ABSTRACT
The general organization of the vertebrate brain is also quite stable and, although there are variations, a fundamental theme makes broad sections of the central nervous system easily identifiable across species. The study of brain evolution is facilitated by the fossilization of the vertebrate skeleton. Early vertebrates are characterized by the evolution of sensory systems that required changes in associated brain structures. In basal vertebrates, rapid escape responses are coordinated by giant neurons located in the rhombencephalon and mesencephalon that send giant axons to the spinal cord. An imaginary line between the pons and tegmentum, establishes the boundary between rhombencephalon and mesencephalon. Thus, the evolution of learning mechanisms in vertebrates appears to be highly conserved because homologous telencephalic areas play similar functions in teleost fish and mammals, despite their dissimilar development. Jerison had obtained a 0.67 value constant for all of his analyses, which suggested that the same factors determined the evolution of relative brain size in vertebrates.
