Electrodes 



Spark discharge 



1 // 



Input gases 



^ Cooling 



condenser fRj^ 



To vacuur 

 pump 



Stanley L. Miller is pictured above with his experimental apparatus that sought to simulate 

 Earth's primordial conditions during life's molecular evolution. The apparatus, shown schemati- 

 cally at right, blended ammonia, hydrogen, methane, and water — thought at the time of the ex- 

 periment to be the primary constituents of Earth's early atmosphere — inside a sealed loop of 

 glass tubes and flasks. The gases, mixed with water vapor in the lower flask, flowed into the 

 upper flask, where electrodes, simulating lightning, sparked the vapor The circulating vapor 

 then condensed and trickled into a collecting trap. After one week, Miller and Urey found that 

 between 10 percent and 75 percent of the system's carbon had formed organic compounds, includ- 

 ing many of the amino acids needed to make proteins. The photograph was made in the mid-1950s. 



Gases mixed 



with water and heated 



Trapped organic compounds 



may have substantially affected early life. Nor is there 

 any fossil record of entities predating the first cells. 



In a sense. Miller and Urey were also heirs to a sec- 

 ond tradition of scientific thought, distinct from 

 that ot Darwin, whose aim can be understood to- 

 day as an attempt to synthesize molecules of prebi- 

 otic significance. Such experiments ciate back as far 

 as 1807, with the work of the French chemist Joseph 

 Louis Proust, as well subsequent chemists, including 

 the Swede Jons Jacob Berzeliiis, the Germans 

 Friecirich Wohler and Adolph Strecker, and the 

 Russian Aleksandr Mikhaylovich Butlerov. All of 

 them attempted to synthesize biologically related 

 molecules under what today would be called prim- 

 itive conditions — though they were not the condi- 

 tions Darwin imagined in his "warm little pond." 



True prebiotic simulations began with Miller and 

 Urey, and others have followed in their wake. All ot 

 them confirmed that amino acids, purines, and 

 pyrimidines — all molecules of biological signifi- 

 cance — readily formed under atmospheric condi- 

 tions thought to be similar to the ones present on 

 the early Earth. Most likely, those molecules would 

 also have formed in the prebiotic soup, along with 

 many other biologically related compounds: urea 

 and carboxylic acids, sugars formed from formalde- 

 hyde, and various hydrocarbons, alcohols, and fat- 

 ty acids, including some known to develop into bi- 



layered membranes — the probable precursors of cell 

 membranes. In addition to all those molecules, oth- 

 er, extraterrestrial molecules may have spiced the 

 prebiotic soup. They would have arrived on Earth 

 aboard fragments of comets, meteorites, and inter- 

 planetary dust, as the chemist Juan Oro of the Uni- 

 versity of Houston first suggested in 1961. 



Yet exactly how those simple organic com- 

 pounds assembled themselves into more com- 

 plex molecules, or polymers, and then into the first 

 living entities remains one of the most tantalizing 

 questions in science. Earths primitive broth must 

 have included a bewildering array ot organic com- 

 pounds, a virtual chemical wonderland that synthe- 

 sized, disintegrated, and absorbed a wide variety' of 

 molecules, in ongoing cycles of transformation. 



One feature of life, though, remains certain: Life 

 could not have evolved without a genetic mecha- 

 nism — one able to store, repHcate, and transmit to its 

 progeny information that can change with time. That 

 condition, of course, does not imply that nucleic acids 

 (the stutf of RNA and DNA) wriggled in the prim- 

 itive oceans, ready to serve as primordial genes. Nor 

 does it suggest that RNA arose completely assembled 

 from simple precursors in a prebiotic soup. Rather, 

 precellular evolution likely resembled a branching 

 tree of chemical transformations. Some of the 

 branches would have become evolutionary dead 



40 



NATURAL HIS IORY February 2006 



