IQ is a measure of one's ability to think, speed of thought, capacity of thought, memory, predictions of the future. A century ago you had to be smarter to survive than now because there were more problems to be solved by the individual. People still built their own homes back then. Grew their own food. Fixed their own cars.
There were more constraints, so people had to be more clever to survive.
Plato tells us this is why Socrates eschewed writing. He considered the written word a crutch and just like walking with crutches all the time would atrophy your legs, writing atrophies the memory.
