both problems was to assume that Hfe had not 

 evolved on Earth, but had come here from some 

 other location — a view that still begs the question 

 of how life evolved elsewhere. 



Crick was neither the first nor the last to try to ex- 

 plain life's origin with creative speculation. Given so 

 many diificult and unanswered questions about life's 

 earthly origin, one can easily understand why so 

 many investigators become frustrated and give in to 

 speculative fantasies. But even the most sober at- 

 tempts to reconstruct how life evolved on Earth is a 

 scientific exercise fraught with guesswork. The evi- 

 dence required to understand our planet's prebiotic 

 environment, and the events that led to the first liv- 

 ing systems, is scant and hard to decipher. Few geo- 

 logical traces of Earth's conditions at the time of life's 

 origin remain today. Nor is there any fossil record of 

 the evolutionary processes preceding the first cells. 

 Yet, despite such seemingly insurmountable obsta- 

 cles, heated debates persist over how life emerged. 

 The inventory of current views on life's origin re- 

 veals a broad assortment of opposing positions. They 

 range from the suggestion that life originated on Mars 

 and came to Earth aboard meteorites, to the idea that 

 life emerged from "metabolic " molecular networks, 

 fueled by hydrogen released during the formation of 

 minerals in hot volcanic settings. 



This flurry of popular ideas has often distracted 

 attention from what is still the most scientifically 

 plausible theory of hfe 's origin, the "heterotroph- 

 ic" theory. The theory holds that the first living en- 

 tities evolved "abiotically" — or from systems of 

 nonliving organic molecules present on the primi- 

 tive Earth. (The term "heterotrophic" was origi- 

 nally coined to describe a kind of metaboHsm in 

 which "nutrients" such as carbon and nitrogen must 

 be obtained from nature as complex organic mole- 

 cules such as amino acids, rather than from ex- 

 tremely simple compoLincHs such as carbon dioxide.) 

 According to the theory, organic molecules such as 

 amino acids were chemically combined in a prebi- 

 otic soup and "cooked" by various sources of en- 

 ergy. True, some of the details of Miller and Urey's 

 recipe for prebiotic soup presented difficulties, such 

 as the ones Crick highlighted. But abandoning the 

 premise of a prebiotic soup when new findings 

 largely support its account oflife's origin is to "throw 

 the baby out with the bathwater." 



One strong argument in favor of the het- 

 erotrophic theory is the surprising variety of 

 biochemical constituents that emerge in laboratory 

 simulations of Earth's prebiotic environment, and 

 the remarkable similarity between them and the 

 constituents of some carbon-rich meteorites. On 



September 28, 1969, for instance, a meteorite 

 landed in Murchison, Australia, carrying near- 

 ly eighty kinds of amino acids. Among them 

 were several amino acids that occur in proteins. 

 Also embedded in the Murchison meteorite 

 were purines, pyrimidines, carbox"ylic acids, 

 and compounds derived from ribose anti de- 

 oxyribose, the sugars present in RNA and 

 DNA. (In tact, ribose is the "R" of RNA, de- 

 oxyribose the "D" of DNA.) Such relics of the 

 early solar system provide insight into the kind 

 of organic chemistry that took place some 4.6 

 billion years ago. 



The similarity between the prociucts of lab- 

 oratory synthesis and the components of the 

 meteorite seems more than accidental. In fact, 

 it offers strong justification for bringing the 

 study of the possible reaction pathways of pre- 

 biological molecules into the laboratory. Per- 

 haps reactions such as the ones Miller and Urey 

 simulated were common throughout the solar 

 system, or at least in a prebiotic soup on Earth. 



What about the criticisms that the highly re- 

 ciucing atmosphere in the Miller-Urey experi- 

 ment was unrealistic? The hydrogen in such an 

 atmosphere, according to the critics, would have 

 escaped into space too quickly to have played 

 any role in atmospheric chemistry. But the crit- 

 ics may have overstated their case. Recent the- 

 oretical models by Feng Tian, an atmospheric 

 chemist at the University of Colorado, Boul- 

 der, and his colleagues suggest that hydrogen m 

 the atmosphere of the early Earth may have es- 

 caped more slowly than planetary scientists pre- 

 viously assumed. So although Earth's primitive 

 atmosphere may not have been as strongly re- 

 ducing as Miller, Urey, and their followers have 

 assumed, it may not have been lacking in hy- 

 drogen, either. The hydrogen would have co- 

 existed with carbon ciioxide. The presence of 

 both gases would have helped forge hydrogen- 

 rich molecules, which would have transformed 

 into organic compounds. 



Certainly, the classical recipe for prebiotic 

 soup requires updating. It must take into ac- 

 count such additional, newly recc-)gnized fic- 

 tors as extraterrestrial organic compounds, 

 minerals such as combinations of iron and 

 nickel with sulfur that act as chemical catalysts, 

 and organic molecules synthesized in hy- 

 drothermal vents. None of those fictors threat- 

 ens the plausibility of a heterotrophic theory 

 as an explanation for the origin of life. 



The heterotrophic theory has also gained sup- 

 port from studies of the capabilities of RNA, 



