Answer:
The Proterozoic Eon
Step-by-step explanation:
The eons are the longest periods of geologic time. The Proterozoic eon is the geological eon from the appearance of oxygen in the atmosphere until the emergence of complex life forms (e.g., marine invertebrates). The Proterozoic eon started about 2.5 billion years ago and ended 541 million years ago. This eon can be divided into three geologic eras: Paleoproterozoic (2,500 to 1,600 million years), Mesoproterozoic (1,600 to 1,000 million years) and Neoproterozoic (1,000 million to 541 million years).