A fascination with the possibility of alien life is not a recent development. In fact, there is evidence that ancient Greeks pondered the possibility of extraterrestrial life at least as far back as the 4th century BC. Additionally, there are some who believe that the stories of legendary Greek gods are in fact an accurate historical retelling of what happened when extraterrestrials first visited Earth.
Understanding the Views of Ancient Greek Philosophers
Anaximander (c. 610 - 546 BC), a pre-Socratic Greek philosopher who lived in Miletus, is thought to be one of the first people to examine the idea of cosmic pluralism—also known as the plurality of worlds. This term refers to the view that there are “worlds” (planets, dwarf planets, or natural satellites existing in addition to Earth) that could potentially harbor extraterrestrial life. As a metaphysical argument, cosmic pluralism predates the scientific Copernican conception that Earth is one of numerous planets in the solar system.
Democritus (c. 460 - 370 BC), primarily remembered for his formulation of an atomic theory of the universe, and Epicurus (c. 341–270 BC), the founder of influential school 
of philosophy known as Epicureanism, both believed it was highly likely that there were other worlds in the cosmos—and that it was only logical that these other worlds would be inhabited.
Anaxarchus (c. 380 - c. 320 BC), a Greek philosopher of the school of Democritus, is said to have shared the view that there were an infinite number of worlds with Alexander the Great, the king of the ancient Greek kingdom of Macedon. According to legend, this caused Alexander to weep with the realization that he had not yet conquered even one of those worlds.
Roman Epicurean poet Lucretius (c. 99 - 55 BC) later expanded on Greek thought with this tantalizing explanation of life beyond our planet, “Nothing in the universe is unique and alone and therefore in other regions there must be other earths inhabited by different tribes of men and breeds of beasts.”
Of course, just as there are people today who believe the idea of extraterrestrial life is absurd, the ancient Greeks did not have entirely unified views. Aristotle (c. 384 - 322 BC) has exerted a unique influence on almost every form of knowledge in the West, and he firmly believed that the existence of other worlds beyond ours was impossible. He, along with his followers, preferred to focus on things that could be directly observed as opposed to merely imagined.
Greek Gods as Extraterrestrials
In Odyssey of the Gods: The Alien History of Ancient Greece, Erich von Däniken explores the idea that the Greek gods were extraterrestrial beings. According to his theory, the alien gods of Olympus interbred with humans to perform genetic experiments and produce mythical creatures like centaurs. He proposes that the ancient Greek temple Delphi was an aircraft refueling station, the ancient cities of Troy and Atlantis were obliterated in a war between aliens or their descendants, and Jason’s search for a golden fleece was in fact the pursuit of a vital aircraft component.
Erich’s ideas are sometimes referred to as the ancient astronauts or ancient aliens hypothesis. Proponents of this hypothesis state that prehistoric visits from intelligent extraterrestrial beings greatly influenced the development of modern cultures, religions, and technologies, as well as basic human biology.
The History Channel’s Ancient Aliens explores the ancient aliens hypothesis in several episodes, including looking at the adopted son of the ancient Greek philosopher Pythagoras as the offspring of the gods in Season 7 and sharing that Poseidon was an alien in Season 8. The series is inspired by and based on the works of von Däniken, as well as Zecharia Sitchin, Graham Hancock, Charles Hapgood, Edgar Cayce, Robert Bauval, and Brinsley Le Poer Trench.