Human Exploitation and Mass Extinction

"The settlement of Australia/New Guinea was perhaps associated with still another big first, besides humans' first use of watercraft and first range extension since reaching Eurasia: the first mass extermination of large animal species by humans. Today, we regard Africa as the big continent of big mammals. Modern Eurasia also has many species of big mammals (though not in the manifest abundance of Africa's Serengeti Plains), such as Asia's rhinos and elephants and tigers, and Europe's moose and bears and (until classical times) lions. Australia/New Guinea today has no equally large mammals, in fact no mammal larger than 100-pound kangaroos. But Australia/New Guinea formerly had its own suite of diverse big mammals, including giant kangaroos, rhinolike marsupials called diprotodonts and reaching the size of a cow, and a marsupial 'leopard.' It also formerly had a 400-pound ostrichlike flightless bird, plus some impressively big reptiles, including a one-ton lizard, a giant python, and a land-dwelling crocodiles."   Guns, Germs, and Steel by Jared Diamond

Diamond continues explaining that "all of those Australian/New Guinean giants disappeared after the arrival of humans." An interesting correlation indeed, but what truly caused such mass extinctions. I've always been fascinated with paleontology; even as a 5 year old child I would sit down and watch a 3 hour long documentary about dinosaurs. So this question really resonates with me in relation to my past ventures into prehistory. I am most familiar with the demise of the large North American mammals which scientists often correlate with the arrival of human beings. However, I've never drawn comparisons with Australia/New Guinea and North America to Africa. It is true that while Africa is now considered synonymous with massive animals, other regions, particularly North America, were at once also home to massive animals   some even being larger as is the case with the Columbian woolly mammoth. Yet, most large animals that did not reside in Africa are now extinct. Some scientists blame this phenomenon on drastic climate change following the end of the ice ages, while others blame human hunter-gatherers. Diamond sides with the theory which blames human beings and elaborates upon the possible reason that African giants still exist while Eurasian, Australian, New Guinean, and American giants perished. According to fossil evidence, human beings evolved in Africa; therefore, they co-evolved with African biology. This means that African animals were familiar with human beings prior to their development of advanced weapons, and, therefore, were familiar with their behavior and regarded them as dangerous respectively   they knew to be cautious. However, as humans diffused across Eurasia and even further, they came across valuable game animals which were not familiar with human behavior. It reminds of an expedition which was taken by a group of scientists into a remote region of rainforest which had never been introduced to human beings. Many of the fauna were much less frightened by the presence of the scientists upon their arrival than what is typically observed. It would be somewhat reasonable to expect a similar reaction amid animals upon the arrival of prehistoric humans. This, combined with the fact that humans had developed advanced weapons and hunting strategies by the time of their arrival, produced a major disadvantage for the exploited fauna.