According to the popular series Ancient Aliens, on H2 (a spinoff of the History channel), extraterrestrial intelligences visited Earth in the distant past, as evidenced by numerous archaeological artifacts whose scientific explanations prove unsatisfactory for alien enthusiasts. The series is the latest in a genre launched in 1968 by Erich von Däniken, whose book Chariots of the Gods? became an international best seller. It spawned several sequels, including Gods from Outer Space, The Gods Were Astronauts and, just in time for the December 21, 2012, doomsday palooza, Twilight of the Gods: The Mayan Calendar and the Return of the Extraterrestrials (the ones who failed to materialize).

Ancient aliens theory is grounded in a logical fallacy called argumentum ad ignorantiam, or “argument from ignorance.” The illogical reasoning goes like this: if there is no satisfactory terrestrial explanation for, say, the Nazca lines of Peru, the Easter Island statues or the Egyptian pyramids, then the theory that they were built by aliens from outer space must be true.

Whereas the talking heads of Ancient Aliens conjecture that ETs used “acoustic stone levitation” to build the pyramids, for example, archaeologists have discovered images demonstrating how tens of thousands of Egyptian workers employed wood sleds to move the stones along roads from the quarry to the site and then hauled them up gently sloping dirt ramps of an ever growing pyramid. Copper drills, chisels, saws and awls have been found in the rubble around the Great Pyramid of Giza, and the quarries are filled with half-finished blocks and broken tools that show how the Egyptians worked the stone. Conspicuously absent from the archaeological record are any artifacts more advanced than those known to be used in the third millennium B.C.

Another alleged aliens artifact is a symbol found in the Egyptian Dendera Temple complex that vaguely resembles a modern lightbulb, with a squiggly filament inside and a plug at the bottom. Instead of featuring archaeologists who would explain that the symbol depicts a creation myth of the time (the “plug” is a lotus flower that represents life arising from the primordial waters, and the “filament” signifies a snake), ancient aliens fantasists speculate that the Egyptians were given the power of electricity by the gods. In this “if this were true, what else would be true?” line of inquiry, it is telling that no electrical wires, glass bulbs, metal filaments or electric power stations have ever been excavated.

On the lid of the sarcophagus of the Mayan king Pakal in Mexico is a “rocketlike” image that Ancient Aliens consulting producer Giorgio Tsoukalos claims depicts the ruler in a spaceship: “He is at an angle like modern-day astronauts upon liftoff. He is manipulating some controls. He has some type of breathing apparatus or some type of a telescope in front of his face. His feet are on some type of a pedal. And you have something that looks like an exhaust—with flames.” According to Mayan archaeologists, however, this depiction shows King Pakal sitting atop the sun monster and descending into the underworld (where the sun goes at night) within a “world tree”—a classic mythological symbol, with branches stretched into the heavens and roots dug into the underworld.

Ancient aliens arguments from ignorance resemble intelligent design “God of the gaps” arguments: wherever a gap in scientific knowledge exists, there is evidence of divine design. In this way, ancient aliens serve as small “g” gods of the archaeological gaps, with the same shortcoming as the gods of the evolutionary gaps—the holes are already filled or soon will be, and then whence goes your theory? In science, for a new theory to be accepted, it is not enough to identify only the gaps in the prevailing theory (negative evidence). Proponents must provide positive evidence in favor of their new theory. And as skeptics like to say, before you say something is out of this world, first make sure that it is not in this world.

Tellingly, in subsequent printings of Chariots of the Gods? the question mark was quietly dropped, and this disqualifier was added on the copyright page: “This is a work of fiction. Names, characters, places, and incidents are either the product of the author's imagination or are used fictitiously.” Gap closed.