Origin of Life

Most living cells today store genetic information in the long-ribbon-shaped molecules of DNA. The information stored in DNA's molecular components is transferred to another ribbon-shaped molecule, RNA, by a process termed transcription. Proteins, including enzymes, are then formed by cellular structures that translate the information on the DNA. The enzymes thus produced facilitate the biochemical cellular functions necessary to maintain life and reproduce. Many scientists believe it is unlikely that all of the components of this complex sequence of events, DNA to RNA to protein, evolved simultaneously. Some scientists propose that, in fact, RNA appeared before DNA. This view has been strengthened by the discovery that some forms of RNA, called ribozymes, can act like non-protein enzymes to catalyze biological reactions. RNA thus may have been capable of ordering amino acids into forming proteins and of replicating itself in an RNA-based arrangement termed the "RNA world."

Scientists who favor the RNA world hypothesis suggest that RNA might have been able to self-replicate even before DNA and protein enzymes had evolved. Single-stranded RNA might have been able to assume a shape that allowed it to line up amino acids in specific sequences to create specific protein molecules. RNA molecules capable of causing amino acids to link up to form a protein could have had an advantage in replication and survival, compared with other RNA molecules. At that point, molecular evolution and natural selection could have taken over in furthering the development of life. RNA that produced useful protein enzymes, for example, would have survived better than that which did not.

Critics of these ideas say that the evidence for self-replicating RNA is weak. Instead, they suggest that other organic molecules, rather than nucleic acids, were the first self-replicating chemicals capable of storing genetic information. According to this idea, these simple hereditary systems were later replaced by nucleic acids during the course of evolution. Since laboratory results in this field are hard to come by, this debate is likely to persist for a long time to come.