Frederick L. Coolidge and Melissa L. Coolidge explore why persons see faces in slices of toast and other places where there are none. A snippet:
The neurological foundation for pareidolia resides primarily in an area of the temporal lobes of the brain called the fusiform gyrus, where humans and other animals (primarily modern great apes) have neurons dedicated to the recognition of faces and other objects. Ontogenetically, it would be important for infants to recognize and distinguish faces for sustenance and sociality. Astronomer Carl Sagan in his 1996 book, The Demon-Haunted World, noted that infants who could not recognize their parents’ faces were less likely to win their parents’ hearts, and thus less likely to prosper. Evolutionarily, it would be critically important to recognize faces (and other objects, both animate and inanimate) immediately for those two reasons and additionally to recognize predators and other threats in the environment.