INTRODUCTION 6 very 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 exam ines 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 sts 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 scien tists have generally dated the oldest rocks to about 4 billion years ago discoveries of some older miner als 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 con tinents oceans atmosphere and biosphere the
INTRODUCTION Earth's origins have occupied the minds of many scientists through the ages NASA
INVESTIGATING THE HISTORY OF EARTH 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 con tained 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 con 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 signifi cantly 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 pres ent saw a rapid expansion and evolution of life It is divided into three major time
INTRODUCTION periods largely on the basis of characteris tic life forms the Paleozoic Mesozoic and Cenozoic eras The Paleozoic began 542 mil lion 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 spe cies 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 dur ing 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 gradu ally assumedits present configuration and phys ical features through such processes as plate tectonics mountain building and continental glaciation Thus though the Phanerozoic rep resents 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
