In nature, a stable RNA molecule takes advantage of an extensive network of tertiary interactions to fold and self-assemble into a functional three-dimensional (3D) structure. Recently, extensive sequence and structural analysis of known RNA 3D structures has revealed that most natural RNA tertiary interactions belong to a rather limited set of prevalent self-assembling modules, which correspond to well-defined structural conformers characterized by sets of conserved and semi-conserved nucleotides. As such, a natural stable RNA can be seen as a complex 3D network that is hierarchically built from prevalent modular 3D networks of smaller sizes. The underlying structural syntax behind modern day RNA architectures could be seen as an informational proto-language that likely emerged during the early evolutionary process of life on Earth. However, reasons behind the emergence of this particular proto-language are largely unknown. Using artificial self-assembling RNA systems, we isolated by in vitro evolution several novel self-assembling modules of RNA. By extensively characterizing their biochemical and biophysical properties, we were able to explore the genotype/phenotype landscape of both natural and in vitro selected self-assembling modules of RNA. Our investigation shed light on the evolution of modularity and self-assembly in RNA. It also provides valuable insights for the design and construction of RNA nanostructures of increasing complexity.