Geoscientists have estimated the earth to be about 4.5 billion years old. As the crust cooled, early geologic processes were largely volcanic, building up continental crust and a primitive atmosphere. Bacterial forms of life have been found in rocks that are billions of years old. Complex oceanic organisms such as trilobites began to appear only about 600 million years ago. From about 66 million to 245 million years ago, dinosaurs and other reptiles flourished all over the world. In contrast, human beings have existed in only about the last 3 million years, less than a thousandth of the age of Earth.