The complexity of fossils in the geologic time scale varies widely depending on the era. The Precambrian time is characterized by simple, single-celled organisms, while the Phanerozoic eon (which includes the Paleozoic, Mesozoic, and Cenozoic eras) is characterized by the emergence of more complex organisms, including plants, animals, and eventually humans. The Quaternary period is the most recent period and is characterized by the emergence of modern humans and the extinction of many large mammals.