There is a subviral world, whose most prominent representatives are viroids. Despite being solely composed by a circular, highly structured RNA of ~250 to 400 nucleotides without protein-coding ability (all viruses code for one or more proteins), viroids can infect and incite specific diseases in higher plants. The RNA of human hepatitis delta virus (HDV), the smallest genome of an animal virus, displays striking similarities with viroids: It is circular, folds into a rodlike secondary structure, and replicates through a rolling-circle mechanism catalyzed by host enzymes and cis-acting ribozymes. However, HDV RNA is larger (~1700 nucleotides), encodes a protein in its antigenomic polarity (the ∂ antigen), and depends for transmission on hepatitis B virus. The presence of ribozymes in some viroids and in HDV RNA, along with their structural simplicity, makes them candidates for being molecular fossils of the RNA world that presumably preceded our extant world based on DNA and proteins.
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