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 school? How would we know what day to celebrate our birthday, or even how old we were? Time exists whether we notice or not, but chronology—the measuring of time—is a human invention. It allows us to plan the future and record the past.

Measuring time is not just a matter of marking off particular 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 happened. Each event in a chronology is given a rough or exact date.

History is usually told as a chronological story of past events. For that reason, historians depend on 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 without a reliable chronology.

The First Chronometric Revolution—Writing

The earliest humans 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, such stories are not very reliable. They tend to change a bit every time we tell them. As a story is retold over generations, many details are changed or dropped. 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, or 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 past history.

Written records give us a picture of the past. However, it is far from the complete picture. For one thing, written records limit our focus. For many years, historians believed that “history” only began when people started to write about things. As a result, historians focused 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.

Written records have other important limitations. They made historians limit their focus to the perspective, or point of view, of a society’s literate people. In ancient times, only a very small percent of the population could read and write. Generally, they were the richest and most powerful members of society. Therefore, historians were only seeing how a very small and privileged group of people viewed events. They were ignoring the opinions and experiences of the majority.

The written record also largely overlooks important developments in the natural world. It ignores 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 time periods. Several scientific discoveries have helped them greatly.

The Second Chronometric Revolution—Radioactivity

In the early twentieth 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 scientist Willard Libby built on Rutherford’s discovery. Libby determined the rate of decay of certain radioactive isotopes. The most important was 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.

Scientists have also found other ways to measure time. For example, with tree-ring dating, a living tree can be used to tell us the story of every season through which it has lived. Take the case of a 1,230-year-old pine tree in Italy, the oldest on record. Its tree rings provide much information about changing weather patterns.

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. We now know more about the history of humans, the Earth and the universe than we ever did before. Much of this new knowledge is due to revolutions in the measurement of time. Who knows what new discoveries and inventions will allow us to learn even more about our history?