[Blame (OP)]

I can't quite see evolution starting with DNA. It looks too complex a system. Could something like a prion spontaneously come into existence in the organic soup that presumably existed before life existed on earth?

The jump from prion based life to DNA strikes me as difficult but at least plausible if we assume that prions evolved to be far more sophisticated then than they are now.

Could be that vulnerability to prion diseases now is a legacy from the time when all replication was that way.       

[Villi]

Totally plausible that it went from protein to DNA. Many amino acids that make proteins are simpler (to memorize) than DNA. There's also a lot more of them compared to DNA bases, suggesting there was a longer period of time for so many to develop than DNA had time to develop. Though we simply don't know what went on billions of years ago on this planet.

[puppypower]

The Miller Experiments, from the 1950's, used simple gases and water, and an electric spark to create many of the precursors of life. The Miller experiments showed how easy it was to make over 20 different amino acids (more than life uses), simple sugars, oils and even tars too complex to analyze during that time. Miller tried to simulate the powerful thunderstorms, due to the infants earth's water based climate, using only the simple gases, scientists postulated were present at that time.   

The nucleic acids, needed for RNA and DNA, did not appear in these experiments. However, pretty much all the precursors for protein based pre-life could be generated by this global scale simulation mechanism for making precursors.

Research by the Miller group notes the formation of seven different amino acids and 11 types of nucleases in ice when ammonia and cyanide were left in a freezer from 1972 to 1997. Research by Stanley L. Miller and colleagues suggested that while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures.

What this suggests is animo acids came first when the infant earth was hotter and weather was very turbulent. As the earth cools and ice is able to form, then all four nucleic acids became available, from heating and freeze cycles of ammonium cyanide. This suggest that protein based pre-life, may have come first, before the earth was ready to make all the nucleic acids.

If you look the thermodynamics of inanimate matter, inanimate matter will seek lower energy and higher entropy. Yet, for life to form we need to concentrate energy rich organic materials; growth, into a sense of order; lowered structural entropy. Since life is going in the opposite direction of inanimate matter, this means the bulk direction of inanimate matter will set up an opposing potential to pre-life. Pre-life will be torn down as fast as it appears.

For pre-life to move forward, we will need an offset, where energy is able to lower and entropy is able to increase, in an exaggerated way. The offset will exaggerate the thermodynamic needs of inanimate matter, so pre-life assembly can slide under the radar. This means some type of protein based digestion/metabolism needs to appear, to act as an offset, so the assembly of inanimate pre-life is not a local thermodynamic liability. 

As an analogy, say you own a farm and hire a new worker. The first morning on the job, worker stays in the kitchen eating your food. If he does nothing but this, he will be fired, since he is going in the wrong way in terms of making you a profit; liability. But after a few rounds of eating, he begins to work in the field, digging the soil faster than any other worker. Now his same eating is not viewed the same, since his value is more than his liability. The assembly of pre-life, since it is still inanimate matter, is a thermodynamic liability. It would be like a rock spontaneously getting warm. This can happen, but there will needs to be a local offset; energy and entropy asset; fire.

Metabolism, even crude, used to grind down energy rich materials, creates a local thermodynamic asset, which allows a thermodynamic liability, implicit of the order for pre-life, to appear.

An analogy is fire. When you have a large fire, it begins to set up a convection where hot air rises and cool air is pulled into the fire. This convection can begin to move materials and suck in materials into the fire. Not all the proteins sucked into the digestion/metabolism center will be consumed, with the smaller easy to burn molecules. The sturdy protein become part of the furnace; so to speak.