Wallace Line by the Sahul Shelf to New Guinea. The biogeographic boundary known as Lydekker's Line, which separates the eastern edge of Wallacea from the Australian region, has a similar origin to the Wallace line. During ice age glacial advances, when the , both Asia and Australia were united with what are now islands on their respective continental shelves as continuous land masses, but the deep water between those two large continental shelf areas was, for over 50 million years, a barrier that kept the flora and fauna of Australia separated from those of Asia. Wallacea consists of islands that were not
Mainland Southeast Asia Thailand, Laos, Cambodia and Vietnam). To the south it forms the Malay Peninsula, on which are located Southern Thailand and Peninsular Malaysia; the latter is variably considered part of Mainland Southeast Asia or separately as part of Maritime Southeast Asia. Mainland Southeast Asia contrasts with Maritime Southeast Asia, mainly through the division of largely land-based lifestyles in Indochina and the sea-based lifestyles of the Malay and Philippine archipelagos, as well as the dividing line between the Austroasiatic, Tai–Kadai, Sino-Tibetan languages (spoken in Mainland Southeast Asia) and Austronesian languages (spoken in Maritime Southeast Asia). The countries of mainland Southeast Asia received