10,000 years ago, when the woolly mammoth was becoming extinct, humans started looking for alternative food sources. So, they turned to plants. Some of these prehistoric plants, like lily bulbs and wild onions required extensive cooking times. Cooking over hot coals didn't cut it so they turned to piles of hot rocks, where they could cook something for days.
Here's a fantastic article from the LA Times about prehistoric slow food.
According to a study done by a team at the University of Wales, there is some similarity in Prehistoric food to what we eat today - except for a hedgehog or two. Nettle Pudding was one of the most popular prehistoric dishes:
Nettle pudding
Ingredients
1 bunch of sorrel
1 bunch of watercress
1 bunch of dandelion leaves
2 bunches of young nettle leaves
Some chives
1 cup of barley flour
1 teaspoon salt
Directions
Chop the herbs finely and mix in the barley flour and salt. Add enough water to bind it together and place in the centre of a linen or muslin cloth. Tie the cloth securely and add to a pot of simmering venison or wild boar (a pork joint will do just as well). Leave in the pot until the meat is cooked and serve with chunks of bread.
Taken from Prehistoric Cooking by Jacqui Wood (Tempus, 2002)