The Earth’s geologic history tells a story about the inception of life and the rise and fall of species, showing life is fragile in the face of gradual and sudden changes to the environment. In this unit, students will learn how rock and fossil observations in combination with radioactive dating techniques have been used to construct a geologic time scale. Students will also wrap their heads around the vastness of geologic time and the brevity of human existence by generating their own geologic time scale analogies. Finally, students will create a geologic timeline that includes the mapping of mass extinctions throughout Earth’s history and consider whether human behavior might lead to the next mass extinction.