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Biogeochemistry is the study of how living organisms influence the chemical processes that shape our planet. It exists at the intersection of biology, geology, and chemistry, and helps explain how life and Earth co-evolve. This chapter lays the groundwork for understanding the fundamental concepts that define biogeochemistry, from the early chemical origins of Earth to the tightly interconnected systems that govern planetary change today.

The Earth formed over 4.5 billion years ago from a swirl of cosmic dust and gas. In those early years, our planet was a chaotic mix of molten rock, unstable atmospheres, and extreme temperatures. As it cooled, chemical processes began forming more stable environments, laying the groundwork for the emergence of life. The Earth's oceans, atmosphere, and crust developed unique chemical profiles, each influenced by volcanic activity, solar radiation, and asteroid impacts. But it wasn't until life appeared that Earth's chemistry began to change in truly transformative ways.

Biogeochemistry focuses on these transformations—how life, from the simplest microbes to complex multicellular organisms, has shaped the Earth's geochemical systems. Microorganisms were the first drivers of global chemical change. They metabolized minerals, produced gases like methane and oxygen, and initiated nutrient cycles that continue today. These cycles—of carbon, nitrogen, sulfur, and others—are central to the biogeochemical framework. They show how elements move through different Earth systems: the biosphere, atmosphere, hydrosphere, and lithosphere.