The first known yurt was seen engraved on a bronze bowl that was found in the Zagros Mountains of southern Iran, dating back to 600 BC, but the felt tent probably did not arrive in Mongolia for another thousand years. When the yurt did arrive, however, it quickly came into widespread use because of its ability to act in concert with the nomadic lifestyle of the Mongols. Most of the Mongol people were herders and moved constantly from southern regions in the winter months to the northern steppes in summer as well as moving periodically to fresh pastures. The yurts' size and the felt walls made them relatively cool in the summers and warm in the winters allowing the Mongols to live in the same dwelling year-round. Disassembling the yurts only took about an hour, as did putting them back up in a new location. This is why there are still some doubts today about the assumption that the yurts have ever been really put on carts pulled by oxen for transporting them from camp to camp, without disassembling them, or if these carts are just a legend. Some travelers, like Marco Polo, did mention them in their writings: "They Mongols have circular houses made of wood and covered with felt, which they carry about with them on four wheeled wagons wherever they go. For the framework of rods is so neatly constructed that it is light to carry." (Polo, 97) Yurts could be heated with dried dung, found in abundance with the traveling herds, so no timber was needed. The felt for the covering was made from wool that was taken from sheep also present in most Mongol herds. The wooden frame was handed down from one generation to the next and seldom had to be replaced.