AP Environmental Science

Answer: C - The correct order of the four eons is: Hadean, Archean, Proterozoic, and Phanerozoic.

Key Takeaway: The Phanerozoic Eon is the current geological era and the one during which the most abundant animal life has existed. The Phanerozoic Eon has covered more than 530 million years, going back to when the first hard-shelled animals appeared. The time before the Phanerozoic Era, formerly called the Precambrian, is now divided into three eras. In order from oldest to newest, they are the Hadean, Archean, and Proterozoic.