Unit 1 : Many Planets, One Earth
Overview Astronomers have discovered dozens of planets orbiting other 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, examine the unique characteristics that make our planet habitable and learn how these conditions were created.

Surfaces of Mars, Moon, Venus, Earth. Source: NASA

Sections:
1. Introduction 2. Many Planets, One Earth 3. Reading Geologic Records 4. Carbon Cycling and Earth's Climate 5. Testing the Thermostat: Snowball Earth 6. Atmospheric Oxygen 7. Early Life: Single-Celled Organisms 8. The Cambrian Explosion and the Diversification of Animals 9. The Age of Mammals 10. Further Reading

Unit 1 : Many Planets, One Earth

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1. Introduction
Earth's long history tells a story of constant environmental change and of close connections between physical and biological environments. It also demonstrates the robustness of life. Simple organisms first appeared on Earth some 3.8 billion years ago, and complex life forms emerged approximately 2 billion years ago. Life on Earth has endured through many intense stresses, including ice ages, warm episodes, high and low oxygen levels, mass extinctions, huge volcanic eruptions, and meteorite impacts. Untold numbers of species have come and gone, but life has survived even the most extreme fluxes. 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. This unit describes how Earth's carbon cycle regulates its climate and keeps surface temperatures within a habitable range. It also examines another central factor: the rise of free oxygen in the atmosphere starting more than 2