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About this unit

What makes life so special? Is there life elsewhere in the Universe? And how exactly did life emerge and diversify? In many ways these remain mysteries, but we do have enough evidence to explore some possible answers. Our exploration will span more than 3.8 billion years of Earth’s history. Defining life is harder than it seems. One answer centers on four inherent qualities: metabolism, homeostasis, reproduction, and adaptation. But what Goldilocks Conditions enabled life to prosper in so many diverse forms? To help answer some of these questions, you’ll chart the remarkable journey of life on Earth, crossing six “mini-thresholds” where life demonstrated distinct new characteristics. You’ll also explore the biosphere and the dynamic, sometimes catastrophic, relationship between life and Earth.