|MadSci Network: Evolution|
You've hit on the most difficult question in evolutionary biology, and it would take a book to summarize some of the ideas. Since we have no fossil data to guide our thinking, we are forced to rely on some clever experiments involving heating mixtures of simple chemicals that were very likely to be found in the early pre-biotic oceans. A creative graduate student named Stanley Miller. exposed a mixture of methane, ammonia, hydrogen gas and water to electrical discharges (simulating lightning) and to his great surprise ended up with a brown solution rich in amino acids and simple carbohydrates. His results have been supported by the chemical analysis of carbon rich meteorites found on Antarctic ice sheets (where they would be protected from contamination of terrestrial biomolecules). These carbonite meterorites, formed in outer space, also contain the same amino acids (both left and right handed isomers; cells contain only left handed amino acids) that Miller produced in his experiment. Amino acids are the building blocks of proteins, the most important biomolecules. Short chains of amino acids (called proteinoids) have been made by heating dry mixtures of amino acids. Proteinoids actually have some weak enzymatic activities, stimulating chemical reactions needed for cell metabolism. Similar work as shown that fatty acids and even the nitrogenous bases found in RNA and DAN can be formed in "pre-biotic" conditions.
Now that we know that the ingredients of life could have formed and accumulated in the early oceans, how did the first cell originate? Cell-like blobs called microspheres have been produced by boiling a mixture of lipids and proteinoids. These simple blobs are still a long way from the complexity (with over a thousand different kinds of proteins) of a bacterium.
This is as far as I can take it, but I should mention that there is evidence that RNA was the first information chemical (before DNA). Recently, RNA was shown to be able to act as an enzyme, performing precise chemical reations such as cutting and splicing RNA molecules at a precise location (these talented RNA's are called ribozymes). Also, RNA molecules can be made from individual bases by first attaching to and lining up along the flat, organized crystals of clay minerals. (Clay is covered with positive charges that would attract negatively charged molecules like RNA bases).
We may never know what caused the first bacterial (or pre-bacterial) cell to form; some people speculate that a bacterial spore drifted to earth from a different planet (but that just begs the question of how that cell originated) Christians such as myself who see the formation of life proceeding gradually by natural processes can easily see opportunities for divine intervention to get the whole thing rolling (just as you need a big bang before galaxies, stars and planets can form). I suspect new insights concerning the creation of life will emerge in the next century (perhaps bacteria-like cells will be found on Mars, which once had rivers and seas, after all.)
Try the links in the MadSci Library for more information on Evolution.