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Suppose that, due to a slightly different angle or speed of drift, Australia gets close enough to Asia to form a land bridge. This doesn't even have to be a permanent land bridge, just one that opens up during ice ages like the Bering Straits. What would evolution wring out of that change?

For non-ASB purposes, we can assume that modern humans are butterflied away. However, Homo erectus could plausibly end up crossing into Australia on the land bridge. We might see the dominant human species in this world come out of Australia rather than Africa, with a higher intelligence developing due to the need to adapt to climactic shifts in Australia.

Otherwise, I think that in most cases marsupials would be replaced by placental mammals. Cats like clouded leopards and tigers would replace predators such as monitor lizards and Thylacines. Still, some marsupials could survive, particularly in harsh environments where it would be harder to placental invaders to edge them out. Kangaroos could survive in deserts, and platypi may survive by virtue of holding on to an extremely specific edge niche.

Any other thoughts?
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