To make sense out of the past, to see potential developmental patterns, there needs to be some form of chronology or time line.Ideally, we need to know when things happened—how far in the past—and how long certain events lasted. One of the common questions that archaeologists ask, and are asked by an inquiring public, is: How old is it? This is one of the key questions in archaeology.
In their textbook Archaeology: The Science of the Human Past, Mark Sutton and Robert Yohe write:
“To be able to document change through time, archaeologists must be able to determine how old things are. The ability to date things allows scientists to build chronologies and to track changes in material remains and behavior throughout history and prehistory.”
In an essay in The Oxford Handbook of Archaeology, A.M. Pollard writes:
“History, and to a great degree archaeology, with its much greater time depth and paucity of written evidence, would be a jumble of unrelated and therefore meaningless observations if we were not able to place particular events into a measured chronological sequence. Establishing cause and effect, difficult enough with such a timeline, would be impossible.”
One of the basic and earliest forms of chronology is based on relative age, that is, knowing that an event occurred before or after some other event. Mark Sutton and Robert Yohe write:
“Much of the dating done in archaeology is relative dating, determining whether something is older or younger than something else.”
For many European Christians the ancient past was characterized by a single event: the large, supposedly world-wide, flood described in the Bible. This concept was based on the belief that the Bible, a collection of oral traditions which had been collected and written down, was a factual history rather than a metaphorical description of the past. In this Biblical relative chronology, evidence of ancient life forms, such as fossils, were described as belonging to the pre-flood era. With a greater understanding of the world from the emerging sciences, particularly from geology, in the eighteenth and nineteenth centuries, the notion of a world-wide flood was shown to be imaginary and therefore of no importance to our understanding of the past.
By the early nineteenth century, museum collections of antiquities were growing and there was a need to bring some kind of order to the chaos of the evidence of the early past. One solution was proposed by Christian Jürgensen Thomsen (1788-1865), curator of the National Museum of Denmark. In his book Exploring Prehistoric Europe, archaeologist Chris Scarre writes:
“Thomsen reorganized the displays in his museum in a sequence depending on whether they had cutting tools of stone, bronze, or iron, and in his Guide to Northern Archaeology he published an account of this chronological sequence that had wide influence and was rapidly translated into German, English, and other European languages.”
While Thomsen popularized the concept of the Three Age System, it should be noted that this idea was first suggested by Nicholas Mahudel (1704-1747) in his 1734 book Three Successive Ages of Stone, Bronze, and Iron.
By the 1860s, the Three Age System (i.e. Stone Age, Bronze Age, Iron Age) was widely used as the chronology for European prehistory. In his chapter in Archaeology: The Key Concepts, Paul Bahn writes:
“Thomsen’s version of the Three Age System became, and remains, the very basis of European prehistoric chronology. It made possible for the first time to bring order where there had previously been chaos—to place objects into a sequence, and to group them according to the period to which they belonged.”
The Three Age System is a relative chronology based on technology. While Thomsen originally regarded it as speculation or hypothesis, later archaeological excavations confirmed this relative dating. In his entry on the Three Age System in the Oxford Companion to Archaeology, Brian Fagan writes:
“Thomsen’s version of the Three Age System became, and remains, the very basis of European prehistoric chronology. It made possible for the first time to bring order where there had previously been chaos—to place objects into a sequence, and to group them according to the period to which they belonged.”“As more sophisticated theoretical formulations came into being, chronologies became more refined and archaeologists uncovered evidence of great biological and cultural diversity in the past, the Three Ages assumed lesser importance. Today, they are used as the broadest of technological categories, more labels of convenience than precision.”
While the Three Age (and later Four Age, with the addition of the Copper Age or Chalcolithic) System does an adequate job of describing the past in Europe, it does not work for other regions, such as Africa and the Americas.
In the field, archaeologists use a type of relative dating known as stratigraphy in developing an initial understanding of the chronology of a specific site. In her chapter in Archaeology: The Key Concepts, Julie Stein explains:
“Stratigraphic succession is the foundation of relative dating in archaeology and is based on the principle that underlying objects or deposits must be older than those which cover them. The relative age determination technique, using the law of superimposition and context, is at the heart of every single archaeological excavation, and almost every other dating technique.”
Julie Stein also writes:
“Stratigraphic succession has been the backbone of dating in archaeology since the foundation of the discipline.”
During the nineteenth century, museum curators and archaeologists, inspired by the use of typology in biology, came to develop typologies of artifacts. From this they then developed a dating method known as seriation. A.M. Pollard explains:
“This is an extension of the everyday observation that artefacts, be they tools, clothing styles, or buildings, change in an observable way over time in a defined geographical (or cultural) region, to the extent that a date can often be guessed from a knowledge of this evolution.”
It is important to keep in mind that the use of seriation is restricted to defined cultural or geographic regions. Stone tools in North America, for example, cannot be dated by comparing them with European stones tools.
In the field, seriation, like stratigraphy, is often used in making an initial estimate regarding the age of a site. In their textbook Archaeology, David Hurst Thomas and Robert Kelly write:
“Seriation was a common technique in the mid-twentieth century, but today it is used mostly where absolute dating methods cannot be employed or are not sufficiently specific.”
Like stratigraphy, seriation is a form of relative dating, but in some areas, such as Egypt, Mesoamerica, and China, where there is a known chronology of events, seriation can provide accurate dating to within 20 years or so under certain circumstances.
As we go back farther in time, back to the Paleolithic (Old Stone Age), seriation becomes less viable as a dating technique as the rate of stylistic change is very slow. The Acheulian handaxe, associated with Homo erectus, is relatively unchanged for many centuries.
In their book A Thesaurus of British Archaeology, Lesley Adkins and Roy Adkins provide a caution regarding the use of seriation:
“Its use as a dating method is based on the assumption that the variations in the object only occur with time and not as a result of other factors, but as this is not always so it has to be used with great caution.”
During the last half of the twentieth century, a number of more precise chronometric forms of data were developed and provided archaeologists and paleoanthropologists more accurate insights into the time lines of the past. One of these is radiocarbon dating which is based on the rate of decay of carbon-14. On July 12, 1948, radiocarbon dating was first used on an Egyptian sample and this is regarded as the birthdate of this dating method. The birth of radiocarbon dating is sometimes called the atomic bomb of archaeological dating. In his book Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, David Reich writes:
“Radiocarbon dating transformed archaeology, making it possible to determine the true age of materials, going beyond what was possible by studying the layering of remains.”
Radiocarbon dating revolutionized archaeology and enabled archaeologists to develop chronologies showing how old sites were and how long they had been in use. With regard to the advantages of radiocarbon dating, Colin Renfrew, in his book Prehistory: The Making of the Modern Mind, writes:
“It was, moreover, a chronology free of any assumptions about cultural developments or relationships, and it could be applied as well to nonliterate societies as to those with written records.”
Colin Renfrew also writes:
“The beauty of the new method was that it could work without any archaeological assumptions about date or time. The sample for dating has to be organic, that is to say of plant or animal material, and found in a stratigraphic context defining the time period to be dated.”
There are a couple of limitations in using radiocarbon dating. First, it simply dates how long ago a carbon-based life form died. This means that it cannot directly date artifacts such as stone tools. In other words, radiocarbon dating is used to date the stratigraphy in which stone tools are found. When stone tools are carelessly removed from their stratigraphic context by looters and souvenir hunters, they cannot be dated and their value to an understanding of the past is destroyed.
While radiocarbon is important in dating many sites, it cannot date materials much older than 50-60,000 years. This means, that radiocarbon dating cannot be used to date materials associated with early humans, such as Homo habilis and Homo erectus. To date materials associated with early humans, paleoanthropologists turn to other dating methods, such as paleomagnetic, potassium-argon, uranium series dating, rubidium-strontium, fission track, and others. In his book The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design, evolutionary biologist Richard Dawkins writes:
“More recently, advances in physics have given us methods to put absolute dates, in millions of years, on rocks and the fossils that they contain. These methods depend upon the fact that particular radioactive elements decay at precisely known rates. It is as though precision-made miniature stopwatches had been conveniently buried in the rocks.”
Today it is unusual for an archaeological site to be dated with a single method. In his book Unlocking the Past: How Archaeologists Are Rewriting Human History with Ancient DNA, Martin Jones writes:
“Archaeologists and earth scientists looking back into the past are accustomed to working with a variety of natural clocks. The basic requirement of such clocks are that they proceed at a knowable pace, as do very many natural processes, and that their pace does not get deflected by what is happening around them. The most important of these are the so-called radiometric clocks.”
A.M. Pollard put it this way:
“The common view now is that a single date for an object or event must always be regarded with suspicion, and that the best dating protocol usually involves dating stratified sequences or objects with defined stratigraphic relationships. Moreover, it is now apparent that the best dates (in terms of accuracy and precision) are often obtained by combining as much information as possible (but particularly stratigraphic relationships) into the analysis, using mathematical methodology published posthumously by the eighteenth-century Nonconformist minister Thomas Bayes (1764).”
In his chapter in Archaeology: The Key Concepts, Paul Pettitt writes:
“Today, the numerous and highly technical dating methods available to archaeologists tell us in a very precise way two pieces of information that are critical to archaeology: how old monuments or artefacts are, and how long in duration major periods or processes in human history have been.”
In trying to understand the past, and particularly the far distant past of the first human ancestors, archaeologists and paleoanthropologists have only bits and pieces of evidence which can be used for seeing the patterns of human evolution and the development of culture. In putting the pieces together so that the patterns can be seen, chronology is vital and this chronology is based on scientific methods from many different fields.