The core of the problem seems to me to be the sense of separation that we have. The gift of a neocortex appears to carry with it the Faustian price of dualism: man/nature, self/other, mind/spirit, man/god. That sense of separation leads us directly to the assumption that we can manipulate our environment at will and manage all the consequences, and that our own needs are paramount over all others' needs (whether those others are human or not). For more on this topic I strongly recommend Charles Eisenstein's remarkable online book
The Ascent of Humanity.
Anarcho-primitivist philosopher John Zerzan places the beginning of the breakdown at the point where we began symbolic manipulation of or environment by developing the concepts of language, numbers and time. This probably began to occur several tens of thousands of years ago. I refrain from "blaming" these developments for our eventual downfall because I suspect they were inevitable given the nature of our brains. Certainly no human civilization could have arisen without them, but to blame them seems as logical to me as "blaming" our opposable thumbs.
Other anarcho-primitivists like Derrick Jensen and Daniel Quinn place the turning point at the development of unsustainable organized (or what Quinn calls "totalitarian") agriculture as opposed to the benign and sustainable horticultural practices of hunter-gatherers. That happened just 8,000 to 10,000 years ago. There's a reasonable case to be made that this transition was a marker of our shift to unsustainable activities, especially since humans had existed on the planet for hundreds of thousands of years without causing the ecological damage we saw immediately upon the development of agriculture.
I feel that focusing on agriculture might be a bit of a red herring, though. Like the development of language, the development of agriculture was almost inevitable, as it similarly depends on the logic and reasoning power of our neocortex. Once language and numbers were in place, agriculture can be viewed as just another step up the slope of technology. Again it's more of a consequence than a cause.
It could be that the current disastrous outcome was written in our stars when the reasoning rind of the neocortex formed on the surface of our brains. That's not to say that doom is inevitable. Humans are very adept at living within limits, and at finding joy and happiness in very constrained circumstances. The fact that the limits we will face in the future are largely of our our own manufacture makes no difference at all. We will do what we do until we can't do it any more, at which point we will do something different. Life goes on.
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Now is the time.
Time to wake up;
Time to understand what is happening;
Time to understand why it happened;
Time to accept the change;
Time to accept that we caused it;
Time to forgive ourselves;
Time to make peace with our personal gods;
Time to reach out;
Time to reach in;
Time to re-connect;
Time to begin the healing;
Time to remember that every ending contains the seed of a new beginning.
Now is the time.