The Origin of Life: Not so Simple (Part II)
Writing in Scientific American Robert Shapiro recounts many criticisms of popular models for the chemical origin of life. Part I recounted why many origin of life theorists reject the possibility that DNA was the first genetic molecule. As noted, Shapiro even takes aim at those who suggest that the Miller-Urey experiment chemistry was important for forming prebiotic molecules on meteorites because studies of these meteorites show "a bias toward the formation of molecules made of fewer rather than greater numbers of carbon atoms, and thus shows no partiality in favor of creating the building blocks of our kind of life." Due to these deficiencies, Shapiro then notes that increasing numbers of prebiotic chemists now turn to RNA as the first form of life. Yet Shapiro observes that some scientists have called the natural origination of the first RNA molecule "the prebiotic chemist's nightmare."
Shapiro explains that "[e]normous obstacles block Gilbert's picture of the origin of life, sufficient to provoke another Nobelist, Christian De Duve of Rockefeller University, to ask rhetorically, 'Did God make RNA?'" The problem is that even given a soup of the right ingredients--sugar, phosphate, nitrogen, and oxygen--the components of RNA can be connected in innumerable different patterns, most of which are not nucleotides like those used in RNA, and the infinitely large majority of which are not even nucleotides.
So how did RNA form? Shapiro notes that those who bank on Miller-Urey chemistry to form the building blocks of RNA are challenged by the fact that such experiments do not produce nucleotides or even simpler nucleosides. Others have tried to recreate RNA in experiments meant to simulate other natural environments, such as some prebiotic soup (the existence of which is doubtful).
While chemists occasionally produce familiar organic molecules through this process, they use experiments which do not necessarily mimic real-earth conditions. Shapiro is therefore skeptical that RNA could be produced in a natural environment because "neither chemists nor laboratories were present on the early Earth to produce RNA." Shapiro thus gives a wonderful golf-analogy to explain the problems faced by those trying to simulate how RNA could be produced naturally:
The analogy that comes to mind is that of a golfer, who having played a golf ball through an 18-hole course, then assumed that the ball could also play itself around the course in his absence. He had demonstrated the possibility of the event; it was only necessary to presume that some combination of natural forces (earthquakes, winds, tornadoes and floods, for example) could produce the same result, given enough time. No physical law need be broken for spontaneous RNA formation to happen, but the chances against it are so immense, that the suggestion implies that the non-living world had an innate desire to generate RNA. The majority of origin-of-life scientists who still support the RNA-first theory either accept this concept (implicitly, if not explicitly) or feel that the immensely unfavorable odds were simply overcome by good luck.Shapiro notes that some scientists are abandoning the RNA world hypothesis: "Many chemists, confronted with these difficulties, have fled the RNA-first hypothesis as if it were a building on fire." Their new solution, and its problems, will be discussed in the next post.
(Robert Shapiro, "A Simpler Origin for Life," Scientific American, February 12, 2007)