originated in ancient Sumer (in Mesopotamia) and spread over the world from there via a process of cultural diffusion. According to this theory, the concept of representing language by written marks, though not necessarily the specifics of how such a system worked, was passed on by traders or merchants traveling between geographical regions. However, the discovery of the scripts of ancient Mesoamerica, far away from Middle Eastern sources, proved that writing had been invented more than once. Scholars now recognize that writing may have independently developed in at least four ancient civilizations: Mesopotamia (between 3400 and 3100 BCE), Egypt (around 3250 BCE), China (1200 BCE), and lowland areas of Mesoamerica (by 500 BCE). Regarding ancient Egypt, several scholars have argued that "the earliest solid evidence of Egyptian writing differs in structure and style from the Mesopotamian and must therefore have developed independently. The possibility of 'stimulus diffusion' from Mesopotamia remains, but the influence cannot have gone beyond the transmission of an idea." Regarding China, it is believed that ancient Chinese characters are an independent invention because there is no evidence of contact between ancient China and the literate civilizations of the Near East, and because of the distinct differences between the Mesopotamian and Chinese approaches to logography and phonetic representation. Debate surrounds the Indus script of the Bronze Age Indus Valley civilisation, the Rongorongo script of Easter Island, and the Vinča symbols dated around 5500 BCE. All are undeciphered, and so it is unknown if they represent authentic writing, proto-writing, or something else. The Sumerian archaic (pre-cuneiform) writing and Egyptian hieroglyphs are generally considered the earliest true writing systems, both emerging out of their ancestral proto-literate symbol systems from 3400 to 3100 BCE, with earliest coherent texts from about 2600 BCE. The Proto-Elamite script is also dated to the same approximate period. Developmental stages A conventional "proto-writing to true writing" system follows a general series of developmental stages: Picture writing system: glyphs (simplified pictures) directly represent objects and concepts. In connection with this, the following substages may be distinguished: Mnemonic: glyphs primarily as a reminder. Pictographic: glyphs directly represent an object or a concept such as (A) chronological, (B) notices, (C) communications, (D) totems, titles, and names, (E) religious, (F) customs, (G) historical, and (H) biographical. Ideographic: graphemes are abstract symbols that directly represent an idea or concept. Transitional system: graphemes refer not only to the object or idea that