Abiogenesis is the proposal that life emerged from non-life. It can be viewed as a special form of spontaneous generation (see "The Origin of Life: Philosophical Perspectives," published in the Journal of Theoretical Biology, 1997, by Michael Ruse). Instead of life arising from non-life on a regular and observable basis, abiogenesis proposes life arising from non-life at some particular point in the ancient, unobservable past. But abiogenesis differs from spontaneous generation in another important way. While spontaneous generation proposed the emergence of a complete, complex cell or organism from organic molecules in one huge jump, abiogenesis draws from gradualism, where the original life forms were much simpler than modern cells and only gradually evolved their present-day form of complexity. Thus, abiogenesis not only places the spontaneous generation of life far in the past, but the life that is generated was supposedly much simpler, thus easier to generate spontaneously.
Essentially, abiogenesis is an exercise in reductionism whereby we attempt to a) conceptualize life in progressively simpler and simpler components and b) imagine that history reversed this process of conceptualization, building up layer and layer of increasing complexity. The goal is to begin with the simplest components (along with minimally specific interactions) that could be generated by nonliving forces but that were also able to give rise to life. In other words, a smooth continuum is sought that shows that the chemistry of life and non-life are bridged by nothing more than the laws of chemistry and physics. Since we've reduced life to simple and messy components, its origin then becomes a problem of accounting for the emergence of the increased complexity and increased specificity. John Maynard Smith and Eors Szathmary define this problem as follows: "How could chemical and physical processes give rise, without natural selection, to entities capable of heriditary replication, which would therefore, from then on, evolve by natural selection?" (The Major Transitions of Evolution, p 17). Others might define the problem as one of self-organization. How do chemicals organize themselves into more complicated forms that eventually reproduce themselves and thus evolve? Or to put it another way, how do we explain the self-organization of organic self-replicators and how did these self-replicators become the cell?
Once the origin of life has been defined simply as a problem of increasing complexity, the basic approach of science is to break it down into three basic phases: the origin of monomers (or basic building blocks); the origin of polymers (chains of basic building blocks); and the origin of cells. Living things are built upon a variety of basic building blocks, including amino acids, the parts that make up nucleotides (sugars, nitrogenous bases, soluble forms of phosphate), and fatty acids. Thus, we need to account for their origin in the same way that we would need to account for the origin of bricks to explain the origin of a brick house. This appears at first glance to be no problem as these basic building blocks are largely thought to have been generated by the type of processes demonstrated by Stanley Miller. That is, as long as there was a reducing atmosphere, with no free oxygen, and an energy source, the monomers would spontaneously appear and accumulate over time. A soup of organic precursors was thus generated and might even have been bolstered from periodic impacts of meteorites and comets containing such precursors.
Once we have our soup of ingredients, the next step is to string them together in a process called polymerization. This step was crucial for two reasons. By stringing together monomers, the next level of complexity, peptides and proteins, along with nucleic acids, could be formed. This would allow two important biological features to emerge on the ancient planet, namely, specific catalysis and the capability to store and transmit information. By joining amino acids together, peptides could form that served to catalyze, or accelerate, chemical reactions thought to be important for the origin of life. However, most scientists who study abiogenesis think such protein catalysis probably came later as the peptides would be formed randomly and have no way to reproduce themselves. Thus, a catalyst may form, but within a few years, be lost for all time. Instead, most scientists look to the molecule RNA.
RNA is made up of four different nucleotides strung together. The four nucleotides are linked together through their sugar and phosphate groups. Each nucleotide differs in that it contains a different nitrogenous base. In turn, the nitrogenous bases on the same strand can fold back and form hydrogen bonds with each other, usually in a specific manner where A binds to U and G binds to C. What this means is that if we have a chain of nucleotides, the chain can adopt certain three-dimensional shapes as a consequence of the nucleotide sequence. For example, if one end of the chain contained eight adenines and the other end contained eight uracils, the two ends could bind together through the nitrogenous base interactions such that a simple loop is generated. This is significant because the ability to adopt different shapes is at the heart of biological catalysis. Thus, RNA could have functioned as a catalyst to speed up certain reactions, and in fact, there is now plenty of evidence to show that RNA can and does function as a catalyst. What's more, the nucleotide sequence of RNA can theoretically be replicated and once replication occurs, RNA can begin to function as information that evolves.
Because of RNA's ability to function as both a catalyst and an information storage molecule with the potential to reproduce itself, RNA is thought to be very important in the origin of life. So important that many scientists have subscribed to the paradigm where there once existed something called an RNA World. The primary reason for the wide-spread acceptance of the RNA World is that RNA gets us out of a thorny problem for life's origin, namely, which came first, the chicken or the egg? Since eggs need chickens and chickens come from eggs, the origin of chickens appears to be a problem. But if chickens at one time evolved from another type of bird, which at one point evolved from a dinosaur, which evolved from an amphibian, etc., the chicken-and-the-egg problem does not truly hatch until we reach the first life forms on the planet, some form of single-celled microbe. Yet here, the problem comes in explaining the origin of the molecule of life, DNA. In order to synthesize DNA we need various proteins. But in order to synthesize the proteins we need the information supplied by the DNA. RNA gets us around this problem because it can theoretically function as a self-replicator. Both the information (in the form of a template) and perhaps the catalytic ability to synthesize an RNA molecule can be found within the RNA itself. Other circumstantial evidence is often cited in support of the RNA world. For example, in addition to its catalytic and information storage ability, RNA is often said to play crucial roles in processes believed to be the most ancient. Furthermore, the synthesis of DNA components (deoxyribose and thymine) does not occur by unique DNA-specific metabolic pathways but instead occurs as the result of peripheral modifications of the pre-existing RNA synthesis pathways. This suggests that DNA arose after RNA as a mere consequence of some minor metabolic modifications.
Once the RNA world had formed, the stage was set for the origin of cellular life. Three main steps would be involved. First, the RNA World generated the process of information-guided protein synthesis similar to modern protein synthesis. Secondly, at some point, all this biochemistry was encapsulated by a membrane (although the exact timing of this encapsulation remains in dispute). And finally, the information-storage property of RNA was transferred to DNA in such a way that it could easily be retrieved.
Book Resources On Abiogenesis
Biogenesis: Theories of Life's Origin by Noam Lahav
Vital Dust: Life As a Cosmic Imperative by Christian De Duve
Life Evolving: Molecules, Mind, and Meaning by Christian De Duve