From Wikipedia

Lost Treasures Of Tibet

Tibetan history, as it has been recorded, is particularly focused on the history of Buddhism in Tibet. This is partly due to the pivotal role this religion has played in the development of Tibetan, Mongol, and Manchu cultures, and partly because almost all native historians of the country were Buddhist monks.
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. The Zhang Zhung are considered the original culture of the Bön religion. By the first 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.

Megalithic monuments dot the Tibetan Plateau and may have been used in ancestor worship. It is unknown whether these monuments were built by ancient Tibetans. Prehistoric Iron Age hill forts and burial complexes have recently been found on the Tibetan plateau but the remote high altitude location makes archaeological research difficult.

Humans inhabited the Tibetan Plateau at least twenty thousand years ago, and some archaeological data suggests humans may have passed through Tibet at the time India was first inhabited, half a million years ago.