The Anthropocene epoch: scientists declare dawn of human-influenced age
theguardian.com
theguardian.com
There has definitely been a change in the nature of mankind's relationship with the Earth's environment, such that human actions have become a dominant factor. However, that happened at some time during the bronze age. Humans have been completely changing biomes and drastically impacting the environment for thousands of years. Huge swaths of forests were cut down, much of the remaining forests were managed, farmland and grazing lands replaced natural habitats, numerous megafauna were hunted to extinction, etc. The difference between the bronze age and today is merely that it's become much more obvious how significant our impact on the world is today.
There's this zeitgeist that mankind was largely at the mercy of nature until the industrial age, but that's based primarily on our foggy view of history, not on reality.
During the neolithic and bronze ages, humans cut down and/or "managed" most of the forests of the world. Read that again, it's accurate. There's almost no such thing as a primeval forest that hasn't been touched by human hands anywhere on Earth, nor has there been for a very long time. In Europe there was a significant reduction in forested land during the bronze age. Not only were forests cut down to make room for planting, but the forests that were left standing were managed for timber et al and for game hunting. In the Americas and Africa, for example, it was quite common for the locals to use fire on huge scales to modify the environment, either for slash and burn agriculture, or for forest management (e.g. to make game hunting easier). The image of the primeval American frontier is one that is based on a recently depopulated post-Columbian exchange New World, which hides the massive extent of the impact that the native americans had on their environment.
This is true for the rain forests, the arctic, the deserts, etc. Which saw massive changes due to the presence of humans. They affected rates of desertification, they became the apex predators in many cases, driving out others, and they extensively modified the local environment (as in the case of rain forests).
We see such things as untouched by human activity primarily because we don't know any better. Our knowledge of ancient history is limited due to lack of record keeping and the difficulty of piecing together data from just artifacts (if we are lucky enough that the artifacts survived and were found during a research dig, which requires luck stacked on top of luck stacked on top of luck), and we often lack the context to know the difference between what a landscape that has seen the impact of pre-industrial human activity looks like and what a truly pristine landscape looks like. There are a lot of areas which people think are natural but are the result of the activities of prehistoric humans (such as many of the unforested regions in Scotland, which were deforested and made into farmland but have lain fallow for centuries and reverted to a semi-wild state, though not a pristine natural state).
The difference today is that we can more readily see the changes happening directly, because we have much better record keeping and can compare observations over recent decades and centuries. And, of course, the pace of change has been accelerating, with impacts on new things. But the start of mankind having a dominant role in the environment happened a long, long time ago.
Of course, there are folks who have a vested interest in denying this fact for a variety of reasons. Environmentalists (and I count myself one, for the record) might find this fact somewhat disconcerting, because if mankind has been a driving force in environmental change for 5 thousand years, maybe that implies that its nothing to worry about, right? I can understand wanting to avoid having that discussion, but it's no excuse for denying the reality. And indeed, sometimes the impact of humans on the environment has had extreme negative consequences in the past, on human beings and on the environment, even going back thousands of years (but, of course, such things can be hard to suss out because of that pesky spotty historical record).
Looking back at, say, the bronze age, it takes a lot of research to figure out the extent of that impact. Looking at today it's blindingly obvious. But that doesn't mean that our impact has been extensive only recently, it only means that it's become blindingly obvious recently.
Since I'm pretty sure it already happened, and quite some time ago, depending on your metric.