When did it start?
Slow cooking dates back to the start of organised society. People living in towns would cook their bread in a shared central wood burning oven situated in the town centre. At the end of the day once all the bread had been cooked, the oven would still be really hot. While it was cooling down, the townsfolk would place their cooking pots into the oven. They would allow their food to cook slowly overnight. The next day, the food would be fully cooked, tender, and full of flavour.
The slow cooking used by the Greeks to make Kleftiko has a similar history. The story goes that when thieves stole a lamb, they would dig a hole and build a fire with lots of wooden logs. Once the logs had burned down, they would place the lamb on top of the smouldering embers, cover the hole with soil and seal it with mud. They did as a way of preventing the smell of their cooking, leading pursuers to where they were. Once the threat of capture had diminished, they would return to remove the lamb, which now had become succulently cooked and tender from the underground oven.
The Taino and escaped slaves (Maroons) in the Caribbean used the same method of cooking to prevent being captured. They would bury wild hogs they had caught in underground ovens. Before so they would cover the hog with salt, wild herbs and spices like pimento and Chillies and then wrap them in leaves. Doing this added great spicy flavour to the meat while it cooked and keep humidity constant while the meat was cooking. This style of cooking led to what has become known today as jerking meat (as for example in Jerk Chicken and Jerk pork)