Geologists have divided the earth history into time units that are largely based on the earth's biotic composition. The modern geological time scale includes both relative and absolute age dating and represents the most up-to-date assessment of the earth's history. In the modern geological time scale, the eras in earth's history when the planet was populated with advanced life forms are referred to as the Paleozoic (meaning ancient life), the Mesozoic (meaning middle life), and the Cenozoic (meaning recent life).