Final answer:
In the context of immune system evolution, vertebrates exhibit both innate and adaptive immune systems, while invertebrates have only the innate system. This indicates that innate immunity evolved first and is ancestral to all animals, with adaptive immunity developing later in the vertebrate lineage.
Step-by-step explanation:
Evolution of Immune Systems in Vertebrates and Invertebrates
The presence of both innate and adaptive immune systems in vertebrates, as opposed to the presence of only the innate immune system in invertebrates, provides insight into the evolutionary history of these systems. Option a) is the most plausible, as it suggests that all animals likely inherited the innate immunity from a common ancestor due to its presence in both invertebrates and vertebrates. Subsequently, the adaptive immunity system evolved later in vertebrates, which allowed them to develop responses that target specific pathogens and retain a memory of these pathogens for more efficient responses upon re-exposure.
Through evolutionary processes, vertebrates enhanced their capacity to fight off pathogens by developing the adaptive immune system in addition to the innate system. The adaptive immune system is characterized by its specificity and ability to remember previous encounters with pathogens, allowing for a quicker and more effective response in future encounters. This system is crucial for survival in diverse and pathogen-rich environments.