The introduction to
The Age of Everything
How Science Explores the Past
From our twenty-first-century perspective, events from the past can often seem impossibly remote. With today’s complex technology and constantly shifting political and economic networks, it is sometimes hard to imagine what life was like even a hundred years ago, much less comprehend the vast stretches of time preceding the appearance of humans on this planet. However, thanks to recent advances in the fields of history, archaeology, biology, chemistry, geology, physics, and astronomy, in some ways even the far distant past has never been closer to us. The elegantly carved symbols found deep in the rain forests of Central America, uninterpretable for centuries, now reveal the political machinations of Mayan lords. Fresh interdisciplinary studies of the Great Pyramids of Egypt are providing fascinating insights into exactly when and how these incredible structures were built. Meanwhile, the remains of humble trees are illuminating how the surface of the sun has changed over the past ten millennia. Other ancient bits of wood are helping us better understand the lives of first inhabitants of the New World. Fossil remains, together with tissue samples from modern animals (including people) suggest that anthropologists may be close to solving the long-standing puzzle of when and how our ancestors started walking on two legs. Similar work might also help biologists uncover how a group of small, shrew-like creatures that lived in the shadow of the dinosaurs gave rise to creatures as diverse as cats, rabbits, bats, horses and whales. The origins of the earth and the solar system are being explored in great detail thanks in part to the rocks that fall from the sky, while the history of the universe can be read in the light from distant stars. The cosmic static that appears on our television sets even allows cosmologists to look back to the very beginning of our universe.
To accomplish all this (and much more besides), scholars and scientists have had to develop a variety of clever ways to figure out when things happened. Without this information, the relics from bygone eras—from impressive stone monuments to humble sticks to feeble starlight—can provide only scattered and almost incomprehensible glimpses of the past. However, once these clues can be arranged and organized in time, the picture becomes much clearer. It becomes possible to evaluate the causes, consequences, nature, and importance of ancient events, and what was once merely an array of random facts takes shape and forms a coherent story.
This book explores how researchers in a wide variety of fields determine the age of things. It grew out of a series of lectures I gave in the spring of 2004 while I was a researcher in the Kavli Institute for Cosmological Physics at the University of Chicago. The talks were part of the Compton Lecture program, which is dedicated to providing the public with information about recent discoveries in the physical sciences. Since at the time I was working as a radio astronomer and cosmologist, it would have been natural for me to discuss the many exciting advances that had taken place in those fields. However, several experts had already given very good lectures on these subjects, and I was encouraged to pursue a different path. I have always been passionately interested in a broad range of academic disciplines—including ancient history, archaeology, evolutionary biology, paleontology, and planetary sciences—and this gave me an opportunity to offer a multidisciplinary series of talks, each one focusing on a different method of dating ancient objects and events, and how it was being used to revise and reshape our view of the past.
Like the original lectures, this book is not intended to provide an exhaustive catalog of every single dating technique. Nor does it present some sort of comprehensive survey of the history of humanity, the earth, and the universe. Instead, it will focus on a few specific points in time and a sample of methods of measuring age. I hope this approach will allow the reader to gain a deeper understanding of the techniques used in many different fields and to appreciate the special challenges involved in doing research on subjects ranging from the origin of the universe to the politics of the Maya lowlands. In addition, the topics included in this book are all very active areas of study. The following chapters should therefore also provide both background and insight into some of the interesting historical, archaeological, biological, and astronomical discoveries being made today.
However, because the topics covered in this book are still the subjects of active research, it is quite likely that additional discoveries will have come to light by the time you are reading this. Furthermore, several of the topics considered here—such as the colonization of the New World and the use of genetic data to measure time—are still very contentious at the moment. For this reason, I have included lists of articles, books, and websites at the end of each chapter. These should enable curious readers to seek out additional information and perspectives on the issues of interest to them.
I also encourage any interested reader to delve into these references for another reason: I am by no means an expert in all of the subjects covered here. I am well enough versed in topics like ancient history and cosmochemistry to follow the published literature and appreciate technical lectures, but my training is primarily in radio astronomy and observational cosmology. Although I have undergraduate degrees in both physics and anthropology, and even though my current job involves processing data from the Cassini spacecraft in orbit around Saturn, I do not have extensive professional experience in ancient history, archaeology, evolutionary biology, planetary science, and optical astronomy.
I am also well aware of the trouble that can occur when a scientist—particularly a physicist—begins to write about subjects outside their field of expertise. Too often, that scientist seems to be under the mistaken impression that their own training gives them a privileged perspective on a topic others have been studying for decades. I don’t want to fall into that trap here, as I have the deepest respect for those who have spent their careers working on these areas. I will therefore tread carefully on other researchers’ territories, and point interested readers to those sources that will allow them to further explore any of the subjects covered in this book.
This book begins with events in human history and from there we will move further back in the past all the way back to the Big Bang. Along the way we will cover a broad range of timescales, from mere centuries to billions of years. To help make sense of all this, I have provided the series of time lines in Figure 1.1 to serve as an overview of the events we will consider here.
The time line at the far left of the figure represents the last 100 years, a time span that most of us can readily comprehend and interpret. Marked on the time line are significant events like the two world wars and the moon landings. World War II and the Apollo missions, along with countless moments between and since, are still in people’s living memories, but times before this are slowly becoming the domain of history.
Each of the subsequent time lines incorporates fifty times as many years as the time line before it. The second time line thus represents 5,000 years, which includes most of recorded human history. The twentieth century occupies only a tiny fraction of this time. Even the signing of the Declaration of Independence in 1776 and Columbus’ expedition in 1492 are comparatively recent occurrences on this timescale. Here we come to the first two topics covered in this book: the politics of the Classic Maya civilization of Central America (about 1,500 years ago); and the construction of the Great Pyramids of ancient Egypt (4,500 years ago). Historical records play a crucial role in our understanding of both of these subjects.
Prior to about 5,000 years ago, however, there were no historical records. Scientists therefore must find other means to measure ages. This prehistoric era is covered in the next time line, which represents 250,000 years. Anatomically modern humans—creatures physically indistinguishable from people living today—first appeared about 200,000 years ago, near the top of this time span. This era includes the last great Ice Age and the dispersal of human beings from their earliest home in Africa throughout the rest of the world. For the bottom part of this time span, carbon-14 dating is a key method of measuring ages. A series of three chapters describe this famous dating technique and how it is being used to study such far-flung topics as the physics of the sun and the arrival of people in the New World.
Well before modern Homo sapiens made their appearance, there were creatures we would recognize as human-like: they walked on two legs like we do and some even fashioned stone tools. The origins of these traits are included in next time line, which covers a span of 12.5 million years. A combination of fossil evidence and DNA data indicate our ancestors first began to walk upright about 6 million years ago. This pivotal time in our evolution is the subject of chapter 7.
The next time line stretches over 625 million years, encompassing the entire age of dinosaurs and even the origin of multicellular animal life. During this lengthy period, many species arose and went extinct, and the characteristics of life on earth changed in a variety of ways. For example, around the end of the age of dinosaurs, a group of shrew-like animals became the diverse array of creatures we now call mammals. As we will see in chapter 8, analyses of the DNA of living animals may be able to shed new light on this remarkable transformation.
Note that the final time line is shorter than the rest. Were it extended to the same length as the others, it would represent 31.25 billion years. Our universe is not that old, so this bar has been shrunk down to begin at the Big Bang, which occurred less than fifteen billion years ago. Well after this point on the time line, we can see the formation of the earth and the solar system, which is the subject of chapter 9. Voyaging even deeper into the past, the last three chapters will discuss the age of the oldest known stars and even the birth of the universe itself.
In addition to this visual depiction of the history of the universe, here are some useful rules of thumb to help keep these various timescales straight: