It is my intention in this post to illustrate the most feasible theories of how life began.
Scenario 1: The Clay Hypothesis
A.G. Cairns-Smith suggests that the first replicating beings were clay crystals. He argues that information is stored in the structure of the crystal, and passed on from generation to generation. Along the way, of course, the clay crystals who have structures that best allow them to reproduce eventually out compete the 'less fit' crystals. Cairns-Smith argues that the structure of the crystal could very well affect the chemistry of the environment it is in. He thinks that in the beginning some crystals were structured so that amino acids or sugars stuck to them. These crystals were benefited by their affinity for the molecules of life, and so the crystal population evolved this way (He speculates as to why and how sugars and amino acids would be useful to clay reproduction in his book, Seven Clues to the Origin of Life). Eventually, evolution worked its way up to clay crystals that produced nucleotides. These Nucleotides were later co-opted to serve as a new genetic system, and the primitive clay apparatus was dropped.
Pros and Cons:
This theory is very speculative and has little experimental evidence to support it. In fact, one recent study suggests that clay is not able to store genetic information very well. Now for the pros of this theory: if it is correct, the odds of life arising on earth would be 100%. Another pro is that it explains why we have not found any truly simple organism (Organic life outcompeted clay life long ago, and organic life by this time was relatively complex). To wrap up: I think a simple approach like this is needed for solving the mysteries of abiogenesis. Maybe Cairns-Smith's basic idea is right, except that clay wasn't the original replicator. Maybe some other self forming structure was.
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Scenario 2: The Primordial Soup Theory
Scientists have been very successful in creating amino acids, nucleic acids, and even sugars under conditions that simulate the early earth (My Physical Science book cites an experiment by Carl Sagan that produced all of these). These experiments have been criticized for using atmospheres of methane and ammonia, which are currently not thought to be abundant in the atmosphere of the prebiotic earth. However, at least a few scientists are arguing that the atmosphere was mainly composed of methane, ammonia, and water vapor, just like the Miller Urey experiment used. Another point to be made is that many local areas, such as those with a lot of volcanic activity, would have had a reducing atmosphere. And finally, even if the prebiotic atmosphere was completely composed of carbon dioxide, hydrogen, and nitrogen, there are experiments that have used this atmosphere and produced amino acids.
Onto the theory: Scientists think that these organic molecules began bonding together to produce bigger and more complex molecules. Eventually, a molecule was formed that could replicate itself, and it evolved into life.
Several ideas exist about how these long molecules formed. Was it in hydrothermal vents? Did they form in ice? Or did it happen in between sheets of Mica? Several ideas exist about what the first replicator was, too. Was it a self replicating peptide, or an RNA molecule? And just how did this replicator evolve into life? This video has the most plausible illustration I have seen so far.
There are lots of less popular (and in my opinion, less plausible) theories on wikipedia. Discover Magazine has an article from a 1995 issue about "protocells" that David Deamer created. That about covers the origin of life theories. Yep.