Imagine a long time ago in your family Do you remember old photographs of your great grandparents Perhaps you have seen an ancestor's portrait that was painted even before photography was invented You may have studied a family tree that gives a few details about your relatives who lived a century or more ago It is probably hard to picture their day to day lives so long ago For humans both now and in the past just a thousand years seems like a vast stretch of time Most people who lived in the 1700s and early 1800s believed that Earth had existed for just a few thousand years Also they believed that the plants and animals they knew had always been the same A few people disagreed They found evidence that suggested that Earth might be many millions of years old They also learned that there had been great changes in Earth's life The evidence was in plain sight in the landscape and the rocks of Earth's surface Old family photographs remind us of past generations It is hard for humans to imagine life stretching back for many thousands or even millions of years 1 Imagine 11
2 Evidence from the Earth 12 One of the first of all sciences was geology the study of the history and structure of Earth Some early geologists had some scientific training but most were just curious amateurs They liked to explore and observe different land forms and rocks on our planet's surface They could see that flowing water wore away or eroded soils and even rocks Clearly a long time would be needed for water to erode hard rocks Geologists saw that some rivers had cut through layer after rocky layer This erosion would take huge amounts of time much more than a few thousand years Geologists also found different kinds of rocks sandstone for example that had once been sand but gradually became solid rock They discovered rocks that had once been liquid flowing lava from volcanoes And they saw layers of rocks that were tilted and twisted by powerful forces in Earth's crust Wherever they looked geologists found evidence that Earth was not a few thousand years old Instead its surface had been changing over a mind boggling span of time perhaps even billions of years More evidence was found in plain sight Lying on beaches or sticking out of rocky cliffs or unearthed in farmers fields were shells teeth bones made of stone They were fossils These objects fascinated and puzzled people What could they be For example what kind of animal left the big three toed footprints in rocks that a Massachusetts farm boy discovered in 1802 People thought that the tracks had been left by ancient birds
that they called Noah's ravens Eventually people learned that the footprints had been made millions of years ago by dinosaurs On Ocean beaches and sometimes far inland people found stony sharp edged triangular objects What were they One answer had been offered more than two thousand years ago by Pliny the Elder a Roman scholar who studied nature He wrote that they were stone tongues that fell to Earth from the moon In 1667 a scientist named Nicholas Steno had a much better idea He examined the mouth of a The Colorado River shifted its course numerous times over a span of several million years as it eroded the wide and deep Grand Canyon 13
