Actually this discovery is another one in which genes are silenced by the organism. One of the functions of such silencing is to suppress the activity of mutated genes. Organisms have quite a few tactics for fixing mutations and errors. It has nothing to do with an 'RNA World' the micro-RNA is coded by DNA just as regular RNA is a transcription from DNA. This is indeed another kick at stupid evolution theory which claimed that all DNA between genes was junk. But then as medved used to say, this is no problem for evolutionists because it does not refute the prime process by which evolutio takes place - abracadabra shazam.
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There's a rumor going around that everything refutes evolution.
If you are speaking of Darwininianism, it really doesn't take this much.