The Palaeolithic and Mesolithic periods saw hunter-gatherers move into the region of Somerset. There is evidence from flint artefacts in a quarry at Westbury that an ancestor of modern man, possibly Homo heidelbergensis, was present in the area from around 500,000 years ago. There is still some doubt about whether the artefacts are of human origin but they have been dated within Oxygen Isotope Stage 13 (524,000 – 478,000 BP). Other experts suggest that "many of the bone-rich Middle Pleistocene deposits belong to a single but climatically variable interglacial that succeeded the Cromerian, perhaps about 500,000 years ago. Detailed analysis of the origin and modification of the flint artefacts leads to the conclusion that the assemblage was probably a product of geomorphological processes rather than human work, but a single cut-marked bone suggests a human presence." Animal bones and artefacts unearthed in the 1980s at Westbury-sub-Mendip, in Somerset, have shown evidence of early human activity approximately 700,000 years ago.