Rock sequences from different geographic areas are correlated and arranged by means of fossil contents, radioactive isotope time clocks, magnetic records, and the oxygen isotope record in ocean sediments. Based on evidence of lead isotopes from the radioactive decay of uranium, the age of the earth is calculated to be around 4.5 billion years. The oldest rocks are at the bottom of the geologic column, and the youngest at the top. The first fossils in the geologic column occur in layers about 3 billion years old and remain rather rare up to layers around 600 million years old, where fossils are found in relative abundance worldwide. These layers represent the Cambrian period, characterized by a wide diversity of animal life.
Trilobites are common, but different fossil assemblages are found in successively younger rocks.
Dinosaurs dominated the Mesozoic era and disappeared at the end of the Cretaceous period about 65 million years ago. At the very top of the column, the Pleistocene Epoch, fossil assemblages show a close similarity with living plants and animals. Fossils further down the column generally show a greater divergence from modern plants and animals. Nearly all scientists agree with these observations. It is the explanations for why fossils are found as they are in the geologic column that are controversial.