soup, which had preceded life. Oparin then de- 

 scribed how organic molecules could have evolved, 

 via simple, ubiquitous fermentation reactions, into 

 precellular systems on the primitive Earth. Such sys- 

 tems, he maintained, couki then have led to cells 

 that survived without oxygen and ted on the pre- 

 biotic soup. 



Not too surprisingly, that line of thinking has 

 sparked disagreement. As recently as 1988, 

 the German chemist Gunter Wachtershiiuser, now 

 a patent attorney in Munich, proposed an alterna- 

 tive "iron-sulfur" hypothesis. Wachtershauser 's 

 core insight was that when iron sulfide (FeS) mix- 

 es with hydrogen sulfide (H2S) to form pyrite 

 (FeSi), the reaction releases copious quantities of 



Forming dimers 



Forming trimers 



Catalyzing 



formation 



of double-stranded 



polymers 



Catalyzing 

 formation of 

 complex polymers 



Forming catalysts 



Self-catalyzing 

 additional catalysts 



Forming dimers ° — ► 



Forming trimers °o — ^ & 



A A A 



A 

 A 



•a 



Catalyzing 

 formation of 

 macromolecules 



Catalyzing 

 formation of 

 complex 

 macromolecules 



Forming 

 complex 

 catalysts 



Self-catalyzing 

 additional 

 complex catalysts 



8 



6> 



o 00 

 o qPo 



°oo\ 



Template matching \\^ ^00^ ^/ 



hydrogen gas (H,). With the release of the hydro- 

 gen, on Wachtershiiuser's view, organic compounds 

 could form from carbon dioxide in the atmosphere. 

 Life began when selt-catalyzing molecular systems 

 emerged from the organic compounds. Experi- 

 ments confirm that the formation of pyrite can in- 

 directly yield a tew organic compounds as well as 

 ammonia (NH,). 



But compared with the variety of biochemical 

 compounds synthesized in simulations such as 

 Miller and Urey's, the process Wachtershauser de- 

 scribed gives rise to only a limited range of mole- 

 cules. Moreover, the Miller-Urey apparatus sought 

 to simulate Earth's real environment shortly after 

 our planet formed from the primordial solar neb- 

 ula. In contrast, there is little empirical support for 

 Wachtershiiuser's hypothesis. 



Unfortunately, since the 

 — — Earth's geologic record 

 fi-om those early times is so 

 sparse, the rocks cannot an- 

 swer the kinds of questions 

 raised by the Miller-Urey 

 and Wachtershauser exper- 

 iments. Most rocks that are 

 more than three billion years 

 old have so thoroughly 

 metamorphosed that life's 

 precursor molecules are no 

 longer detectable. There is no 

 direct evidence of Earth's en- 

 vironmental conditions at the 

 time of life's origin, either. 

 No one knows the tempera- 

 ture of the early Earth, its 

 ocean acidity, the composi- 

 tion of its atmosphere, or any 

 other factors that 



& & 



& 



& 



Competition and 

 selection of 



"genetic" polymers 



Life's precursor molecules built up over 

 at most a few hundred million years. The 

 schematic diagram indicates several kinds of chemical re- 

 actions that led, over perhaps several "generations " 

 (blue, red, and green, respectively) to increasingly elabo- 

 rate molecular complexes. (As the keys beneath the two 

 leftmost panels indicate, the products of one generation 

 become the building blocks for the next.) Among those 

 complexes, some began to carry out functions associated 

 with the basic molecules of life. At some stage, complex 

 polymers emerged that could store and transfer information 

 via template matching. Such "genetic" polymers ultimately became encap- 

 sulated within cell-like membranes formed by lipid molecules. The resulting cell-like 

 complexes thereby housed self-replicating molecules capable of multiplying — and hence evolv- 

 ing — genetic information. Many specialists consider the emergence of genetic replication to be 

 the true origin of life. 



Catalyzing 

 formation of lipids 



Self-replication 

 and information 

 storage 



. O 



o ^00 



OA O. 



February 2006 NAn'R AI HISTORY 



39 



