There is an article discussing this here in that was produced by Natural Society. An excerpt:
"I would wager that if an average citizen from Athens of 1000BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive ..."
I am a bit skeptical that Athens existed in 1,000 BC, but we will let that pass. The main thing I would note is that an early citizen of Athens could easily handle the male pronoun being sometimes used with reference to a male, sometimes with an object of a male gender, and sometimes referring to an object of uncertain gender. Context would allow an easy determination. Today's academics are no longer capable of this feat, hence the need to use "he or she" all the time.
The article next jumps to "Darwin's theory", apparently oblivious to the fact that Darwin never defined a theory, nor has anyone ever actually used it to solve a problem, nor will you find this mythical theory defined in any textbook. Of course an Athenian from 300 BC would immediately recognize that what Darwin proposed to be a scientific theory, "It just happens", was actually coined by Epicurus who learned it from Forest Gump.
But let us continue to find out what else is lowering our intelligence:
"The Water Supply, Fluoride is Lowering Your IQ"
"Pesticides are Diminishing Intelligence"
"Process Foods, High Fructose Corn Syrup Making People 'Stupid'"
Horrors! I consumed all of these this morning!
The comment section provides more samples of intellects in various stages of decline. At this point in life, I think I will just accept my fate and not worry too much about my rapidly diminishing intellectual powers.