Some clever geology has dated the first existence of liquid water on Earth to be about 4.4 billion years ago. Some clever selenology has dated the preponderance of the craters on the moon to have originated before 3.8 billion years ago, meaning the last bits of formation of planets was still occurring then. Life may have originated a million times during the period between 4.4 billion and 3.8 billion years, most of it being extinguished by the next large asteroid making impact. So common dates for the origination of life start around 3.8 billion and go up to 3.4 billion, when there are fossils of some tiny microorganisms.
Many interesting theories have been devised for the origination of life, since Pasteur proved that it didn't just spontaneously form before your eyes. Many of them involve water, with the water being a premordial soup with lots of organics, or being circulated through sea vents so lots of minerals with excess energy would be there, and others. Another theory involves clay, and there are likely many other theories which have not become so popular, but likely most of them involve water.
Since it seems to be very hard to get a cell to form, maybe horizons need to be expanded. Recall that there is a catch-22 going on with cellular life. You need to have some genes and enzymes to make the various organics in the cell, including the cellular membrane, but you need the cellular membrane to hold the genes and enzymes together. Which came first, some DNA, aka the Replicator Theory, or some membranes.
The Replicator Theory says something replicates, and then it gradually makes other stuff which helps replication. This seems quite reasonable, and given the billions of years for evolution to be experimenting with things, something could come out of it. But no one can come up with a Replicator.
It has also been downright difficult to come up with a membrane that is easy enough for a simple replicator to make, or which could accidentally form. Other theories, like the sea vent ones, have energy as the crux of the origination, and some are lumped together as 'metabolism' theories, which says that the first thing which happens is something organic starts absorbing energy from the peculiar water environment at a deep sea vent, or somewhere else interesting.
Sea vents are fascinating places, but no one has found any simple life there, nor anywhere else. It's all gone. So, perhaps analogs of existing places is not the key to figuring out the puzzle of life origination. Instead, let's consider what existed back in the days right after the asteroid bombardment got infrequent enough to allow some calm. Were there a lot of organics around? Nowadays, anything organic is consumed by some organism, but back then before there were any organisms, there were likely a mixture of different organic molecules.
Hark back to the concept of physical chemistry, and recall the concept of miscibility. This simply says that some liquids do not mix together in any proportion. Immiscible liquids can be extreme, and almost none of one kind can intermingle with the molecules of the other kind, and vice versa. Or it can be partial, with some of one kind going into solution in the other, but only a limited amount. The children's chemistry set experiments has the experimenter putting two immiscible liquids together, shaking them vigorously to mix them, and then seeing the utter failure of the shaking to cause the two to mix. You have one solution above another, separated by a liquid-liquid boundary called the meniscus. There are a lot of organic molecules that mix well with water, being miscible, and a lot of organic molecules which do not, and are mostly or totally immiscible. Everyone knows the example of gasoline spilled on a rain puddle. It doesn't mix.
Most simple molecules which do not mix with water do mix with each other, but not all. This means that on early Earth, with organics and water co-existing, there would be pools of each of them. Water forms lakes and oceans, and maybe the immiscible organics did as well. In other words, there on top of the lake or ocean, was a layer of organic molecules. Some other organic molecules are miscible with both, and there is a preference they have which says how much of the molecule is in the water ocean and how much is in the organics ocean, or perhaps the organic layer on top of the ocean. There could be, in our imagination, a lake wholly organic with no water there, but having a whole ocean of organics, like Europa does, seems to be asking a lot. Maybe there were huge quantities of organics back then, but maybe not.
In the organic layer, there is a process of chemical change happening, as some energy source is tapped and different organics are made. Perhaps it is lightning hitting the upper surface. Recall there is no free oxygen, so nothing catches on fire. Perhaps it is a volcano, a meteoroid, our old friend the deep sea vent, or something else. But the organics in the organic ocean are changing, and if something is made which is not miscible in the organic ocean but is in the water ocean, it might migrate, or make a third layer on top. If there were strong winds and waves which made connection between the deeper water ocean and this hypothetical third layer, the third layer would mix in to the water ocean.
Likewise, if something was produced in the water ocean which was immiscible, it might rise up and join with the organics ocean. In both cases, there would be material remaining. if something new that was miscible in the water ocean was made in the water ocean, it would stay there. Likewise for organic miscible new things made in the organic layer.
At the miniscus, things which are in the organic ocean soup are contacting things which are in the liquid ocean soup. Suppose some hydrophobic molecule, miscible only in the organic ocean, contacts some hydrophilic molecule, miscible only in the water ocean, and forms a bond, perhaps with some connection between them that is more tolerant of both. These combined molecules stay at the meniscus, with their orientation being dictated by keeping the hydrophilic one deep in the water ocean and the hydrophobic one up higher in the organic ocean. If the molecule is stable, it would stay there for some time, and perhaps as time progressed, there would be more and more of them. The most stable ones would remain longest, and those, perhaps few types, that were extremely stable might begin to build up their numbers large enough so two would come into contact through horizontal drift. Can they form a linkage of some kind? Given enough permutations, evolution may have found a way to keep two, and therefore many together.
What we have is an elementary membrane. It is not a cell membrane, as there is no cell and nothing to put inside it. But it is a film with the right kind of structure. One of the main stumbling blocks to the origin of life pops out of elementary physical chemistry and considering the different environment of the early Earth. Could the membrane be other than flat? If the molecule comprising one side of the membrane has less cross-sectional area than the one on the other side, there would be a tendency for the membrane to curve, and tend to spherical. At the meniscus, there would be some bubbles of one liquid poking into the other. Perhaps they would close up, and be stable. If so, what would happen if thousands of square kilometers of oceans, plus some lakes, were just full of these microscopic spherules.
Here is a good environment for the replication concept. If one of the trillions of spherules has some DNA, RNA, TNA ,PNA or whatever inside it which is complex enough to make itself and the two components of the membrane, the whole business of life origination may be wrapped up. But still, no one has come up with anything to put inside the spherules yet.
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