We study folding and structure of RNA and DNA as modulated by sequence, covalent damage, anti-cancer drugs, proteins, other nucleic acid molecules. The oldest assembly in biology is the ribosome, which is a primary focus of our efforts. Ancient ribosomal structure and function, from beyond the root of the tree of life, can be inferred from extant structure/function combined with phylogeny, evolutionary theory, biophysical chemistry, bioinformatics and molecular biology. We use all of these approaches to construct models of ancient ribosomes, which we then study by biochemical methods. Three-dimensional structure, being more conserved over evolutionary time than sequence, offers some of the most important guideposts in our journeys down the base of the tree of life.