It seems a bit naïf to assume that nothing happened during that time. Gaining a whole lot more sentience is a big adjustment. We have to build language, build teaching methods, figure out how, to cooperate via trade and so on, master parenting of sentient creatures, notice patterns in the year and seasons and maybe decades and so on. I mean maybe nothing happened, but it seems like the first order assumption would be there is a lot that goes into modern society and it took time to develop. We can see for ourselves that new inventions and methods can speed up cultural rates of change and adaptability, so it makes sense the groundwork would be filled in slowly.