The evolutionary origin of plant and animal microRNAs

MicroRNAs (miRNAs) are a unique class of short endogenous RNAs, which have become known in the past few decades as major players in gene regulation at the post-transcriptional level. Their regulatory roles make miRNAs crucial for normal development and physiology in several distinct groups of eukaryotes including plants and animals. The common notion is that miRNAs have evolved independently in those distinct lineages, but recent evidence from non-bilaterian metazoans, plants, and various algae raise the possibility that the last common ancestor of these lineages might already have employed an miRNA pathway for post-transcriptional regulation. In this Review we present the commonalities and differences of the miRNA pathways in various eukaryotes and discuss the contrasting scenarios of their possible evolutionary origin and their proposed link to organismal complexity and multicellularity.