According to Jared Diamond’s argument in Chapter 1 of the book, the megafauna of the Americas and Australia became extinct soon after human beings arrived in those areas. However, this is not a good answer to this question. There is no dispute about the fact that the extinctions occurred. What is important about Diamond’s argument is that it holds that the coming of the human beings caused the extinctions to happen.
It is a matter of scientific fact that the megafauna died out soon after the humans arrived. On p. 42 of the paperback edition of the book, Diamond tells us that there is no fossil evidence of megafauna in Australia in the last 35,000 years. On p. 46, he tells us that the American megafauna became extinct somewhere between 17,000 and 12,000 years ago.
However, there is some dispute as to why the extinctions occurred. Some people believe that the animals went extinct because the climate changed. But Diamond thinks this is a silly argument. As he says on p. 43:
Personally, I can’t fathom why Australia’s giants should have survived innumerable droughts in their tens of millions of years of Australian history, and then have chosen to drop dead … when the first humans arrived.
Thus, according to Diamond, human beings killed off the megafauna of these areas.