Between 4500 and 3500 years ago, partially intrusive Neolithic populations in the riverine basins of mainland Southeast Asia began to form mounded settlements and to develop economies based on rice cultivation, fishing, hunting, and the domestication of animals, especially pigs and dogs. A number of these sites have been excavated in recent years and they are often large mounds that can attain several meters in depth, comprising successive layers of alluvial soil brought in periodically to serve as living floors. The site of An Son is of this type and lies in a small valley immediately north of the Mekong Delta in southern Vietnam. Excavated on five occasions since 1978, and most recently in 2009, it was occupied from the late third into the late second millennium b.c . An Son has produced evidence that attests the domestication of pigs and dogs in all layers apart (perhaps) from the most basal one, which was not investigated in 2009, together with the growing of rice of the subspecies Oryza sativa japonica, of Chinese Neolithic origin. The oldest pottery has simple incised and punctate zoned decoration with parallels in central Thailand, especially in the basal phases at Nong Nor and Khok Phanom Di. From its middle and later occupation phases (1800–1200 b.c .), An Son has produced a number of supine extended burials with finely decorated pottery grave goods that carry some unique forms, especially vessels with wavy or serrated rims. The An Son burials represent a Neolithic population that expressed a mixture of both indigenous Hoabinhian and more northerly (probably Neolithic southern Chinese) cranial and dental phenotypes, perhaps representing a likely ancestral population for some of the modern Austroasiatic-speaking populations of mainland Southeast Asia.