The anatomical similarity annotations are used to define evolutionary relations between anatomical entities described in the Uberon ontology. The annotations are currently focused toward the concept of historical homology, meaning that they try to capture which structures are believed to derive from a common ancestral structure. These annotations follow practices similar to the Gene Ontology consortium guidelines to capture evidence lines. Each statement about homology is captured as a single annotation, providing reference, UBERON or Cell Ontology term to capture the anatomical entity, NCBI Taxonomy term the ancestral taxon, ECO term the evidence type, and Confidence Information Ontology term the confidence in the annotation.