Captioned: "Illustration of a wheel is in The Cart of the Felines from a bas-relief of the city of Ur." The invention of the wheel falls in the late Neolithic, and may be seen in conjunction with other technological advances that gave rise to the early Bronze Age. Evidence of wheeled vehicles appears from the second half of the 4th millennium BC, in Mesopotamia (Sumerian civilization), the Northern Caucasus (Maykop culture) and Central Europe, so that the question of which culture originally invented the wheeled vehicle is still unsolved. Early wheels were simple wooden disks with a hole for the axle. Because of the structure of wood, a horizontal slice of a tree trunk is not suitable, as it does not have the structural strength to support relevant stresses without failing; rounded pieces of longitudinal boards are required. The invention of the wheel has also been important for technology in general, important applications including the water wheel, the cogwheel, the spinning wheel, and the astrolabe.