[nmstoker]

Life forms are effectively locked into the current set of chiral molecules. For a cell to flip, it would need all the corresponding mirror changes to happen as well, since the behaviours are interlinked - this makes it something that would be highly unlikely to occur due to evolution and why it's only realistic if carried out by people all at once.

[geysersam]

That argument would be a good one if the time scale was shorter.

But over the course of the entire evolution of life on earth...

It's hard to argue that the step from chirality A to B is larger than all the other insanely unlikely steps evolution has taken. From earliest proto-life to complex multicellularity and beyond.

[nmstoker]

The issue is not that one chirality has to flip and you can accumulate change over long time periods, it's that every occurrence of chirality had to flip at once or the things won't work together: so the DNA has to mysteriously all get reversed (v difficult when all the DNA/RNA machinery is handed in the other direction), plus all the supporting items need to flip at that same point for all sorts of other cell functions that interlink (energy supply, waste handling etc etc).

But if your argument is given the extraordinary timeframes then why didn't life evolve again separately in the other chirality, so it wasn't descended from the earlier cells but was just a new unrelated line, then that is harder to generalise about but I recall hearing that there are sometimes slight advantages to one chirality over the opposite in certain cases as the reaction rates can differ (this is the same concept behind kinetic resolution). Perhaps life based on the less effective form would get crowded out by the more effective form, but I would defer to experts here as it's not my area.

[fabian2k]

The problem for new life with a different chirality would be that it would have to compete with the existing organisms. And the existing ones are already far, far more optimized than any new kind of life would be. So this is not comparable to the way life originally developed, where no competition existed and the entire environment was very different.