Bread has been an important part of the human diet for thousands of years. Evidence suggests humans started baking bread at least 30,000 years ago by cooking grain mixtures on heated rocks. Prehistoric people had already developed the practice of making gruel from water and grains, making baked bread a small evolutionary step. A 2010 study found traces of starch from roots in prehistoric tools, indicating roots were ground into flour and mixed with water before being cooked on heated stones to make one of the earliest forms of bread.