Have too many cooks spoiled 

 the prebiotic soup? 



By Antonio Lazcano 



wenty-five years ago, Francis Crick, who 



codiscovered the structure of DNA, pub- 



-A_ hshed a provocative book titled Life Itself: Its 

 Origin and Nature. Crick speculated that early in 

 Earth's history a civilization from a distant planet 

 had sent a spaceship to Earth bearing the seeds of 

 lite. Whether or not Crick was serious about his pro- 

 posal, it dramatized the difficulties then plaguing the 

 theory that life originated from chemical reactions 

 on Earth. Crick noted two major questions for the 

 theory. The first one — seemingly unanswerable at 

 the time — was how genetic polymers such as RNA 

 came to direct protein synthesis, a process funda- 

 mental to life. After all, in contemporary life-forms, 

 RNA translates genetic information encoded by 

 DNA into instructions for making proteins. 



The second question was. What was the compo- 

 sition of Earth's early atmosphere? Many planetary' 

 scientists at the time viewed Earth's earliest atmos- 

 phere as rich in carbon dioxide. More important, 

 they were also skeptical about a key assumption 

 made by many chemists who were investigating life's 

 origin — namely that Earth's early atmosphere was 

 highly "reducing, " or rich in methane, ammonia, 

 and possibly even free hydrogen. In a widely pub- 

 licized experiment done in 1 953, the chemists Stan- 

 ley L. Miller of the University of California, San 

 Diego, and Harold C. Urey had demonstrated that 

 in such an atmosphere, organic, or carbon-based, 

 compounds could readily form and accumulate in 

 a "prebiotic soup." But if a highly reducing atmos- 

 phere was destined for the scientific dustbin, so was 

 the origin-of-life scenario to which it gave rise. 

 In Crick's mind, the most inventive way to solve 



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NATURAL HISTORY February 2006 



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