The Blueprint Problem
Life's defining features are growth, replication and evolution. To get an organism, its composition has to be "organized" in a reliable fashion. That is, there have to be instructions in terms of a template. As mentioned, the earliest types of instruction for making additional molecules of the "same" may have been based on mineral templates. An organic compound "replicated" by sticking to and growing on clay surfaces, for example. The suitable mineral surfaces would have been those that have grid spacing and electric charges defining a scaffolding or mold for some fairly complex molecules. The molecules would have grown according to the "instructions" from the mineral surface, and popped off when ready, like bottles off an assembly line. The molecules best constructed to take advantage of the assembly line process would become abundant. They would "learn" (by competition) how to grow from a template.
In an additional step, portions of mineral surfaces might cover themselves with certain organic molecules that stay in place rather than popping off. These might produce complementary molecules at a much faster rate than the inorganic surface alone. A kind of one-way reproduction would ensue. However, when such a molecular offspring became a new template cover itself, by finding a mineral surface to serve as cover for, the cycle would be closed so that selection pressures could make the system evolve. Once the offspring itself churned out complementary molecules, the system could rapidly grow in complexity, through collaboration between replicating molecules. Once replicating molecules became abundant, small non-replicating molecules ("food") became scarce. It was then an advantage for the replicating molecules to collaborate with enzymes attacking large molecules for food. This collaboration also insured against being eaten! Thus, a Great Arms Race was on among (almost) living things, leading into an ever-widening cycle of new inventions in attack and defense and resulting in innovations for collaboration, growth and replication.
The modern highly evolved means of replication associated with DNA and RNA are of such complexity that it staggers the imagination. The odds of creating DNA from scratch in a broth of randomly interacting amino acids hast been estimated as less than 1 in 10 to the power of a hundred. Even if every star in every galaxy in the known universe had a planet suitable for life, chances of DNA forming on any one of the planets would still be less than one in ten thousand. If the road to intelligent life requires DNA to spring intact from a broth of amino acids, we humans must surely be alone in the Universe!
It is more reasonable to assume that since Life exists (we are here), it is likely that Life can arise. But even RNA is far too complicated to have spontaneously emerged. We must envision a start with simple, short sequences of proteins that can replicate. This hypothetical protein nucleic acid (PNA) must have preceded RNA which preceded DNA. And we must then allow for natural selection to take over. Experiments on the problem of self-replicating molecules are being performed by Leslie Orgel at the Salk Institute and his collaborators, and by Jerry Joyce and others at the Scripps Research Institute. Both scientists are NSCORT/Exobiology principal investigators. A web page on this research can be found at: UCSD Exobio
