Human Gene Age Dating Reveals an Early and Rapid Evolutionary Construction of the Adaptive Immune System

T cells are a type of white blood cell that play a critical role in the immune response against foreign pathogens through a process called T cell adaptive immunity (TCAI). However, the evolution of the genes and nucleotide sequences involved in TCAI is not well understood. To investigate this, we performed comparative studies of gene annotations and genome assemblies of 28 vertebrate species and identified sets of human genes that are involved in TCAI, carcinogenesis, and aging. We found that these gene sets share interaction pathways, which may have contributed to the evolution of longevity in the vertebrate lineage leading to humans. Our human gene age dating analyses revealed that there was rapid origination of genes with TCAI-related functions prior to the Cretaceous eutherian radiation and these new genes mainly encode negative regulators. We identified no new TCAI-related genes after the divergence of placental mammals, but we did detect an extensive number of amino acid substitutions under strong positive selection in recently evolved human immunity genes suggesting they are coevolving with adaptive immunity. More specifically, we observed that antigen processing and presentation and checkpoint genes are significantly enriched among new genes evolving under positive selection. These observations reveal evolutionary processes of TCAI that were associated with rapid gene duplication in the early stages of vertebrates and subsequent sequence changes in TCAI-related genes. The analysis of vertebrate genomes provides evidence that a "big bang" of adaptive immune genes occurred 300-500 million years ago. These processes together suggest an early genetic construction of the vertebrate immune system and subsequent molecular adaptation to diverse antigens.