The evolutionary foundations of transcriptional regulation in animals.

The development of a single-celled zygote into a complex, multicellular animal is directed by transcription factors and regulatory RNAs that coordinate spatio-temporal gene expression patterns. Given the morphological complexity of animals, some prior work has hypothesized that the origin of animals required the evolution of unique and markedly complex transcriptional regulatory mechanisms. Such postulated animal innovations include the evolution of greater numbers of transcription factors, new transcription factor families, distal enhancers and the emergence of long non-coding RNAs. Here, we revisit these explanations in light of new genomic and functional data from diverse early-branching animals and close relatives of animals, which provide essential phylogenetic context for reconstructing the origin of animals. These experimental models also offer examples of how some animal developmental pathways were built from core mechanisms inherited from their protistan ancestors. These new data provide fresh perspectives on whether animal origins entailed fundamental innovations in transcriptional regulation or whether, alternatively, a gradual accumulation of smaller changes sufficed to generate the complex developmental and cell differentiation mechanisms of early animals.