The second microRNA was identified in Gary Ruvkun’s lab in 1999, so it was a good six years before the second one was found, also in C. elegans. Really, the watershed discovery was when Ruvkun showed that let-7, the other microRNA, was actually conserved perfectly in sequence amongst all the bilaterian animals. So that meant that let-7 microRNA had been around for, what, 500 million years?

And so it was immediately obvious to the field that there had to be other microRNAs – this was not just a C. elegans thing. There must be others, and that quickly emerged to be the case.