The Proterozoic aeon is a geological time period that lasted from 2.5 billion to 541 million years ago. It is often referred to as the middle era of Earth's history, marked by significant changes to the planet's atmosphere and the evolution of early life forms. Some synonyms for the Proterozoic aeon include the middle Precambrian or simply the Proterozoic era. This period is also divided into three parts: the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. Due to the significant changes that occurred during this time, the Proterozoic aeon is of great interest to geologists, biologists, and paleontologists.