If writing is viewed as a means of communicating between humans, using conventional, visible marks, then writing spans the entire history of visual communication—from early pictographic (picture writing) beginnings to alphabetical writing. The oldest picture writing identified thus far was found in Mesopotamia (present-day Iraq), the valley between the lower Tigris and Euphrates rivers. This writing, called cuneiform, dates back to the last centuries of the fourth millennium B.C. (c. 3700 B.C.). The finding consisted of about a dozen pictures inscribed on both sides of a limestone tablet. The characters are made up of wedge-shaped strokes; the system was probably created by the Sumerians, a non-Semitic people whose origins are unknown; they probably immigrated to southwest Asia (Mesopotamia) from the East. Cuneiform pitographs closely resemble Egyptian hieroglyphics, picture script developed by ancient Egyptian priests and perfected by the first Egyptian dynasty (3110 to 2884 B.C.). Egyptian hieroglyphics consisted of some 600 symbols (phonograms). Cuneiform and hieroglyphic systems were the predecessors to the alphabet.