Lymphoid tissue inducer cells (LTi) are a relatively new arrival on the immunological cellular landscape, having first been characterized properly only 15 years ago. They are members of an emerging family of innate lymphoid cells (ILCs). Elucidation of their function reveals links not only with the ancient innate immune system, but also with adaptive immune responses, in particular the development of lymph nodes and CD4+ T cell memory immune responses, which on one hand underpin the success of vaccination strategies, and on the other hand drive many human immunologically mediated diseases. This perspective article is not an exhaustive account of the role of LTi in the development of lymphoid tissues, as there have been many excellent reviews published already. Instead, we combine current knowledge of genetic phylogeny and comparative immunology, together with classical mouse genetics, to suggest how LTi might have evolved from a primitive lymphocytic innate cell in the ancestral 500-million-year-old vertebrate immune system into a cell critical for adaptive CD4+ T cell immune responses in mammals.