The possibility of an earlier modern human migration out of Africa—at least as far back as 220,000 y

[sculptor]

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The marsupial populations of Australia are not just unusual. They are natively found nowhere else in the world. Think kangaroos, koalas, wombats, echidnas, platypus, antechinus, bilby, quokkas ... the list goes on and on.
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Also worth mentioning is that how isolated the continent has been in the past is also relevant. The continents aren't in the same places today that they were 100 million years ago, for instance. You have to take continental drift into account. Also, with continental drift comes changes in climate, which can be fatal to many species. Antarctica used to be near the equator.
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Ahem
marsupials seem to have evolved in south america
when australia, new zealand, and south america were connected to antarctica
they migrated
Australia's isolation allowed them to flourish without having to compete with placental mammals

We still have a few left in the americas

I wonder what fossils could be found under the antarctic ice?

addendum to above
monotremes seem to have evolved in Australia (or New Guinea?)