Final answer:
Adaptive immunity developed in vertebrates after diverging from a common ancestor with invertebrates. The common ancestor had an innate immune system, and then vertebrates evolved the more complex adaptive system, which enables a highly specific response to a wide variety of pathogens.
Step-by-step explanation:
At what point in evolution did adaptive immunity develop? This is a question that delves into the complex history of the vertebrate immune system. Evidence suggests that the innate immune system evolved first. Both invertebrates and vertebrates share a common ancestor that possessed an innate immune system. As the two lineages diverged, it was the vertebrate lineage that developed the more complex adaptive immunity. Over time, adaptive immunity continued to evolve exclusively in vertebrates, incorporating mechanisms such as the production of antibodies and the maturation of specialized B and T cells responsible for immune memory and highly specific pathogen targeting.
The reason vertebrates evolved an adaptive immune system rather than an innate system with specific responses to pathogens is related to the adaptive system's ability to respond to a wide variety of pathogens through the development of up to 10¹¹ different receptors. This incredible specificity and flexibility allow the adaptive immune system to effectively resolve infections through rapid clonal expansion, seroconversion, and the establishment of immunological memory following the primary adaptive response.