into different spans of time (such as periods, epochs, and eons).
Witnessing evolution in the fossil record
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.
Chapter 2
Observing Earth through a Scientific Lens
IN THIS CHAPTER
Geology is one of many sciences that study the natural world. Before moving on to the details of geologic science, I want to spend a little time sorting out what exactly science is and does. In this chapter, I describe the elements of science and the scientific method, and I explain how you do science every day perhaps without even realizing it!
Realizing That Science Is Not Just for Scientists
Science is not a secret society for people who like to wear lab coats and spend hours looking into microscopes. Science is simply the asking and answering of questions. Any time you make a decision by considering what you know, collecting new information, forming an educated guess, and figuring out whether your guess is right, you participate in acts of science.
Take a very simple example: choosing a shampoo. You’ve probably tried different types or brands of shampoo, observed how each one leaves your hair looking and feeling, and then decided which shampoo you wanted to purchase the next time. This process of observation, testing, and decision-making is all part of the scientific approach to problem-solving. You follow this process every day in multiple situations as you make decisions about what to buy, what route to take in your car, what to eat for dinner, and so on.
Using a Methodical Approach: The Scientific Method
Scientists seek to answer questions using a sequence of steps commonly called the scientific method. The scientific method is simply a procedure for organizing observations, making educated guesses, and collecting new information. The scientific method can be summarized as the following steps:
1 Ask a question. Scientists begin by asking, “Why does that happen?” or “How does that work?” Any question can be the start of your scientific journey. For example, “Why are my socks, which used to be white, now colored pink?”
2 Form a hypothesis that answers your question. A hypothesis is a proposed answer to your question: an educated guess based on what you already know. In science, a hypothesis must be testable, meaning that you (or someone else) must be able to determine if the hypothesis is true or false through an experiment. For example, “I think my socks turned pink because I washed them with pink laundry soap.”
3 State a prediction based on your hypothesis that can be tested. Using the proposed explanation in your hypothesis, form a prediction that you can test. For example, “I predict that if I wash a white T-shirt with pink laundry soap, it will turn from white to pink.”
4 Design an experiment to test your prediction. A good experiment is designed to best answer your question (see the upcoming “Testing your hypothesis: Experiments”