The study of language origin and divergence is important for understanding the history of human populations and their cultures. The Sino-Tibetan language family is the second largest in the world after Indo-European, and there is a long-running debate about its phylogeny and the time depth of its original divergence(1). Here we perform a Bayesian phylogenetic analysis to examine two competing hypotheses of the origin of the Sino-Tibetan language family: the 'northern-origin hypothesis' and the 'southwestern-origin hypothesis'. The northern-origin hypothesis states that the initial expansion of Sino-Tibetan languages occurred approximately 4,0006,000 years before present (BP; taken as AD 1950) in the Yellow River basin of northern China(2-4), and that this expansion is associated with the development of the Yangshao and/or Majiayao Neolithic cultures. The southwestern-origin hypothesis states that an early expansion of Sino-Tibetan languages occurred before 9,000 years bp from a region in southwest Sichuan province in China(5) or in northeast India(6), where a high diversity of Tibeto-Burman languages exists today. Consistent with the northern-origin hypothesis, our Bayesian phylogenetic analysis of 109 languages with 949 lexical root-meanings produced an estimated time depth for the divergence of Sino-Tibetan languages of approximately 4,200-7,800 years bp, with an average value of approximately 5,900 years bp. In addition, the phylogeny supported a dichotomy between Sinitic and Tibeto-Burman languages. Our results are compatible with the archaeological records, and with the farming and language dispersal hypothesis(7) of agricultural expansion in China. Our findings provide a linguistic foothold for further interdisciplinary studies of prehistoric human activity in East Asia.