Infants once relied on a stable evolutionary sequence—spontaneous labor at full term, vaginal birth, and immediate, extended breastfeeding—to provide layered maternal immune protection during early life. Modern obstetric and cultural shifts, including rising cesarean deliveries, increased labor induction, and delayed or reduced breastfeeding, have altered this landscape. These changes decrease the late‑pregnancy transfer of maternal immunoglobulin G (IgG) and can limit postnatal delivery of secretory immunoglobulin A (IgA), leaving many infants with less maternal protection than nature once provided. Such shifts in delivery and nursing behavior can also affect the early gut microbiome, which plays a critical role in immune education and pathogen defense. While maternal antibodies can only buffer early vulnerability, vaccines supply infants and young children with long‑lasting immunological reference points at a moment in human evolution of unprecedented vulnerability. In this context, vaccines not only fill a modern‑day evolutionary gap but improve upon it by providing early, reliable protection against an array of diseases.