In 1816 he was appointed head of 'antiquarian' collections which later developed into the National Museum of Denmark in Copenhagen. While organizing and classifying the antiquities for exhibition, he decided to present them chronologically according to the three-age system. Other scholars had previously proposed that prehistory had advanced from an age of stone tools, to ages of tools made from bronze and iron, but these proposals were presented as systems of evolution, which did not allow dating of artifacts. Thomsen refined the three-age system as a chronological system by seeing which artifacts occurred with which other artifacts in closed finds. In this way, he was the first to establish an evidence-based division of prehistory into discrete periods. This achievement led to his being credited as the originator of the three-age system of European antiquity.