Environmental overuse and abuse by human populations is nothing new. Consider the Pleistocene Overkill. Fossil records show that most large mammals became extinct in the Americas and Australia in a relatively short time period. Interestingly it coincides with the appearance of the first human populations on these previously uninhabited land masses.
Separated from human populations for millennium the Americas and Australia developed a unique flora and fauna distinctly different than that of Eurasia and Africa. It was also a flora and fauna with no contact and therefore no knowledge of human populations. Eventually populations in Eurasia and Africa became large enough to spark competition in the form of armed conflict over prime real estate. The losers and the adventurous struck out for parts unknown.
When they came to the Americas and Australia the flora and fauna had no idea how to deal with these curious and very determined intruders. So for the most part they ignored the harmless looking creatures. The migrants took complete advantage of this situation, the end result being mass extinction.
At first consideration this might seem like a ridiculous hypothesis – that relatively small and ill equipped bands of humans could have caused the extinction of rather large and scary animals such as mammoths and saber toothed tigers. But the coincident of the first signs of humans and mass extinction at the same time in history is hard to ignore. Imagine this scenario. It is hard for a band of ill equipped humans to kill a mammoth or buffalo or tiger in head to head combat. So the hungry hunters used their big brains to figure out a less risky way of getting dinner. Instead of going after a single buffalo in a herd of 1000's, a very dangerous thing to do, they would light a fire behind the entire herd and run them all off a cliff. At the bottom of the cliff they would select a couple of the dead or near dead and eat their fill – thus leaving 1000's unused. The next time they were hungry for buffalo they would practice a similar technique.
Efficient? From their standpoint, yes indeed. Environmentally devastating? Absolutely. An additional point to keep in mind is that these animals had never had contact with humans and had no innate sense to fear the very benign looking humans. This gave the hunters a tremendous advantage in the element of surprise – no sneaking needed – just walk up and bonk them on the head. Once they are on to your game then you run them off a cliff.
So the early humans very well could have been responsible for the mass extinctions of the large mammals of the Americas and Australia leaving very few which had potential for domestication. This would set these two continents back dramatically in the ultimate competition between the biotas of the Old World and New World when they would eventually collide via Columbus and crew. Just something to think about.