The Origin of Life: Not so Simple (Part III)
This post will provide a final discussion of an article in Scientific American entitled "A Simpler Origin for Life" by Robert Shapiro. Part I explained why the Miller-Urey experiment and the DNA-first hypothesis is deficient. In Part II, I explained Shapiro's apt criticisms of the RNA-world hypothesis. Those who have abandoned the RNA-world hypothesis still seek a self-replicating molecule to qualify as the climax of chemical-origin of life scenarios--the "pre-RNA world." However, Shapiro observes not only that "no trace of this hypothetical primal replicator and catalyst has been recognized so far in modern biology," but also that "the spontaneous appearance of any such replicator without the assistance of a chemist faces implausibilities that dwarf those involved in the preparation of a mere nucleotide soup."
The reason that producing such a special self-replicator is so difficult is that a self-replicating molecule would have to incorporate nothing but the right nucleotides (or nucleotide-analog molecules) in a long chain, never splitting into two chains and never incorporating other random organic molecules which would mess up replication. He explains: "There is no reason to presume than an indifferent nature would not combine units at random, producing an immense variety of hybrid short, terminated chains, rather than the much longer one of uniform backbone geometry needed to support replicator and catalytic functions."
Shapiro doesn't even begin to address the problem of getting the "nucleotides" of this "pre-RNA" molecule in an order such that self-replication is possible. Again, his analogy is apt:
Probability calculations could be made, but I prefer a variation on a much-used analogy. Picture a gorilla (very long arms are needed) at an immense keyboard connected to a word processor. The keyboard contains not only the symbols used in English and European languages but also a huge excess drawn from every other known language and all of the symbol sets stored in a typical computer. The chances for the spontaneous assembly of a replicator in the pool I described above can be compared to those of the gorilla composing, in English, a coherent recipe for the preparation of chili con carne. With similar considerations in mind Gerald F. Joyce of the Scripps Research Institute and Leslie Orgel of the Salk Institute concluded that the spontaneous appearance of RNA chains on the lifeless Earth "would have been a near miracle." I would extend this conclusion to all of the proposed RNA substitutes that I mentioned above.Shapiro's preferred solution is to redefine the first life as a thermodynamic state--not a molecule--but an energy driven network of chemicals which can reduce entropy in a system, and most importantly, can grow, reproduce, and therefore evolve under natural selection. Shapiro notes that some people would call this a "metabolism first" system rather than a hereditary molecule first system. But like many proponents of the RNA world or pre-RNA world hypotheses, Shapiro gives scant explanation for how these life-like metabolic networks can come into existence naturally, and he gives no details as to how these thermodynamic states produce real life--life as we know it today. Calling for complex molecules like nucleotides to arise for functions other than their current function as the letters of the language of life doesn't sound so "simple."
(Robert Shapiro, "A Simpler Origin for Life," Scientific American, February 12, 2007)
Shapiro admits that many details are missing from his hypothesis, but perhaps further research will solve them. We'll just have to wait and see...