1. Unit 1 : Many Planets, One Earth
Overview
Astronomers have discovered dozens of planets orbiting stars, and space probes have explored many parts of our solar system, but so far scientists have only discovered one place in the universe where conditions are suitable for complex life forms: Earth. In this unit, we will examine the unique characteristics that make our planet habitable and learn how these conditions were created.

2. • What transitions has Earth gone through and why?
Essential Questions
• How is it that a one-time collection of cosmic dust has gradually changed over time and produced this incredible planet with its tremendous biological diversity?
• What transitions has Earth gone through and why?
• How does evidence of the deep past inform us about today’s conditions?
• How does the Earth’s geological carbon cycle act as a thermostat?
Seattle , Washington June 2012 Pamela R. Cox

3. Introduction
To understand why Earth has been so conducive to life, we need to identify key conditions that make it habitable and ask why they exist here but not on neighboring planets.

4. Scientists believe simple organisms first appeared on Earth
Earth’s long history tells a story of constant environmental change and close connections between physical and biological environments.
Scientists believe simple organisms first appeared on Earth
some 3.8 billion years ago and complex life forms emerged
approximately 2 billion years ago.

5. High and low oxygen levels Mass extinctions Huge volcanic eruptions
3. List the 6 intense stresses life on Earth has endured through the ages.
Ice ages
Warm episodes
High and low oxygen levels
Mass extinctions
Huge volcanic eruptions
Meteorite impacts

6. Humans have been on Earth a relatively ( short / long ) time.

7. 5. Life on Earth will persist in spite of human impacts
5. Life on Earth will persist in spite of human impacts. List 3 impacts humans have on the Earth.
water, land and air pollution, carbon emissions, fertilizers, pesticides, deforestation, strip mining, clearing land for farming, building cities, paving roads, industrial outputs, garbage dumps, smoking, burning of fuels; gasoline, coal .and many others
6. Human impacts add to a natural level of ongoing environmental change.