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18.7 Pre-RNA World

The quest to understand the origins of life on Earth has led scientists to take a hard look at life's first bio molecules. What was the nature of the first bio molecules? How and when did molecules transition from being merely a random mixture of chemicals to actual living things? Many biologists agree that life could not have arisen from nonliving chemicals to single-celled living organisms in a single step. Instead, it has been proposed that there may have been an intermediate life form that was a cellular. The discovery of self-catalyzing RNA molecules in the 1980s by Noble-prize winning Thomas R. Cech lent credibility to suggestions that the first living organisms may have been self-replicating RNA molecules. This "RNA World", as termed in 1986 by biologist Walter Gilbert, has become one prominent conjecture about the origin of life on Earth. However, RNA skeptics contend that there are too many problems with RNA for it to have been the molecule responsible for the transition from chemical to biological. These scientists have proposed a variety of mechanisms and molecules by which the transition from chemical to living might have transpired in a world existing before RNA. This world, the pre-RNA world, is subject to much interesting discussion.

Over 40 years ago, A. Graham Cairns-Smith proposed that the first molecule with replicating capability was not organic like RNA at all; rather, life arose from inorganic irregularities. His model detailed the participation of inorganic clays in creating a replicating system capable of storing information. He imagined a clay surface with irregularities, such as an unusual distribution of anions (negatively charged ion). If a particular arrangement of ions in the surface could direct the synthesis of another layer on top of the surface with the same irregularities, Cairns-Smith considered this successful replication. Natural selection would come into play when the number of ions in a layer influences how quickly and efficiently the new layer can be made. Since self-replication through this process is likely to be highly inaccurate, this model has long been considered implausible. So much so that no one has yet tested it in the lab.

Other scientists have proposed situations in which molecules more similar to RNA may have been the first life molecules on Earth. Of these, two come to the forefront of the pre-RNA world propositions. The first, pyranosyl RNA (pRNA) is similar to the RNA currently found in living organisms. The main difference is that instead of the five-member sugar ring ribose, pRNA utilizes a six-member ring that has an extra carbon atom. When linked together into strands, pRNA can engage in base pairing just like RNA (i.e. cytosine base pairs with guanine). Also, double-stranded pRNA does not twist around itself in the same way that double stranded RNA or DNA does. This would be important in a world without proteins to help keep strands from getting tangled during replication. Unfortunately, scientists have not yet discovered how the six-member ring would have been synthesized on early Earth.

The second alternative to RNA is a molecule that completely forgoes having a sugar at all. Instead of a sugar-phosphate backbone, peptide nucleic acid (PNA) relies on a protein-like backbone coupled with nucleic acid bases for side chains. Just as RNA and pRNA, peptide nucleic acids can engage in complementary base pairing. PNA was designed using computer-assisted model building; therefore it is still unclear exactly whether or not a PNA polymer could be formed. If successfully accomplished within the lab setting, PNA might become the new focus for origins of life researchers.

Regardless of the true nature of the molecules that bridged the transition between chemistry and biology, tracing life on Earth back to its primordial origins will undoubtedly yield valuable insights into the origins of life on other planets. As we expand our knowledge of other worlds, their chemistries and conditions, we might recognize something that distinguishes itself as more than a set of spontaneous chemical reactions.