lower interior of the island were the last to be settled (as shown by the presence of charcoal particles), possibly due to the prevalence of human diseases, such as plague, malaria, and dysentery. The entire island was not fully colonized by humans until the beginning of the second millennium CE. The extinction of Madagascar's megafauna, including the giant lemurs, was one of the most recent in history, with large lemur species like Palaeopropithecus ingens surviving until approximately 500 years ago and one bone of the extinct Hippopotamus laloumena radiocarbon dated to about 100 years BP. An even wider extinction window for the subfossil lemurs, ranging up until the 20th century, may be possible if reports of unidentified animals are true. As recently as the early 17th century, dwindling populations of subfossil lemurs may have persisted in coastal regions where tree-cutting and uncontrolled fires had less of an impact. By that date, the Central Highlands' forests were mostly gone, with the exception of scattered forest fragments and strips. Along the northwest coast, forms such as Archaeolemur may have survived for more than a millennium after the arrival of humans. This is supported by radiocarbon dates for Archaeolemur from the Ankarana Massif dating to 975 ± 50 CE as well as archaeological data that show there was little human activity in the area until a few centuries ago, with low human population density along the northwest coast until nearly 1500 CE. Hypotheses In the 20th century, six hypotheses for explaining the extinction of the giant subfossil lemurs have been proposed and tested. They are known as the "Great Fire", "Great Drought", "Blitzkrieg", "Biological Invasion", "Hypervirulent Disease", and "Synergy" hypotheses. The first was proposed in 1927 when Henri Humbert and other botanists working in Madagascar suspected that human-introduced fire and uncontrolled burning intended to create pasture and fields for crops transformed the habitats quickly across the island. In 1972, Mahé and Sourdat proposed that the arid south had become progressively drier, slowly killing off lemur fauna as the climate changed. Paul S. Martin applied his overkill hypothesis or "blitzkrieg" model to explain the loss of the Malagasy megafauna in 1984, predicting a rapid die-off as humans spread in a wave across the island, hunting the large species to extinction. That same year, Robert Dewar speculated that introduced livestock outcompeted the endemic wildlife in a moderately fast series of multiple waves across the island.