The appendix: a repeatedly reinvented organ that helps immunity and gut microbes

The article explains that the appendix is not a simple leftover but has evolved independently in multiple mammal lineages (convergent evolution) and contains gut-associated lymphoid tissue that trains the immune system and may shelter helpful microbes after infections. In modern clean environments its selective value is modest, appendectomy does not impair fertility, and medicine treats the organ mainly to prevent disease rather than as an essential feature of health.