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 birthday? Time exists whether we notice or not, but chronology—the measuring of time—is a human invention. It allows us to record the past and plan the future.

Our measuring of time is not just a matter of marking off years, days, hours, minutes or seconds. Establishing a chronology is equally important. A chronology is a listing of events in the order in which they occurred, with each event assigned a rough or exact date.

History is usually told as a chronological story of past events. For that reason, historians depend on good and accurate chronologies. Of course, history is far more than a simple chronology. Historians need to interpret events and give them meaning. Nonetheless, they cannot do this convincingly without a reliable chronology to support their claims.

The First Chronometric Revolution—Writing

You might think humans had to learn to write before they could start keeping track of time. Actually, they began using time-tracking methods long before the written word was developed. Paleolithic and Neolithic peoples used oral storytelling to pass down information about when and in what order things had happened. This type of storytelling was used by many different cultures to tell the history of their people. Unfortunately, orally transmitted stories are not very reliable. They tend to change a bit every time we tell them. As a story is retold over generations, certain details become exaggerated, changed or invented. Based on such stories, it would be very hard to tell when, or in what order, historical events had actually happened.

The invention of writing changed this situation. It was the first chronometric—time-measuring—revolution. Once people began to record events using a written language, the information became fixed in the historical record. It then became possible to construct a fairly reliable chronology of history.

Although writing gives us a more detailed picture of the past, it is far from a complete picture. For one thing, it limits our focus. For many years, historians believed that “history” began when people started to write things down, while “pre-history” was everything that happened before the invention of writing. As a result, historians tended to focus on cultures with a developed system of writing. Communities that existed before the written record were often ignored. So were those that did not develop writing at all.

In addition, by focusing on the written word, historians limited their study of a particular community to the perspective, or point of view, of its literate people. In ancient times, those who could write had far more education and privilege than the average person. Usually, they made up only an extremely small portion of the population. Therefore, historians were only seeing how a very limited group of people viewed events. They were ignoring the opinions and experiences of the majority.

A focus on the written record also overlooks important developments in the natural world. It ignores both plant and animal evolution.

How can we truly understand the past if our source material has such a limited perspective? Modern historians have done their best to gain a fuller understanding of past societies and time periods. Several scientific discoveries have helped them greatly.

The Second Chronometric Revolution—Radioactivity

In the early 20th century, British scientist Ernest Rutherford discovered that atomic nuclei lose energy over time, by giving off radiation. This process is known as radioactive decay. Later, American chemist Willard Libby built on Rutherford’s discovery. Libby determined the rate of decay of certain radioactive isotopes such as carbon-14.

Isotopes are slightly different forms of an element. Regular carbon is carbon-12. It has 6 protons and 6 neutrons in its nucleus. It is stable and does not decay over time. Carbon-14 has 6 protons and 8 neutrons in its nucleus. Those extra 2 neutrons are why it is radioactive and unstable and decays.

Suddenly, there was a way to accurately date the remains of things that lived thousands of years ago. All living things contain carbon and when a plant or animal dies, the isotope carbon-14 begins to decay. By measuring the rate of decay, scientists can determine the age of the plant or animal. Libby published his findings in 1946. His discovery marked the beginning of the second chronometric revolution.

Of course, there are limits to carbon-14 dating. It is only accurate for organisms that are less than 60,000 years old. After 60,000 years, so much carbon 14 is gone that there is not enough left to measure. The good news is that there are other isotopes that can be traced 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 dendrochronology, or tree-ring dating, a living tree can be used to tell us a story of every season it has lived through. In the case of a 1,230 year-old pine tree in Italy, the oldest on record, that’s a lot of information.

Then there is genetic dating. This method allows scientists to trace the chronology of animal and human evolution through the study of genetic codes.

Finally, there is Accelerator Mass Spectrometry (AMS), which was invented in the late 1970s. This technology represents a major advance in radioactive isotope dating. It uses particle accelerators to more quickly and accurately measure decay.

Historians can now date events that happened long before writing developed. Before the 1960s, they could only go back about 5,000 years, to when writing first began. Without the 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?