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Why is there a major difference between mammal species on opposite sides of Wallace’s Line?

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World map using color coding to show the distribution of major floral kingdoms, subkingdoms, and regions. Key floral kingdoms include Boreal (green), Neotropical (pink), Paleotropical (yellow), South African or Capensic (purple), Australian (orange), and Antarctic (blue). Boundaries for subkingdoms, regions, and divisions are marked with different colored lines, and major regions such as Indoafrican, North American, and Malesian are labeled across continents. — biogeographic region Floral kingdoms, subkingdoms, and major regions of the world.

The major difference between mammal species on opposite sides of Wallace’s Line is primarily a result of the long-term geologic history and the isolation of continental landmasses.

For millions of years, Australia and New Guinea were separated from Southeast Asia by a wide expanse of ocean, allowing their respective biota to evolve independently. While the western side of the line is characterized by placental mammals—such as monkeys, deer, elephants, and rhinoceroses—the eastern side is dominated by marsupial mammals, including kangaroos, cuscuses, and bandicoots.1

This faunal divide persists because the two regions remained geographically distinct for a vast period, only coming into closer proximity through plate tectonics and continental drift relatively recently in geologic time. While some highly mobile groups like bats and rodents have successfully crossed the gap, the deep-seated evolutionary separation remains evident in the stark contrast between the mammalian populations found on either side of the boundary.12