CAIRO, Egypt (Reuters) -- Egypt's ancient pyramids are probably a byproduct of a decision to build walls around the tombs of kings, a leading expert on early Egyptian royal burials said on Wednesday. Guenter Dreyer, director of the German Archaeological Institute in Cairo, said he based his theory on similarities between Egypt's first pyramid, built at Saqqara south of Cairo for the Pharaoh Zozer in about 2650 BC, and the structure of the tomb of one of his immediate predecessors. The Saqqara pyramid, known as the Step Pyramid because of its unique shape, began as a flat mound about 25...