Primitive terrain of the early 

 Earth set the stage for life's evolution. 

 Fueled by raw materials from volcanoes, meteorites, 

 and undersea thermal vents, and energized by lightning, cosmic 

 rays, and the planet's own internal heat, life's precursor molecules probably 

 formed in a "soup" of prebiotic organic compounds about 4 billion years ago. 



which have shown that RNA may have played a far 

 broader role during life's evolution than it does in life 

 today. In 1982 the molecular biologists Thomas R. 

 Cech, now at the Howard Hughes Research Institute 

 in Chew Chase, Marv'land, and Sidney Altman of 

 Yale University^ independently discovered that RNA 

 molecules can act not only as messengers and repos- 

 itories of information, but also as enzymes, which cat- 

 alyze chemical reactions. The discover)' of such "ri- 

 bozymes" gave strong support to the idea that RNA 

 might ha\'e both stored information and catalyzed re- 

 actions in the first Hving organisms — a hypothesis first 

 put forth independently in the late 1960s by Carl R. 

 Woese of the University of Illinois at Urbana-Cham- 

 paign, Leslie Orgel of the Salk Institute for Biologi- 

 cal Studies in La JoUa, CaUfornia, and Crick himself. 



If true, the hypothesis suggests that an "RNA 

 world" may have preceded life as it occurs today. In 

 such a world, RNA would have performed many 

 functions that other molecules, including DNA and 

 proteins, have now assumed. If such an RNA world 

 preceded life's development, it would help explain 

 how such biological functions as protein synthesis 

 and genetic information storage and replication may 

 have begun. 



The history of modern thinking about the ori- 

 gins of life begins with the eighteenth-centu- 

 ry naturalist Jean-Baptiste de Monet, chevalier de 

 Lamarck, Charles Darwin's most distinguished pre- 



decessor. Darwin himself was reluctant publically to 

 address the question of life's origin. But the idea that 

 living organisms evolved from lifeless matter became 

 widespread soon after the publication of Darwin's 

 Origin of Species in 1859. Darwin expressed his pri- 

 vate views on the matter m 1871, in a letter to the 

 Enghsh botanist J.D. Hooker. Life, Darwin famously 

 wrote, may have started in a 



warm little pond, with all sorts of ammonia and phos- 

 phoric salts, light, heat, electricity, etc. present, that a pro- 

 teine compound was chemically formed ready to under- 

 go still more complex changes. 



The nineteenth-century German zoologist and 

 evolutionist Ernst Haeckel perhaps best epitomized 

 the leading scientific beliefs after Darwin. The first 

 life-forms, he contended, had been plantlike mi- 

 croorganisms, capable of photosynthesis, that had 

 evolved directly out of nonliving matter according 

 to physical laws. 



In 1924, the Russian plant biochemist and evo- 

 lutionary biologist Aleksandr I. Oparin questioned 

 Haeckel's scheme. Oparin could not reconcile his 

 Darwinian view — that simple organisms had grad- 

 ually evolved into more complex ones — with the 

 prevalent belief that life had suddenly appeared on 

 Earth with a self-sustaining metabohsm. So he pro- 

 posed an alternative scenario. He posited that a long 

 period of abiotic synthesis on early Earth had caused 

 organic compounds to accumulate in a prebiotic 



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NATURAL HISTORY February 2006 



