The Fallacy of Diffusionism When explorers examined the remains of an advanced Mexican culture, they concluded that a superior race must have come from elsewhere to build the palaces and pyramids: the theory of diffusionism was born. This program charts the 150-year search for civilization’s origins, which most 19th and early 20th century archaeologists believed to be a single source. Though questioned in the 1950s by Henri Frankfort, the diffusion theory persisted, as this program shows, and received interpretations ranging from Thor Heyerdahl’s idea that the Mexican pyramids were built by the Egyptians to notions that they were made by aliens—all under the premise that the ancient Mexicans could not have done it themselves.