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
CHAPTERS EARTH's ORIGINS he origin of Earth has been a subject of great interest among scientists for a long time By the late 1800s geolo gists and biologists had collected evidence suggesting that Earth is at least hundreds of millions of years old much older than they had thought Geologists reasoned that vast stretches of time were needed for the slow work of erosion and sedimentation to have had the effects they apparently had Biologists felt that a long time was also needed for evo lution to have resulted in the great diversity of life seen today There were dissenting opinions though Physicists such as Lord Kelvin calculated that Earth apparently still molten inside should have cooled to a solid throughout if it were more than about 40 million years old Similarly astronomers figured that the Sun could not be more than a few tens of mil lions of years old They believed that the Sun shone because of energy from gravitational contraction so it could not have sustained its energy output for so long without a drastic reduction in size 10
EARTH's ORIGINS The discovery of radioactivity by Henri Becquerel in 1896 led to a solution to both problems Earth could be heated internally by radioactive decay of elements such as ura nium deep inside and this could last billions of years The Sun it turned out shines by nuclear fusion giving it a potential lifetime of at least 10 billion years Radioactive dating of rocks played a cru cial role in calculating Earth's age Some isotopes forms of an element with different atomic masses of some elements such as uranium decay by emitting radiation in the form of smaller particles such as electrons and alpha particles This leaves behind a suc cession of daughter products such as lead The rates of decay have been measured with great precision in laboratories By measur ing the ratios of the daughter products to the original radioactive elements and assum ing that the materials were trapped in a rock from the time it solidified the age of the rock can be calculated Rocks from the seafloor are rarely found to be older than 200 million years but those from the continents are occasionally found to date back more than 3 billion years In 2001 Australian researchers announced the dis covery of a small grain of zircon a mineral II
