Earliest man did not cook his food. Whatever he was able to find in nature, he gathered up and ate raw. And this was simply because he had no way of cooking his food. He didn't know how to make fire.
Even when man learned how to make fire, he used it at first only for warmth and to frighten away wild animals. Cooking may have been discovered by accident. Some of the animals he killed may have been thrown near the fire. Or meat he was eating may have fallen into the glowing embers. The surface of the meat turned brown. It smelled good and tasted good—and man discovered his food would be improved by cooking.
One of the earliest means of cooking was on the hot stones around an open fire. Pits lined with stones and glowing coals formed the first oven for primitive man. Soon it was built above the ground, with an outlet for smoke, a draft, and a stone across the front opening to hold in the heat.
Man learned how to boil foods in pits lined with a large hide or skin. This was filled with water and heated to the boiling point by red-hot stones.
Primitive kettles were made by smearing clay over reed baskets and letting it harden. These kettles were placed over the fire for cooking foods, either with or without water.
So early man, many thousands of years ago, worked out the two main methods of cooking: by baking or roasting in dry heat, and by boiling or steaming in moist heat.