Hasanlu was first excavated in 1935-36 for commercial purposes. In 1936, the famous explorer and archaeologist Sir Aurel Stein carried out a six-day excavation at the site, which he published in Old Routes of Western Iran (1940). In 1947 and 1949 commercial excavations were again carried out at the site. These early excavations drew attention to the site as a potentially rich source of information on Iron Age Iran — a particularly important period since scholars believe the transition from the Bronze to the Iron Age marks the point when Indo-Aryan populations, ancestors of the Medes, Persians and modern Iranians, first entered the region. It was not until 1956 that long-term, scientific excavations were started at the site by Robert H. Dyson, Jr., a young Near Eastern archaeologist at the University of Pennsylvania Museum. The Hasanlu Project would go on to complete 14 seasons of excavation at Hasanlu Tepe and carry out work at several surrounding sites, the most important being Hajji Firuz Tepe, Dalma Tepe, Dinkha Tepe, Pisdeli Tepe, and Agrab Tepe. These excavations revealed a long occupation sequence spanning the early pottery Neolithic, the time of the first farming villages in the region, to the Medieval period. The most important contribution was the light shed on cultural developments during the early Iron Age (1400-800 B.C. or Hasanlu Period V and IV). At this time, a new archaeological culture appears in northwestern Iran. At Hasanlu, entirely new types of burial customs, ceramics, and architecture were introduced. These developments set the stage for the cultures of the later Iron Age such as the Medes and Achaemenid Persians.