An ancient human skull was recovered in 2009 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. Stone artifacts including Hoabinhian types have been found at sites dating to the Late Pleistocene in northern Laos. Archaeological evidence suggests an 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 and 10th centuries.