Oh, and here's a disclaimer for you all: this post is going to look a lot like a negative argument against the naturalistic paradigm, and also a lot like the God of the Gaps fallacy. If you have a problem with that, well, so do I. But as I said earlier, I still find the argument somewhat compelling, just not scientifically publishable. (See part 1 of this series of posts.)
There are four crucial parts of a naturalistic scenario for the origin of life that we currently lack evidence for. First, life must arise from a "primordial soup", or a concentrated solution of biological precursor molecules. Second, life must arise gradually over many, many years. Third, life must initially be very simple (whatever that means), only later evolving to be complex. And fourth, life must arise from identifiable biochemical reaction pathways that can be deduced from laboratory experiments.
The primordial soup, or prebiotic soup as some would call it, was a hypothesized aqueous mixture of all the biochemical molecules necessary to form life. I am not sure who first proposed this idea, but I am certain that it was a hypothetical construct for which we never had an evidence, yet somehow was made to be as fact in popular literature (see for example the series finale of "Star Trek TNG") and in textbooks. Darwin talks about it in a letter he wrote to a colleague in 1871:
...[W]e could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, heat, light, electricity, etc. present, that a protein compound was chemically formed, ready to undergo still more complex changes...And so the primordial soup was just some "warm little pond", or ocean, with a bunch of the necessary life chemicals in it, such as sugars, amino acids, and nucleobases (the molecules that give DNA sequences their identity; the A's, G's, C's, and T's).
Well, it turns out that not only do we still have no evidence for a prebiotic soup, but we actually have evidence against there ever existing such a hypothetical mixture. I must confess that I don't know a ton about the science behind these observations, but the chemical signatures from the oldest known rocks show evidence of life, rather than non-life processes (such as prebiotic chemistry). This leads us directly into the second point: how long did it take for life to arise?
Oh boy, I didn't realize how long this post was going to get. We'll have to save discussion of the other this and the other two topics for later.