Question

what sequence of events resulted in this region having animals and plants that are different from those found anywhere else?

Answer 1

THE WEATHER CHANGES.
THE DEATH OF THE SPECIES ANYWHERE ELSE.
THE POPULATION CHANGES.

Answer 2

Australia is the right answer.

Australia has been very isolated and that resulted in its growing number of species that can not be found anywhere. The series for these events would be Australia's geographical separation (the fact that Australia separated from the other areas around 99 million years ago), and secondly, the constant  fluctuations in climate resulted in population bottlenecks, for instance, Australia initially was very hot and then as a result of climate fluctuations it experienced a colder period.