It is unknown whether the Ambrones, who together with the Cimbri and Teutones threatened Rome around 100 BC, stemmed from this island which back then was still connected to the mainland by a land bridge. In the early Middle Ages the island was colonised by the Frisians. The oldest known record of Amrum island has been found in the "Danish Census Book" of King Valdemar II of Denmark from 1231. Next to salt making, agriculture, fishery and whaling, merchant shipping was one of the main sources of income for a long time. Hark Olufs, a sailor from Süddorf who had been enslaved by Algerians in 1724, advanced to the rank of a General until he was allowed to return to his native island in 1736. During the late 19th century, tourism became a rapidly emerging business on Amrum and effectively changed the island's economical structures. During the Middle Ages, Amrum as well as all of North Frisia proper belonged to the so-called "Uthlande", the Outer Lands, which only successively became parts of the Danish realm or the Duchy of Schleswig. After the conflicts between the Danish kings and the counts of Schauenburg about the rule over Schleswig, Amrum and western Föhr became an enclave of Denmark and contrary to neighbouring areas, it was not any longer a part of the Duchy of Schleswig. This state endured until 1864, when Denmark lost Schleswig to Prussia after the Second Schleswig War. For a brief period after that war Amrum was ruled together by Prussia and Austria, yet in 1867 the island came under Prussian rule and was made a part of the province of Schleswig-Holstein. At first, Amrum formed a municipality within the district of Tondern. In 1920, the Schleswig Plebiscites resulted in a clear majority vote for Amrum staying with Germany, while Tondern fell back to Denmark. Until 1972, Amrum belonged to the "Südtondern" district which then merged into the newly created district of Nordfriesland. During the 19th century, Amrum still had a considerably lower population than today. Church records from 1821 to 1833 show an average population of 587, a census in 1860 noted 642 inhabitants, and in 1871, the population had dropped to 571. Among other factors, the decrease owed to the fact that large parts of Amrum's population had emigrated — mainly to the United States.