Figure 2. Simplified vertebrate phylogenetic tree emphasizing those taxa possessing single-domain Igs or TCRs.

Blue boxes display animal groups shown, or predicted, to have single-domain variable regions. There are five known single domains, at least four of which were derived by convergent evolution, holocephalin HCAb, elasmobranch IgNAR, monotreme/marsupial Igμ, and camelid HCAb. The numbers on the left indicate divergence time (millions of years ago) for the various vertebrate taxa.