In 1863, after traveling extensively in the Malay Archipelago, the British naturalist and explorer Alfred Wallace noticed that the species he encountered drastically changed past a certain point. Located in modern Indonesia, running through the Lombok Strait, it is like a 35km wide invisible line, that animals would not cross, a line that would later be called the Wallace Line.
When Australia separated from Antarctica, it initiated significant geological events that shaped the climate and the physical geography of the surrounding areas. This separation opened up a deep oceanic trench around Antarctica, allowing the formation of the Antarctic Circumpolar Current (ACC), which significantly cooled the Earth’s climate. This cooling effect was felt differently across various regions, leading to distinct environmental conditions on either side of the Wallace Line.
The resulting environmental disparity created a natural barrier where the Asian and Australian tectonic plates meet, fostering different ecosystems. Warmer, humid conditions in Southeast Asia contrast with cooler, drier conditions in Australia. Consequently, species evolved to adapt to these climates, leading to high biodiversity and endemism on each region. The deep waters of the Lombok Strait further reinforced this separation, making it difficult for species to migrate, thus preserving each side unique evolutionary paths of flora and fauna.
The discovery of the Wallace Line was the first of several biological boundaries that resulted in species looking very different on each side. This discovery helped Alfred Wallace align his thinking with the theory of Natural Selection, which was being advanced by Charles Darwin during the same era. The discovery of the Wallace Line provided strong support for that theory and offered the best explanation for why there are no Kangaroos outside Australia.
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