The importance of adaptive immunity has been highlighted by the protection afforded by vaccination in the face of the current coronavirus pandemic. Much has been learned about the origins and subsequent evolution of the antigen-specific receptors used by this crucial arm of the immune response. Among recent discoveries are the early appearance of three lymphocyte lineages in vertebrates with antigen-specific receptors based on leucine-rich repeats in jawless fish but on immunoglobulin (Ig) domains in jawed vertebrates (1), and the proto-RAG transposon which created split genes that could recombine to generate diversity in the three antigen-specific receptors of jawed vertebrates (2). In contrast, there has been little agreement on the origin and subsequent evolution of cell surface molecules encoded by the major histocompatibility complex (MHC), which play central roles in adaptive immunity as the targets of T cell recognition. By discovering the W genes as a proposed intermediate in the evolution of MHC class I and class II genes, the paper by Kazuhiko Okamura, Hans Dijkstra, Kentaro Tsukamoto, Keiichiro Hashimoto and their colleagues in the current issue of Proc. Natl. Acad. Sci. USA (3) provides a welcome advance.