Considering how much we know about the origins and subsequent development of our species, there's a lot we still don't know. Not scientifically, because we can find that out with anthropology and biology, but culturally. The time before man began keeping written records, is shrouded in mystery, and we can only make educated guesses within context.
Officially known as the Ancient Astronaut Theory, the thinking goes like this: aliens visited Earth a very long time ago and left evidence. The original Ancient Astronaut theory posits that we ourselves are descendants of aliens - that is, our species originated on another planet, and a small population was left here to colonize Earth.
Or perhaps we are the descendants of offspring created when aliens mated with pre-humans. Or maybe we were already here and our alien visitors left an indelible mark on our collective psyche. Fans of Stargate: SG-1 already understand the concept, but proponents of the Ancient Astronaut Theory cite many examples of possible evidence.
Monoliths and Other Ancient Structures
Judging by the structures they left behind, early humans must have been geniuses (or superhuman). Giant structures - Stonehenge, the Nazca lines and pyramids and temples scattered around the planet - made of enormous blocks of stone from faraway places, designed with precision engineering, and cut with laser-like accuracy. How did they manage that? Aliens!
How did a society with only the crudest tools and materials quarry and transport these things? And the maoi faces of Easter Island have extremely elongated skulls...
Yet they feature perfectly straight precision cuts and perfect 90-degree angles that are still sharp enough to cut your finger, even after 1,000 years' exposure to the elements. Achieving something similar would take expert-level craftsmanship, even with modern tools.
Oh, and Giza and the great central pyramid at Teotihuacan share the same dimensions. And a pyramid at Tikal, Guatemala and one at Angkor Wat in Cambodia are virtually identical, down to the decorative scrollwork on the top. Weird, considering these cultures could not have had contact with one another.
From cave paintings to Renaissance art, flying saucers and spacesuits have been a subject of art long before man went to the moon. Cave paintings in the U.S., Australia, North Africa, Italy and France depict humanoid figures wearing what appear to be space helmets.
The ancient people of Ecuador sculpted figurines that look suspiciously like an Apollo astronaut, and the residents of 5,000 BC Iraq made figurines that look exactly like the stereotypical gray aliens we know from television today.
The 15th-century Ghirlandaio painting "The Madonna With Saint Giovannino" shows what is clearly a flying saucer in the sky to the right of the Virgin Mary, and the artist even painted a man and a dog looking up at it. The 14th-century altar painting at the Visoki Decani Monastery in Kosovo shows what looks to be two spacecraft engaged in a high-speed chase.
With the exception of old nature-based Pagan religions, all religions feature Gods that live in and come from the sky. Considering that ancient people used "God" to explain away things they didn't understand is it really such a leap to suppose that alien visitors were thought to be Gods as they descended from the sky with all their "magic" technology?
A gamut of religious texts - including the Book of Ezekiel, the Sanskrit Vimanas, the Mayan Popol Vuh and Chilam Balam, the Hindu Ramayana and the Book of Genesis all feature references to Gods coming to Earth in "sky chariots" amid smoke, fire and thunder. You know, like when the space shuttle takes off?
The television show Stargate: SG-1 makes the case that the famous deities of yore were nothing more than arrogant visitors who enslaved the human race and even kidnapped a few to colonize other planets. While it's really easy to dismiss that theory as sci-fi fantasy, is it really that much of a stretch?
No, the Ancient Astronaut Theory gets absolutely no respect from the mainstream scientific community - but their theories about these things aren't exactly airtight either. Read the work of Erich von Daniken, Robert K. G. Temple, Zecharia Sitchin and Giorgio A. Tsoukalos and decide for yourself.