Chemists have found a possible explanation for one of the biggest holes in our theories of how life began. We still lack a complete solution to life's appearance, but when the question you are tackling is as significant as this one, every step forward is giant news.
Creationists love to mock scientists by pointing out their inability to explain how life could arise from non-living chemicals. Several competing theories have been presented, none of which are complete. The most popular is known as the RNA World Hypothesis, that the pre-existing chemicals combined to form strands of RNA, which went on to replicate themselves and achieve increasing complexity.
RNA is made from four nucleotides; adenine, guanine, cytosine, and uracil. Chemists have set to work trying to replicate the conditions on the planet 4 billion years ago, to see if these molecules can be made without help. “Considerable progress has been made toward potentially prebiotic syntheses of [cytosine and uracil],” Harvard University's Professor Jack Szostak and graduate student Seohyun Kim write in the Proceedings of the National Academy of Sciences.
Adenine and guanine are a different matter. Reactions designed to produce them without pre-existing life forms drown what little they make in by-products incompatible with RNA formation. Since four letters of code are needed, that's a problem.
One way around this is if the original forms of proto-RNA didn't use the modern four nucleotides, getting started with something simpler and swapping some of the originals once life was well under way. Szostak, Kim, and co-authors attempted to make an RNA approximation using a class of molecules known as 8-oxo-purines that resemble RNA's constituents, but have recently been shown to be more easily produced.
The proto-RNA Szostak and Kim made using 8-oxo-purines didn't work very well, replicating slowly and inaccurately. However, at the same time, the pair also investigated a different version, originally intended as a control. This comparison version, which used the molecule inosine instead of guanosine, behaved in all the ways RNA should, replicating quickly enough to avoid degradation and with even higher accuracy than versions that contain guanosine.
The authors write; "We propose that inosine could have served as a surrogate for guanosine in the early emergence of life."
We haven't yet managed to produce inosine through non-biological means, but the paper notes it could easily be formed from adenine. Clearly, this makes the question of where adenine comes from even more important, but it's better to have one problem than two.
Once this is solved, a complete story of life's origins will require an explanation of how, and indeed why, RNA shifted from having inosine to guanosine in its core components.