Ancestral protein reconstruction allows the resurrection and characterization of ancient proteins based on computational analyses of sequences of modern-day proteins. Unfortunately, many protein families are highly divergent and not suitable for sequence-based reconstruction approaches. This limitation is exemplified by the antigen receptors of jawed vertebrates (B-and T-cell receptors), heterodimers formed by pairs of Ig domains. These receptors are believed to have evolved from an extinct homodimeric ancestor through a process of gene duplication and diversification; however molecular evidence has so far remained elusive. Here, we use a structural approach and laboratory evolution to reconstruct such molecules and characterize their interaction with antigen. High-resolution crystal structures of reconstructed homodimeric receptors in complex with hen-egg white lysozyme demonstrate how nanomolar affinity binding of asymmetrical antigen is enabled through selective recruitment and structural plasticity within the receptor-binding site. Our results provide structural evidence in support of long-held theories concerning the evolution of antigen receptors, and provide a blueprint for the experimental reconstruction of protein ancestry in the absence of phylogenetic evidence. protein evolution | protein structure | directed evolution | antibody | homodimer T he adaptive immune system of jawed vertebrates provides specific responses to pathogens and forms long-lasting immunological memory of past encounters (1). Key components of this system are B and T lymphocytes and their cognate receptors: B-cell receptor (membrane-bound or secreted as antibodies) and T-cell receptor. Both receptors are heterodimeric molecules formed by the association of Ig domain building blocks, which then assemble into higher order complexes. This behavior is exemplified by the canonical Y-shaped IgG antibody molecule, composed of two heavy chains (each containing four Ig domains) and two light chains (each containing two Ig domains) (2). Within the N-terminal Ig domains of the receptors, hypervariable complementarity determining regions (CDRs) mediate contact with antigen (Fig. 1A). The diversity and specificity of these receptors is based on the genetic rearrangement of variable (V), diverse (D) and joining (J) gene segments, mediated by recombination-activating gene enzymes, with further diversity introduced through somatic hypermutation (3).Although the molecular mechanisms and structural features of rearranging Ig antigen receptors are well documented, only limited insights have been gained into their evolutionary origins (4, 5). Reconstructing the evolutionary history of antigen receptors using molecular phylogenetic analysis has proven difficult due to high levels of evolutionary divergence (6-8). Indeed, Ig receptor genes are completely absent from the genomes of invertebrates and jawless vertebrates [from which jawed vertebrates diverged some 500 million years ago and which have developed an alternative means of adaptive immunity bas...