How on earth did life start?

One possibility was validated a few weeks ago. Scientists, after about 2 years of testing, confirmed that essential amino acid building blocks required for life (protein making and make-up) could survive space and could have arrived via a comet impacting earth millions of years ago.

http://www.newscientist.com/article/dn17628-found-first-amino-acid-on-a-comet.html

So, why weren't amino acids available on earth to begin with? I mean, why does a comet have to come down to provide for us? It seems like amino acids can be created from simpler compounds (ie. CO, NH3, H2O), or, in other words, organic carbon matter and water. Also, since amino acids are fairly complex, you'd assume some energy as well. It seems that this process is not exclusive to comets.

Good article here with further discussion.

"
Other researchers say the finding appears to be solid, although some are skeptical of its significance. Robert Shapiro, a professor emeritus and senior research scientist in chemistry at New York University, says that because of their low concentration, extraterrestrial nucleobases were unlikely to have played much of a role in kick-starting life. "They're a subunit of a subunit of DNA," he says. "My opinion is that their amounts were utterly unimportant and insignificant." He says he would be more impressed if whole nucleosides—bases plus sugars—were found in meteorites in concentrations similar to those of amino acids.

And researchers may yet discover ways that Earthly chemistry—perhaps around hydrothermal vents—could have generated nucleobases and other compounds."

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Ok, so amino acids could have come from a variety of sources. Now, how did they combine further to produce RNA / DNA? or why? More specifically, what kind of outside pressures would result in their formation (against entropy) of larger amino acids and eventually chains? .. more later, as work is calling.