They probably had a lot more time on their hands, with a culture where sitting to watch tv was an unavailable form of avoidance entertain.

I can’t imagine this would be too difficult to figure out. Heat with a consistent supply of fuel should burn at about the same temperature, right? So, if you can create separate chambers where heat must travel through a medium like stone before it can heat the next chamber, you can somewhat control the heat… meaning you’ll have chambers of different temperatures, each decreasing as the chamber gets further from the fire source.

There’s probably ways to test the temperature without a thermometer, too. For example, how long does it take a small cup of water to start boiling? If you touch a weed to the side of the chamber, does it instantly light aflame or simply char? Etc.