Why human societies developed so little during 300k years
The sheer number of people with free time and who are able to communicate long distances with others who share their interests (whether practical or frivolous) seems to often be left out of these discussions.  Right now there are a billion people using electronic devices, engaged, productively or recreationally, with other human beings.  The elite of Ancient Rome, the people literate, with some free time, measured in perhaps the tens or hundreds of thousands.  So one hour of our collective mental wankery today is equivalent to tens of thousands of hours of it in Ancient Rome, assuming all other things were equal (which they are not).In other words, I don't think it's a coincidence things started changing rapidly after the invention of agriculture, when the human population started to steadily increase.