Ever wonder what came first, the chicken or the egg? Astro biologists studying the origins of life on Earth struggle with that question everyday, only in a slightly different context. They are asking questions not about the living organisms themselves, but rather the nature of the molecules that were the precursors for life. On Earth, the molecular basis for the storage of genetic information in living organisms is deoxyribonucleic acid, or DNA. The instructions contained in molecules of DNA are expressed by the organism with the use of RNA to make proteins that, in turn, are necessary to mediate reactions in the cell. Without RNA, DNA would not be translated into proteins. And without proteins, the necessary reactions could not be catalyzed. These observations as well as others led scientists to question the nature of the first information containing molecules for life. Some scientists contend that RNA was the original information storing molecule of choice and that DNA came to overtake its roles. Other scientists believe that there were other molecules even before RNA that were utilized by the first life forms. Those that believe that RNA was the first molecule with this function believed that RNA, instead of proteins, could catalyze all the reactions necessary for replication. They refer to the time when RNA served this function as the "RNA World".
Why might a molecule other than DNA have been the first information containing molecule? DNA is a very large and complicated molecule and is more stable when two strands come together to form the double helix. It cannot replicate without the help of RNA and enzymatic proteins to catalyze the necessary reactions. DNA also requires the assistance of proteins to unwind its two strands for replication and to keep the strands from getting tangled up during replication.
RNA, on the other hand, is frequently found as a single strand of nucleic acids. Its backbone structure is manufactured in fewer steps than DNA. And because it is comprised of a four letter alphabet, it too can contain hereditary information. In 1983, two scientists, Thomas Cech and Sidney Altman, independently discovered that some enzymes could be made entirely of RNA instead of protein. These enzymes, now known as ribozymes, have lent credibility to the notion that RNA might play a catalytic role in its own replication. This would eliminate the need for proteins to catalyze polymer formation. If this were the case, then one might imagine an early Earth where short strands of RNA-like molecules were spontaneously formed, some of which were able to partially or completely catalyze their own replication. This scenario could have evolved over time by the introduction of random mutations introduced by copying error. Eventually, DNA would have come to assume the role of RNA due to its greater fidelity in replication.
It sounds plausible, but skeptics point out some valid flaws. First of all, the sugar molecule that is used to make RNA molecules is ribose. During attempts to simulate the random formation of organic molecules in the lab, scientists have not been able to produce a reaction that produces a high yield of ribose instead of a random mixture of sugars. Even if scientists find a spontaneous reaction that readily produces ribose in high quantities, they would then have to account for the rapid rate at which sugars would have decomposed in early Earth-like conditions. Finally, like many bio molecules, sugars are manufactured with a "handedness". How would life have selected one structure of sugar out of a mixture that was exactly half "right handed" and half "left handed"?
If not RNA, then what was the first information containing molecule? Some scientists have proposed a pre-RNA world, one in which molecules similar to RNA would have served the role of informational macromolecule. In particular, some have suggested a molecule that doesn't contain sugar at all. These molecules, called peptide nucleic acids, or PNAs, bind to DNA and have a different backbone than RNA. However, there is little evidence to suggest that PNA can form polymers as readily as RNA.
Without being able to travel back in time to observe early Earth, the unraveling of the history of life will be a slow, process. Even if the RNA world becomes the single accepted theory, scientists will then have to explain how the RNA world came to exist in the first place! And until we have a firm grasp of the physically conditions on early Earth, we won't be able to fully predict how prebiotic molecules formed. The search for the origins of life continues.