Investigating The History of The Earth

Investigating The History of The Earth

Introduction

Every day we walk across its surface, admiring its variety of plants, animals, and landscapes. Earth has taken billions of years to evolve into the planet we now inhabit. This volume examines this long history, from Earth’s origins in a cloud of gas and dust to its place as a planet teeming with life. It addresses questions that scientists have long debated: how, why, and when did Earth form and develop into this complex and intriguing planet? 

Scientists believe that Earth, as well as the rest of the solar system, began to form about 4.6 billion years ago from a gas and dust cloud called a nebula.Under the force of gravity, the nebula’s particles came together to form objects that grew larger and larger, eventually forming the planets. At first Earth’s surface was unstable, covered with seas of molten rock. As the surface cooled, the crust— Earth’s solid outer layer—began to form. No one knows when the first crust formed: though scientists have generally dated the oldest rocks to about 4 billion years ago, discoveries of some older minerals have led some to believe that stable crust existed hundreds of millions of years earlier. 

To help study Earth’s history, scientists have devised a scale of geologic time. The scale provides a framework for studying the evolution of the continents, oceans, atmosphere, and biosphere—the“zone of life” that consists of all living things. Earth’s surface consists of layers of rock formed from pebbles, sand, and mud deposited by water, wind, or glaciers. The oldest layers are lower down, and the more recent layers are on top. In studying these rocks, scientists found that each layer contained distinctive fossils and that the layers were similar all over the world. 

They then classified and dated each layer of rock and their fossils to create the geologic time scale. Geologic time is divided into eons, each of which is divided into smaller units of time. The oldest eon is the Archean, which lasted from about 4 billion years ago to 2.5 billion years ago. During this period the first living things appeared. Because the atmosphere contained little oxygen, however, life was limited to microorganisms that could survive without it. Oxygen levels increased significantly during the Proterozoic eon, which lasted from about 2.5 billion years ago to 542 million years ago. Bacteria, fungi, simple plants, and complex organisms, including the first animals, evolved. 

The Phanerozoic eon, stretching from about 542 million years ago to the present, saw a rapid expansion and evolution of life. It is divided into three major time periods largely on the basis of characteristic life-forms: the Paleozoic, Mesozoic, and Cenozoic eras. The Paleozoic began 542 million years ago with the Cambrian explosion, an extraordinary diversification of marine animals, and ended 251 million years ago with the extinction of most marine and land species. 

The Mesozoic era began 251 million years ago and ended about 66 million years ago. The ancestors of major plant and animal groups that exist today first appeared during the Mesozoic, but this era is best known as the time of the dinosaurs. The Cenozoic era, the Age of Mammals, began at the end of the Mesozoic and continues to the present. During the Phanerozoic eon, Earth gradually assumed its present configuration and physical features through such processes as plate tectonics, mountain building, and continental glaciation. 

Thus, though the Phanerozoic represents only about the last one-eighth of time since Earth’s crust formed, its importance far exceeds its relatively short span of time.

With such strong evidence of a constant fluctuation of life, land, and climate, we are apt to be in the midst of more change. We may not know exactly what changes will occur next, but we can learn from the history of our lively and varied planet.

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Investigating The History of The Earth