Which eon spans roughly 2.5 billion to 540 million years ago and includes the evolution of multicellular organisms and the atmosphere beginning to fill with oxygen?

Which eon spans roughly 2.5 billion to 540 million years ago and includes the evolution of multicellular organisms and the atmosphere beginning to fill with oxygen?

• A. Proerozoic Eon
• B. Phanerozoic Eon
• C. Archeozoic Eon
• D. Paleontologist

Answer: (A)

The main idea here is identifying the eon on the geologic time scale that matches this time span and these developments. The period from about 2.5 billion to 540 million years ago is the Proterozoic Eon. During this immense interval, Earth's atmosphere gradually became rich in oxygen, a process linked to the Great Oxygenation Event and subsequent changes that allowed more complex life to evolve. By the late Proterozoic, multicellular organisms appear, setting the stage for the dramatic diversification of life in the following Phanerozoic Eon. In contrast, Archean is earlier than 2.5 billion years ago, and Phanerozoic begins after roughly 541 million years ago and features abundant animal life. The option that is not a geologic interval is a person, not a time span. So the best match for this description is the Proterozoic Eon. (Note the correct spelling: Proterozoic.)

The main idea here is identifying the eon on the geologic time scale that matches this time span and these developments. The period from about 2.5 billion to 540 million years ago is the Proterozoic Eon. During this immense interval, Earth's atmosphere gradually became rich in oxygen, a process linked to the Great Oxygenation Event and subsequent changes that allowed more complex life to evolve. By the late Proterozoic, multicellular organisms appear, setting the stage for the dramatic diversification of life in the following Phanerozoic Eon.

In contrast, Archean is earlier than 2.5 billion years ago, and Phanerozoic begins after roughly 541 million years ago and features abundant animal life. The option that is not a geologic interval is a person, not a time span. So the best match for this description is the Proterozoic Eon. (Note the correct spelling: Proterozoic.)