The most extreme theory is that Australian Aboriginal and Torres Strait Islander people were completely responsible for the extinction of these animals through extensive hunting. This theory is largely based on the overkill hypothesis of the Americas, where hunters travelled through the land exterminating megafauna. The overkill hypothesis is largely discredited (and not just in Australia), as there have been no confirmed discoveries of kill sites, sites that are found in other contexts around the world and associated with megafauna hunting. The sites of Cuddie Springs in New South Wales, and Keilor in Victoria, display some evidence of associations between Aboriginal stone tools and megafauna remains, but do not prove conclusively the overkill theory. Furthermore, the coexistence of Aboriginal populations with the megafauna tends to contradict the overkill hypothesis. These writers suggest "threshold (for megafauna dieoff) was crossed between 26,000 and 15,000 yr B.P. when the arid area expanded further than usual and water resources in the woodland areas were severely reduced", although this finding is disputed by Roberts "et al." It is clear from paleobotanical and palaeontological evidence that the extinction coincided with great environmental change. The high-resolution chronology of the changes supports the hypothesis that human hunting alone eliminated the megafauna, and that the subsequent change in flora was most likely a consequence of the elimination of browsers and an increase in fire.