Intensive horticulture has been practiced in the area for some 9,000 years, starting in fertile drained swamps and moving later to hillsides when sweet potatoes became available to replace taro as the staple crop, an event estimated to have occurred within the last few hundred years. Trading networks brought shell valuables, plumes for decoration, salt, and stone axe blades from distant parts. Europeans discovered the area in 1933 as part of an exploratory drive in search of gold in the highlands creeks. The brothers Michael, James, and Danny Leahy and the Australian Patrol Officer James L. Taylor were prominent in the process of discovery and initial pacification. Mount Hagen was established as a center for mission activities, trade, and administration. Until the 1950s, major contact with the outside world was by air. Nowadays the Highlands Highway to the coastal port of Lae on the north coast of Papua New Guinea is the chief channel for goods to enter and leave. Until 1975, Papua New Guinea was under Australian colonial control, and Western Highlands was a district. At independence, the districts became Provinces and from the late 1970s these gained their own provin cial assemblies and governments in addition to the National Parliament.