Final answer:
Mammalian brains are homologous structures due to their common ancestry and similar anatomical features. The presence of a cerebral cortex, optic lobes, and corpus callosum in mammals supports this homology. These structures have evolved to perform different functions in different species, but their underlying similarities indicate a shared ancestry.
Step-by-step explanation:
Mammalian brains are examples of homologous structures because they share a common anatomical structure and function. The common ancestry of mammals is evident in the similarities of their brain structure, such as the presence of the cerebral cortex, optic lobes, and corpus callosum. These structures are inherited from a common mammalian ancestor and have been modified over time to adapt to different functions in different species.