Final answer:
Chemoreceptors are likely the oldest sensory receptors due to their fundamental role in survival, found even in primitive organisms lacking complex nervous systems.
Step-by-step explanation:
The oldest sensory receptors, as evidenced by their existence in primitive animals that do not have formalized nervous systems, are likely chemoreceptors. Chemoreceptors are specialized for the detection of chemical stimuli, which include substances that contribute to the senses of taste and smell. These sensory receptors play a fundamental role in the survival of organisms by enabling them to sense their environment and interact with it, such as finding food or avoiding toxins. Primitive organisms, which may lack a complex nervous system, still require the ability to detect chemical signals in their environment to survive, making chemoreceptors crucial. Moreover, because the ability to detect chemical changes in the environment is so basic to survival, it predates the evolution of more sophisticated sensory structures.