Complex sentence: The first miRNAs were characterized in the early 1990s, but miRNAs were not recognized as a distinct class of biologic regulators with conserved functions until the early 2000s.
Split into simpler sentences: The first miRNAs were characterized in the early 1990s. However, miRNAs were not recognized as a distinct class of biologic regulators with conserved functions until the early 2000s.