Unit 1: Many Planets, One Earth // Section 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 billion years ago. Next we briefly survey the evolution of life on Earth from simple life forms through the Cambrian explosion and the diversification of multicellular organisms—including, most recently, humans. This unit also describes how scientists find evidence in today's geologic records for events that took place millions or even billions of years ago (Fig. 1).
Figure 1. Evidence of glaciation in seaside rocks, Massachusetts
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Humans are latecomers in geologic time: when Earth's history is mapped onto a 24-hour time scale, we appear less than half a minute before the clock strikes midnight (footnote 1). But even though humans have been present for a relatively short time, our actions are changing the environment in many ways, which are addressed in units 5 through 13 of this course. Life on Earth will persist in spite of these human impacts. But it remains to be seen how our species will manage broad-scale challenges to our habitable planet, especially those that we create. As history shows, Earth has maintained conditions over billions of years that are uniquely suitable for life on Earth, but those conditions can fluctuate widely. Human impacts add to a natural level of ongoing environmental change.