pathway which existed first and things which could use the energy to build structures formed around the source.
These two theories are the same thing. The replication of the first variety has to be fueled by an energy source, which had to have existed before the replication could take advantage of it. The energy source in the second doesn't become life until there is something which can replicate itself, using that energy. The real questions are in the details.
Once deep sea vents were discovered in 1977, they became a favorite speculation as to the source of energy. Hot fluxes of seawater, having found a way down to very hot layers, would come up with the minerals from deep in the earth. At the vent, they would meet cool seawater, saline, with dissolved gases, and any chemist should be able to come up with a combination that has available energy. Which combination was the one that led to the first replicating molecules or combinations of molecules? We can call that Puzzle number 1.
What was the first molecule that could take advantage of that energy and replicate itself, exactly or approximately? What exactly is replication here? We have a saturated system when the hot vent water is quenched by the cool seawater – things precipitate out. Any supersaturated solution precipitates, and anybody who had a childhood chemistry set knows, chemicals often prefer to saturate onto substrates made up of the same chemical. Do organic compounds do this? Is this replication? If the compound forms a pillar, of the type shown around the sea vents of today, and one piece breaks off to land somewhere else, and begin its own precipitation, is that replication?
If there is no physical connection with the solid structure of the vent, how would chemicals in the ocean stay around the vent, the source of energy, long enough to do any substantial replication. Hot water would rise and move away from the vent. The nature of the connection, however, is still unknown.
Does the available free energy in the vent flow serve to modify such a deposition? If it did, and the rate of deposition increased, it would be something analogous to evolution. We have the interplay of three things here, all the result of the flow out of the sea vents: one is an energy source, one is a mineral source, and one is a set of templates upon which structures can be formed. Puzzle number 2 is the first organic compound or combination of compounds that do the replication. Educated guesses say it might have been RNA, but there are other suggestions as well, GNA, TNA, PNA, and more.
With just these two puzzles solved somehow by nature, the kind of very primitive evolution can continue to find other chemicals and combinations that can take a greater advantage of the energy in the sea vent flow, both quantitatively by replicating, and qualitatively by mutating or simply being modified by chance, into something which makes better use of the energy. A sea vent colony can evolve. Initially autotrophs will be the only things, but then heterotrophs will evolve to consume the hydrocarbon in the bodies of the autotrophs.
So far, there has been no discovery of fossil chemicals buried deep in the sea vent structures, and probably there never will be. With no records, perhaps the DNA of the creatures that live there now may give some clues, but it is not at all clear how to extrapolate back from existing DNA to some pre-DNA molecules, very simple ones at that, which existed at the beginning of life.
Most of us remember when there were no exo-planets known, and a big mystery of the universe was whether they existed, how many there were, and where they were. Then the first one was discovered, and the second, and now thousands are known. Perhaps the same fate awaits us on the origin of life. Maybe there are a dozen different combinations of energy-producing chemical reactions and self-replicating molecules. Perhaps it is very easy for life to form around such a deep sea vent, and it doesn't take many centuries at all to do it.
It would be nice to know if there are lots of ways to originate life, or just one. The author of this blog has suggested elsewhere that going to ten or twenty deep sea vents in all different parts of the oceans and seeing if they have identical DNA coding and genes would tell us a lot about the difficulty of life forming. To date, no one seems to have done this, but the equipment to take samples does exist.
One other item has been plaguing those who think about the origins of life. If life began on a deep sea vent, and was completely dependent on the energy pouring out of the subterranean piping there, how is it going to migrate to the surface? If it doesn't get to the surface, and live there a long, long time, chlorophyll isn't going to evolve, as chlorophyll is a complicated molecule and a lot of intermediate steps need to be taken to get to it. Fortunately, we have Hawaii.
The Hawaiian Island chain, starting with the Midway islands, is what a deep sea vent might grow up to be. The Midway islands are small little atolls, and there are reefs that never made it to the surface. But they are volcanic in origin, meaning that some hot water had to come out of them, a la deep sea vents. It seems that the source of the heat, a magma flow deep in the mantle, has been migrating along a fairly straight line, leading to larger and larger islands, with the finale being the island of Hawaii, at the southeast end of the progression. Each of these were once completely underwater. Some still have active volcanic flows. Once such a flow gets to with a few tens of meters from the surface, all those solar photons, chock full of available energy, are available, and once whatever chemotrophs there were evolve the capability of using this source of energy, they can cut themselves free and float through the oceans, spreading the seeds of life everywhere.
Trying to figure out the exact steps that evolution or the chemical pre-evolution discussed above took is like trying to figure out what automobiles were designed, and perhaps mocked up, before a new model was formed. There are few records of these trials, and probably no records of the intermediate steps. They weren't the million-seller successes, and so there aren't any left around to find and examine. The best bet is laboratory experiments, and these are costly and probably take a long, long time. Once we figure out how to do simulated chemical experiments, things should be quicker, but until that happens, the puzzles are likely to remain unsolved.
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