In 2009 an ancient human skull was recovered from the Tam Pa Ling cave in the Annamite Mountains in northern Laos; the skull is at least 46,000 years old, making it the oldest modern human fossil found to date in Southeast Asia.[19] Archaeological evidence suggests agriculturist society developed during the 4th millennium BC. Burial jars and other kinds of sepulchers suggest a complex society in which bronze objects appeared around 1500 BC, and iron tools were known from 700 BC. The proto-historic period is characterised by contact with Chinese and Indian civilisations. According to linguistic and other historical evidence, Tai-speaking tribes migrated southwestward to the modern territories of Laos and Thailand from Guangxi sometime between the 8th–10th centuries.[20]