Final answer:
The initial migration into Australia occurred during the Pleistocene epoch, around 40,000 to 50,000 years ago, facilitated by lower sea levels that allowed early humans to cross over open water using rafts.
Step-by-step explanation:
The initial migration into Australia is believed to have occurred during the Pleistocene epoch. This period of time extends from 2.58 million to 11,700 years ago, during which early humans started to spread out from Africa to populate other parts of the world. The migration to Australia was made possible by lower sea levels during the last ice age, which facilitated crossing over open water using rudimentary watercraft or rafts. It's noteworthy that the arrival of humans in Australia about 40,000 to 50,000 years ago is associated with the extinction of several large mammals that were indigenous to the continent, suggesting a significant impact of human activity on local ecosystems.