It is debatable as to at which point the world became biotic, did it begin with the DNA, RNA or some other precursor?
Some scientists like the idea of a naked gene lying somewhere on a primordial beach, others like the idea of oil globules trapping primordial soup, there are many theories but one the main things to keep in mind when thinking of this early earth is the prebiotic chemistry is somewhat different from that which occurred after the Earth acquired a biosphere. For a start the atmosphere would have either been an unreacting inert environment or a reducing one, completely different from todays oxidising atmosphere.
This means that there where no established metabolic pathways requiring certain molecules so organic material would have been able to accumulate rather than being gobbled up by pre-existing life. But as discussed in THE PRIMORDIAL SOUP and REPLICATION, PROBABILITY AND WHAT IS LIFE? these molecules where very numerous and mostly not very useful. So how was the chaff sorted from the wheat?
Some of the fossils found by Bill Schopf in Australian rocks 3.5 billion years old. Even by this early date, cellular structures can be seen clearly. (J. William Schopf.)
There are several theories on how this occurred, e.g.:
One theory states that at one point the surface of the planet could have been covered with an apparent metres thick oil slick, which could have in itself acted as a sort of filter. In the world of chromatography porous gels can be used as the smaller molecules take longer to filter through as they tend to take longer routes though the gel i.e. the have more of a horizontal component to their movement than the larger molecules. It is thought that the oil slick could have acted like these porous gels to produce size ordered molecules.
Another theory states that due to the immense tidal range of the young Earth, this would have sorted the molecules into those least and most resistant to beginning moved as discussed in ORGANIC VS INORGANIC. These molecules, due to stochastical processes would have ended up with some replicating abilities but would have still been at the mercies of their surrounding chemical environment!
What would these replicating molecules have been? Interesting research shows that they may well have been Thioesters, these are esters which instead of O- groups have S– which is consistent with the reducing atmosphere. It also very likely that RNA could have easily cannibalised this molecule as it took over this pre RNA world. How would it have done this?
RNA has a sugar phosphate backbone!
It seems likely that in a world with replication there would have been Darwinian selective forces at work, and anything that keeps the molecules slightly distant from the outside world would have increased their chances of survival. Sugar attached to the outside of the molecule would have offered this protection as would disappearing inside globules of lipoproteins!
A three dimensional model of the double helix of DNA
This would have eventually lead from thioesters to RNA which in the absence of proteins has been shown to self catalyse, meaning that unlike DNA it can be self-sustaining. It therefore seems a relatively small jump from this RNA world to the DNA one but this is not necessarily the case as DNA relies on lot of other molecules in order to function. The chances are that these other molecules including peptide chains approximately 100 bases long would have had to collect in one place to spawn the DNA world.
This would be the most likely point at which the lipid globules would come into play!
But would the first life have existed in the DNA, RNA or pre-RNA world?