In prehistoric times, the Sepik-Ramu Basin was flooded with salt water, this inland sea probably reached its maximum extent 5,000 to 6,000 years ago when it reached as far westward as Ambunti. The sea then began to drop gradually until it attained its present level around 1,000 years ago. During that span of time the Sepik Basin with its young floodplains began to develop and became separated from the Ramu Basin by the Bosman Plateau. Linguists point out that the Ndu Family of languages had a common ancestry, which suggests a common settlement history. Linguistic evidence also suggests that the Ndu speakers moved into the Sepik Plains from the south of the river. The Abelam evidently migrated northward during the last few centuries until after World War II, although there is much debate about where the Abelam came from and when they began moving north. Except for sporadic contacts with hunting parties from Indonesia, the first direct contact with the outside world occurred immediately before World War I, when the Abelam were discovered by the German ethnologist Richard Thurnwald who was traveling through Abelam Territory on his way over the Alexander Mountains to the north coast. Before long, European goods (and also diseases) had reached the Maprik area. Soon missionaries arrived as well, and by 1937 an Australian patrol post (Maprik) was established, land was cleared for an airstrip, and a road to the coastal town of Wewak was built. World War II brought drastic changes to the Abelam way of life; thousands of Japanese, Australian, and American soldiers fought bloody battles on Abelam territory using technology unknown to the Abelam. The establishment of further patrol posts, missionary stations, trade stores, and schools, the substitution of a cash economy based on wage labor for the indigenous subsistence economy, and the development of flourishing towns led Abelam life in new directions. In precolonial times the Abelam—not as a whole group but as many individual villages—had already had continuous relations with neighboring groups. Those with the Plains Arapesh were the most highly esteemed because the Arapesh villages supplied them with valuables, shell rings, and other shell ornaments in exchange for pigs. Relations with the Boiken in the east, the Sawos in the south, and different groups in the west were restricted more or less to border villages.