Trade and communication networks established by Indian groups in the 15th century A.D. linked the Southwest to Mesoamerica, the Plains and the Pacific littoral; these routes were later used by the Spanish and Americans, and today major highways follow ancient Indian routes. The main east-west route had major termini at Cibola (near Zuni) in the west and Pecos in the east; the north-south trunk road followed the Rio Grande from Taos south to the El Paso area, and was later extended by the Spanish to southern Chihuahua and called the Camino Real. From their arrival in the 1530's the Spanish, primarily concerned with reaching and controlling Indian settlements and resource areas, used Indian routes. Trails were widened for cart and pack animal transportation, but few other modifications were made. In the 16th century, the region split into two administrative divisions: upper (New Mexico) and lower (Sonora); the Camino Real fell into disuse until its reestablishment in the late 18th century. Before and after arrival of the Americans, new roads were built in previously unsettled areas because decline in Indian populations led to relocation of important centers, new technology opened up uninhabited regions, and railroads and automobiles demanded flatter terrain than the old roads. (MH)