Читать книгу Geology For Dummies - Alecia M. Spooner - Страница 24

The most fascinating story told in the rock layers is the story of Earth’s evolution. To evolve simply means to change over time. And indeed, the earth has evolved in the 4.5 billion years since it formed.

Both the earth itself and the organisms that live on Earth have changed through time. In Chapter 17, I briefly explain the biological understanding of evolution. Much of modern understanding about how species have changed through time is built on evidence from fossilized or preserved life forms in the rock layers. Fossilization occurs through different geologic and chemical processes, but all fossils can be described as one of two forms:

 Body fossils: Remains of an organism itself, or an imprint, cast, or impression of the organism’s body.

 Trace fossils: Remains of an organism’s activity, such as movement (a footprint) or lifestyle (a burrow) but without any indication of the organism’s actual body.

Earth did not always support life. In Chapter 18, I describe the very early Earth as a lifeless, hot, atmosphere-free planet in the early years of the solar system’s formation. It took billions of years before simple, single-celled organisms appeared, and their origins are still a scientific mystery.

Simple, single-celled life ruled Earth for many millions of years before more complex organisms evolved. Even then, millions of years passed with soft-bodied life forms that are difficult to find in the fossil record. It wasn’t until 520 million years ago that the Cambrian explosion occurred. Chapter 19 describes this sudden appearance of shell-building, complex life as well as the millions of years that followed when life was lived almost entirely in the oceans until amphibians emerged on the land.

Chapter 20 delves into the Age of Reptiles, when dinosaurs ruled the earth and reptiles filled the skies and seas. During this period, all the earth’s continents were connected as Pangaea, Earth’s most recent supercontinent. But before the Age of Reptiles ended, Pangaea broke apart into the separate continents you recognize today. Evidence for Pangaea is still visible in the coastal outlines of South America and Africa — indicating where they used to be attached as part of the supercontinent.

In relatively recent time, geologically speaking, mammals took over from reptiles to rule the earth. The Cenozoic era (beginning 65.5 million years ago), which we are still experiencing, is the most recent and therefore most detailed portion of Earth’s history that can be studied in the geologic record (the rocks). Many of the most dramatic geologic features of the modern Earth, such as the Grand Canyon and the Himalayan Mountains, were formed in this most recent era. In Chapter 21, I describe the evolution of mammal species (including humans) and the geologic changes that occurred to bring us to today.

At various times in the history of Earth, many different species have disappeared in what scientists call mass extinction events. In Chapter 22, I describe the five most dramatic extinction events in Earth’s history. I also explain a few of the common hypotheses for mass extinctions, including climate change and asteroid impacts. Finally, I explain how the earth may be experiencing a modern-day mass extinction due to human activity.