The Tibetan language has its own script which it shares with Ladakhi and Dzongkha, and which is derived from the ancient Indian Brāhmī script. Starting in 2001, the local deaf sign languages of Tibet were standardized, and Tibetan Sign Language is now being promoted across the country. The first Tibetan-English dictionary and grammar book was written by Alexander Csoma de Kőrös in 1834. History Early history Humans inhabited the Tibetan Plateau at least 21,000 years ago. This population was largely replaced around 3,000 BP by Neolithic immigrants from northern China, but there is a partial genetic continuity between the Paleolithic inhabitants and contemporary Tibetan populations. The earliest Tibetan historical texts identify the Zhang Zhung culture as a people who migrated from the Amdo region into what is now the region of Guge in western Tibet. Zhang Zhung is considered to be the original home of the Bön religion. By the 1st century BCE, a neighboring kingdom arose in the Yarlung valley, and the Yarlung king, Drigum Tsenpo, attempted to remove the influence of the Zhang Zhung by expelling the Zhang's Bön priests from Yarlung. He was assassinated and Zhang Zhung continued its dominance of the region until it was annexed by Songtsen Gampo in the 7th century. Prior to Songtsen Gampo, the kings of Tibet were more mythological than factual, and there is insufficient evidence of their existence. Tibetan Empire The history of a unified Tibet begins with the rule of Songtsen Gampo (604–650CE), who united parts of the Yarlung River Valley and founded the Tibetan Empire. He also brought in many reforms, and Tibetan power spread rapidly, creating a large and powerful empire. It is traditionally considered that his first wife was the Princess of Nepal, Bhrikuti, and that she played a great role in the establishment of Buddhism in Tibet. In 640, he married Princess Wencheng, the niece of the Chinese emperor Taizong of Tang China. Under the next few Tibetan kings, Buddhism became established as the state religion and Tibetan power increased even further over large areas of Central Asia, while major inroads were made into Chinese territory, even reaching the Tang's capital Chang'an (modern Xi'an) in late 763. However, the Tibetan occupation of Chang'an only lasted for fifteen days, after which they were defeated by Tang and its ally, the Turkic Uyghur Khaganate. The Kingdom of Nanzhao (in Yunnan and neighbouring regions) remained under Tibetan control