Introduction

Can you imagine what life would be like if we couldn’t tell time? How would we mark the days or years? How would we know when to be at work or what day to celebrate our birthdays? Time exists whether we notice or not, but chronology—the measuring of time—is an essential construct in our lives that allows us to record the past. History is usually told as a chronological story of past events. Accurate chronology is required for historical thinking. In addition to being able to tell a full and complete history of past events, the knowledge of time also allows historians to make sense of these events. As historians interpret events, they can develop meaningful narratives and arguments about the past.

While chronology helps historians put events in sequence, simply getting the dates and times right is not history. As historian Bob Bain explains, “History depends upon interpretation and meaning-making, but correct chronology is essential to enable historians to draw strong conclusions, make reasonable historical interpretations, and develop compelling arguments. Chronology is not history, but history depends upon good and accurate chronology.”

The First Chronometric Revolution—Writing

You might think humans had to learn to write before they could start keeping track of time. Actually, they had already been using time-tracking methods long before the written word. Paleolithic and Neolithic peoples passed down information orally through storytelling. This type of oral storytelling became tradition and was used by many different cultures to tell the history of their people. Unfortunately, stories come with creative license and tend to change a bit every time we tell them. That’s great for keeping it interesting but may not be so great for passing down accurate, reliable information. As a story is retold over generations, certain details like that week of rain might turn into a yearlong hurricane. Once people began to record events using a written language, this information became etched in the historical record.

Although writing gave us a more detailed picture of the past, it is far from a complete picture. Many historians of the past liked to say that “history” is anything that got written down, and “pre-history” is everything that happened before the invention of writing. Experts in this field tended to focus on eras and locations that had a developed system of writing in place. That works well if you only want to learn about communities that had developed writing.

However, it limits your area of study to certain periods of time and geographic locations. Communities that existed before the written record, or those that did not develop writing at all, were often ignored.

Also, by only using the writing of a particular community to find out what it was like, you limit your study to the perspective of literate people. Those who could write in ancient agrarian states were usually those with the most education and privilege. This population of literate people, especially in ancient times, was extremely small and in many societies, it was a privileged position. Therefore, historians were only seeing how a very limited group of people viewed or recorded events. It would be as if, one million years from now, historians look back on the twenty-first century by only looking at the perspective of school teachers. Yes, teaching is a big profession and that includes about two percent of the total population (at least in the U.S.). But this kind of study would leave out the other 98 percent of people. Another problem is that writing is only a human activity. When you rely purely on written records you leave out aspects of the natural world. Plants and animals are then excluded from the historical record. How can we truly understand the past when our source material only covers a fraction of what was taking place over centuries of time?

The Second Chronometric Revolution—Radioactivity

In the early twentieth century, New Zealand-born British scientist Ernest Rutherford discovered radioactive decay, which would help to bring about a revolution in terms of dating artifacts. Later, an American chemist named Willard Libby was able to build upon the research of Rutherford and many other scientists including Marie Curie. He determined the rate of decay of certain isotopes like carbon-14. Suddenly there was a way to measure and date the remains of things that lived thousands of years ago. Libby published his findings in 1946, which marks the beginning of the second chronometric revolution. Libby won the Nobel Prize in Chemistry in 1960 “for his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science” (The Nobel Foundation, 2018).

Organic material contains carbon and when an organism dies, the isotope carbon-14 begins to decay. Scientists can measure the rate of decay of carbon-14 to determine the age of the organic material. Of course, there are limits to carbon dating, since carbon-14 is only accurate on organisms that are less than 60,000 years old. The good news is that there are other isotopes with longer half-lives that can go much further back. For example potassium-40 can go to 1.3 billion years, uranium-238 to 4.5 billion years, and rubidium-87 to 49 billion years.

Scientists have also found a variety of other ways to measure time. For example, with tree-ring dating (dendrochronology) a living tree can be used to tell us a story of every season it has lived through. In the case of the 1,230-year-old pine tree in Italy, the oldest on record, that’s a lot of information. Then there is genetic dating, a method that can determine the chronology of evolutionary biology. Genetic dating allows scientists to trace animal and human origins through their genetic codes. Finally, there is the powerful invention of Accelerator Mass Spectrometry (AMS) from the late 1970s. This major advance in radioactive isotope dating uses particle accelerators to more quickly and accurately measure decay and date organic materials.

Experts in many disciplines can now date events that happened long before writing began. Before the 1960s, we could only go back about 5,000 years ago, when writing first began. Today we know that the Earth is 4.5 billion years old and the universe is over 13.8 billion years old. Without these advances of the second chronometric revolution, we would not have the Big History narrative you learned about in Unit 1. We now know more about the history of humans, the Earth, and the universe than we ever did before, due to these revolutions in the measurement of time. Who knows what new discoveries and inventions will allow us to learn even more about our history.