The three-age system is a system of classifying human prehistory into three consecutive time periods, named for their respective predominant tool-making technologies:
It was introduced by the Dane Christian Jürgensen Thomsen in the 1820s in order to classify artifacts in the collection which later became The National Museum of Denmark.
Thomsen argued that nobody would have used stone tools if bronze ones had been available and that similarly, no one would have wanted to use bronze tools if there had been iron ones around instead. Reasoning that the advances must therefore have come in chronological sequence, he was the first to suggest a workable system of dating artefacts and sites. Such a system was revolutionary and a vast improvement on the disorganised nature of previous prehistoric archaeology.
Later, the Stone Age was divided into the Palaeolithic, the Mesolithic and the Neolithic and further subdivisions were introduced to divide all the ages into early, mid or late (or lower, middle and upper in the case of the Palaeolithic) sections. In some cultures, archaeological evidence has made it necessary to add a Chalcolithic or Copper Age period between the Neolithic and the Bronze Age. The term Megalithic does not refer to a period of time and merely describes the use of large stones by ancient peoples from any period.
Advances made in the fields of seriation, typology, stratification and the associative dating of artefacts and features permitted even greater refinement of the system. However it still remained that no precise numerical date could be given to finds using the three age system, they could only be placed in a relative sequence. Elaborate efforts were often made to align European and Near Eastern sequences with the datable chronology of Ancient Egypt; more direct and convincing scientific dating methods such as carbon dating were not invented until the mid twentieth century.
The three age system has been difficult to apply fully outside Europe. Cultures developed at different rates or missed out some of the stages of development altogether. Amazonian tribes in South America remain in the Neolithic for example, while there was no Bronze Age south of the Sahara and people living there went straight from using stone to iron.
It also soon became apparent that the switches from one age to another did not happen quickly or decisively. Flint tools remained in use in a limited fashion into the Iron Age in Europe and early metal items often appear in what should technically be the Neolithic.
Although the three age system has been rendered less and less accurate by archaeological discoveries, it still remains the bedrock of prehistoric archaeology as the terms have become ingrained in people's minds, including those of archaeologists. Their clarity and explicability means that the field and the often incomprehensibly long periods of time involved in prehistoric archaeology can also be more easily conveyed to the public.