In 1924, the Russian plant biochemist and evolutionary biologist Aleksandr I. Oparin questioned Haeckels scheme. Oparin could not reconcile his Darwinian viewthat simple organisms had gradually evolved into more complex oneswith the prevalent belief that life had suddenly appeared on Earth with a self-sustaining metabolism. So he proposed an alternative scenario. He posited that a long period of abiotic synthesis on early Earth had caused organic compounds to accumulate in a prebiotic soup, which had preceded life. Oparin then described how organic molecules could have evolved, via simple, ubiquitous fermentation reactions, into precellular systems on the primitive Earth. Such systems, he maintained, could then have led to cells that survived without oxygen and fed on the prebiotic soup.
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 lifes development, it would help explain how such biological functions as protein synthesis and genetic information storage and replication may have begun.
The heterotrophic theory has also gained support from studies of the capabilities of RNA, which have shown that RNA may have played a far broader role during lifes evolution than it does in life today. In 1982 the molecular biologists Thomas R. Cech, now at the Howard Hughes Research Institute in Chevy Chase, Maryland, and Sidney Altman of Yale University independently discovered that RNA molecules can act not only as messengers and repositories of information, but also as enzymes, which catalyze chemical reactions. The discovery of such ribozymes gave strong support to the idea that RNA might have both stored information and catalyzed reactions in the first living organismsa hypothesis first put forth independently in the late 1960s by Carl R. Woese of the University of Illinois at UrbanaChampaign, Leslie Orgel of the Salk Institute for Biological Studies in La Jolla, California, and Crick himself.