Intelligence and knowledge are two separate things. Plato didn't know the Earth orbits the sun; every modern knuckle-dragger does. That doesn't make the knuckledragger SMARTER than Plato; it just means that he's been told a fact that no one was around to tell Plato.
When talking about witchcraft, I will sometimes ask my students if they believe in the existence of invisible beings that can take possession of their bodies, inflict diseases on them and maybe even kill them unless they take measures to cleanse and purify themselves of these dangerous beings' influence. Most give me dull stares, which should say something to the intelligence question right there; most of those who DO respond, say that of course they don't believe in such superstitious nonsense - until it occurs to someone that I'm talking about bacteria. Now, the existence of bacteria rests on much sounder evidence than the existence of witchcraft, of course - but most of us do not OURSELVES have mastery of that evidence. We believe in germs rather than witches not because we figured it out ourselves but because authorities we trust - scientists, doctors, teachers, parents - TOLD us so and we accept their say-so. Recognizing that should make us much humbler about how "smart" modern people really are.
Some aspects of what we call intelligence, perhaps most, are skills, not genetic givens. People often fail to understand something not because they're genetically inferior (whatever that means) but because they haven't cultivated the skill. Finals are coming up, and they usually demonstrate this.
I'm going to get heaps of undergraduate papers. I'll read a paper that says, "According to Jane Doe, 'feudalism was the central institution of medieval society,'" with (hopefully) a footnote to a reading that was assigned.
That will strike me as odd because I'm quite sure that was not Prof. Doe's view. Looking up the reference in Doe's book, I then find the following sentence, at the end of a long discussion of medieval life: "These considerations should make us cautious to embrace the view that feudalism was the central institution of medieval society."
These are college students, the theoretically more intelligent (at least more educated) sector of society, failing at very basic reading; getting the opposite of an author's meaning from a sentence because they're baffled by a subordinate clause, or by nuance and complexity in general. They have trained themselves, by their practices, to scan for keywords rather than read for meaning. That's not genetic capability, that's skill development. Their incapacity isn't going to prevent them getting jobs and mates and having kids, it isn't going to win them any Darwin awards. But it doesn't support any notion of people getting smarter over the ages.