A theory exists that Earth was visited by extraterrestrials in the distant past, the evidence for which includes ancient texts, artifacts and works of art. This concept has been widely discussed by writers such as Erich von Daniken and television shows such as "Ancient Aliens." However, many academics are dismissive of the ancient astronaut theory, pointing out a number of flaws in its methods and assertions.

Bad Science

The idea that aliens might have visited the Earth in the past is not in itself unscientific. Many professional scientists believe that extraterrestrial intelligence and interstellar travel are theoretical possibilities. What is unscientific, however, is the methodology adopted by ancient astronaut proponents, who attempt to fit evidence to theory instead of fitting theory to evidence. This fundamental flaw was pointed out by John T. Omohundro in an article in the Skeptical Inquirer as long ago as 1976. Professor Omohundro goes on to argue that theorists like Von Daniken fail to apply the principle of Occam's Razor, according to which the simplest explanation for the evidence is the most likely explanation.

False Perspective

The aim of the professional historian is to understand ancient writings and artifacts in terms of the culture that produced them. Ancient astronaut theorists, on the other hand, have a tendency to interpret everything in terms of modern culture. An example, cited by Professors Ronald Wyllys and Rory Coker of the University of Texas at Austin, is Erich von Daniken's interpretation of a seventh-century carving at Palenque in Mexico as an astronaut in a space capsule. In fact, the carving is the depiction of a well-known Mayan king of the time, which is replete with traditional Mayan iconography.

Underestimating Human Ingenuity

A common flaw in ancient astronaut theories is to assume that humans of the past were less resourceful, less ambitious and less intelligent than those of the present day. For example, ancient knowledge of the movement of planets in the night sky -- which was highly accurate -- is taken as evidence of extraterrestrial visitors. In fact, all that is required is careful and systematic observations made night after night for many years. Similarly, it requires no knowledge of interstellar space to construct a temple that is aligned to the stars -- simply a reverence for the night sky.

Lack of Real Evidence

Ultimately the biggest flaw in the ancient alien theory is the lack of evidence for it. A nuclear reactor, a microchip or even a synthetic plastic bottle would indicate a level of technology that could not be explained by mainstream theories. Stone carvings that look vaguely like nuclear reactors or microchips are not in the same category, especially if there is a more mundane interpretation of the object depicted. The ancient astronaut theorists may be correct in their assertion that the Earth has been visited by extraterrestrials in the past, but convincing evidence for the theory has yet to be found.