The Aslian branch of Austroasiatic is recognised as the oldest recoverable language family in the Malay Peninsula, predating the now dominant Austronesian languages present today. In this paper we address the dynamics of the prehistoric spread of Aslian languages across the peninsula, including the languages spoken by Semang foragers, traditionally associated with the 'Negrito' phenotype. The received view of an early and uniform tripartite break-up of proto-Aslian in the Early Neolithic period, and subsequent differentiation driven by societal modes is challenged. We present a Bayesian phylogeographic analysis of our dataset of vocabulary from 28 Aslian varieties. An explicit geographic model of diffusion is combined with a cognate birth-word death model of lexical evolution to infer the location of the major events of Aslian cladogenesis. The resultant phylogenetic trees are calibrated against dates in the historical and archaeological record to extrapolate a detailed picture of Aslian language history. We conclude that a binary split between Southern Aslian and the rest of Aslian took place in the Early Neolithic (4000 BP). This was followed much later in the Late Neolithic (2000-3000 BP) by a tripartite branching into Central Aslian, Jah Hut and Northern Aslian. Subsequent internal divisions within these sub-clades took place in the Early Metal Phase (post-2000 BP). Significantly, a split in Northern Aslian between Ceq Wong and the languages of the Semang was a late development and is proposed here to coincide with the adoption of Aslian by the Semang foragers. Given the difficulties involved in associating archaeologically recorded activities with linguistic events, as well as the lack of historical sources, our results remain preliminary. However, they provide sufficient evidence to prompt a rethinking of previous models of both clado- and ethno-genesis within the Malay Peninsula.