Earlier quoted context omitted.
> that sort of thing must happen frequently in nature Individual chiral molecules can happen naturally, sure, but not entire organisms. From the report: In a mirror bacterium, all of the chiral molecules of existing bacteria—proteins, nucleic acids, and metabolites—are replaced by their mirror images. In the fitness landscape there is an absolutely enormous gulf between standard and mirror bacteria, large enough that…
Nature has built a lot of bacteria over the millennia. It probably has experimented with just building the whole thing backwards. That sounds simpler to do than, say, evolving a cogwheel (which does happen occasionally) or actually developing some of these molecules in the first place. It'd be weird for evolution to struggle so much on trying such a simple concept somewhere. This is a process that naturally figured o…
While "building it backwards" sounds simple to us, it's not simple at all in evolutionary space. Evolution operates over a fitness landscape, where every configuration of an organism has some fitness (essentially, probability of reproducing). It can make good progress as long as there are paths that don't require traversing areas of impractically low fitness. The problem in this case is that between "normal" and "mirror" is an area of absurdly low fitness, which evolution shouldn't be expected to be able to cross.