Imagine That Scientists Have Discovered a New Planet Apparently Devoid

Imagine that scientists have discovered a new planet apparently devoid of life, but having ice caps located at the poles. These ice caps reveal that the planet's atmospheric O₂ levels sharply increased about 1,000,000 years ago, but then quickly dropped 50,000 years ago. What could have accounted for these changes in atmospheric O₂ levels?

A) Photosynthetic organisms evolved on the planet 100,000 years ago, but went extinct 50,000 years ago.
B) Photosynthetic organisms evolved on the planet 50,000 years ago.
C) Oxygen-based mammalian life evolved on the planet 100,000 years ago, and photosynthetic organisms evolved 50,000 years ago.
D) Oxygen-based mammalian life evolved on the planet 100,000 years ago.