In historical linguistics, the history of the Chinese language includes the various changes over time of the Chinese language in its various incarnations. Earliest known origins of the Chinese language date back 4,500 years. Modern day characters had not been introduced until centuries later, leaving many components of Chinese language quite obscure and unknown. Sino-Tibetan ancestry Chinese is part of the Sino-Tibetan language family, a group of languages that all descend from Proto-Sino-Tibetan. The relationship between Chinese and other Sino-Tibetan languages is an area of active research and controversy, as is the attempt to reconstruct Proto-Sino-Tibetan. The main difficulty in both of these efforts is that, while there is very good documentation that allows for the reconstruction of the ancient sounds of Chinese, there is no written documentation of the point where Chinese split from the rest of the Sino-Tibetan languages. This is actually a common problem in historical linguistics, a field which often incorporates the comparative method to deduce these sorts of changes. Unfortunately the use of this technique for Sino-Tibetan languages has not as yet yielded satisfactory results, perhaps because many of the languages that would allow for a more complete reconstruction of Proto-Sino-Tibetan are very poorly documented or understood. Therefore, despite their affinity, the common ancestry of the Chinese and Tibeto-Burman languages remains an unproven hypothesis. Categorization of the development of Chinese is a subject of scholarly debate. One of the first systems was devised by the Swedish linguist Bernhard Karlgren in the early 1900s. The system was much revised, but always heavily relied on Karlgren's insights and methods. Old Chinese Old Chinese, sometimes known as "Archaic Chinese", is genetically related to all current dialects of Chinese. The first known use of the Chinese writing system is divinatory inscriptions into tortoise shells and oracle bones in the Shang dynasty (1766–1122 BC). In the later early and middle Zhou dynasty (1122–256 BC), writing which descended from the Shang is found texts of which include inscriptions on bronze artifacts, the poetry of the Shijing, the history of the Shujing, and portions of the Yijing. The phonetic elements found in the majority of Chinese characters also provide hints to their Old Chinese pronunciations. The pronunciation of the borrowed Chinese characters in both Japanese and Vietnamese also provide valuable insights. Old Chinese was not wholly uninflected. It possessed a rich sound system in which aspiration or rough breathing differentiated the consonants,