The dumbing down of American is most evident in the slow decay of substantive content in the enormously influential media, the 30 second sound bites (now down to 10 seconds or less), lowest common denominator programming, credulous presentations on pseudoscience and superstition, but especially a kind of celebration of ignorance.
For all I know we may be visited by a different extraterrestrial civilization every second Tuesday, but there's no support for this appealing idea. The extraordinary claims are not supported by extraordinary evidence.
Many statements about God are confidently made by theologians on grounds that today at least sound specious. Thomas Aquinas claimed to prove that God cannot make another God, or commit suicide, or make a man without a soul, or even make a triangle whose interior angles do not equal 180 degrees. But Bolyai and Lobachevsky were able to accomplish this last feat (on a curved surface) in the nineteenth century, and they were not even approximately gods.
The Platonists and their Christian successors held the peculiar notion that the Earth was tainted and somehow nasty, while the heavens were perfect and divine. The fundamental idea that the Earth is a planet, that we are citizens of the Universe, was rejected and forgotten.
For all its material advantages, the sedentary life has left us edgy, unfulfilled, even after 400 generations in villages and cities, we haven't forgotten: The open road still softly calls like a nearly forgotten song of childhood.