In Guns, Germs, and Steel, Jared Diamond says that the Great Leap Forward ended up changing the environments of Australia and the Americas by allowing humans to reach those two landmasses. Once humans got to Australia and the Americas, Diamond says, they hunted all of the megafauna found there to extinction.
The Great Leap Forward occurred about 50,000 years ago. This was a huge change in the mental abilities of our ancestors. Before the Great Leap, there was no art. There were relatively few kinds of tools. All the tools that existed were made of stone. People did not have weapons like bows and arrows or even spear-throwers that allowed them to kill animals from a safe distance. In short, people were rather primitive before the Great Leap. Diamond speculates that people were not able to speak modern languages before this time.
The Great Leap Forward allowed people to spread rapidly around the world. They suddenly were able to create technology to allow them to succeed in many more places. They even managed to develop technology that allowed them to cross open ocean to reach Australia. These sorts of things were not possible before the Great Leap.
When people spread into Australia and the Americas, they came with “modern” technology, including more effective weapons. These weapons (and presumably new mental abilities) allowed people to hunt animals very effectively. They were so effective, Diamond says, that they killed off all of the megafauna that had lived in Australia and the Americas. This is how the Great Leap Forward led to a major change in the environments of these places.