In the landmark book Guns, Germs, and Steel: The Fates of Human Societies by Jared Diamond, the author discusses the extinction of large animal species from the Australia/New Guinea area and in the Americas in Chapter 1: "Up to the Starting Line." Although Australia/New Guinea now has animals no larger than 100-pound kangaroos, it used to have giant kangaroos, cow-sized marsupials that resembled rhinos, giant leopards, 400-pound flightless birds, and giant snakes, lizards, and crocodiles. America used to have many massive animals, including elephants, camels, and giant ground sloths.
In both areas, Diamond attributes the demise of large animals to the arrival of human hunters. Diamond argues that on the African continent, where large animals still thrive, humans and large animals evolved together. The animals developed wariness against the encroachment of human hunters that enabled the various species of megafauna to survive. In Australia/New Guinea and the Americas, however, nomadic humans came upon the large animals relatively suddenly and quickly hunted them to extinction. Diamond refers to this as the overkill hypothesis.
As Diamond points out, the overkill hypothesis "has not gone unchallenged." Researchers in the Australia/New Guinea area, for instance, point out that no direct fossil evidence has been discovered of large mammals killed by humans. Proponents of the hypothesis argue that if these mammals became extinct swiftly, "within a few millennia some 40,000 years ago," it is not surprising that no evidence has been found. In the Americas, spear points have been found with the bones of large animals, proving that humans did indeed hunt them.
The primary alternate theory to explain the extinction of large animals around the world has to do with the animals dying off due to a change in climate. In the case of Australia/New Guinea, it might have been a severe drought, while in North America it might have been the encroachment of an ice age. Diamond counters the climate change argument by pointing out that in these areas large animals had already faced and survived climate change. Before humans arrived, large animals in Australia/New Guinea had encountered and lived through multiple droughts. Large animals in the Americas had survived multiple ice ages. It was only when humans arrived on the continents that they became extinct.