I'm a bit late in joining this conversation but I do enjoy talking about Evolution. Through my education, this is how i've come to understand human evolution to come about for our higher intelligence. Keep in mind that I do not have a degree and I've only done research on my own through textbooks, online, etc.
I'll give you the gimme everyone desires before reading a long post. The key to our intelligence was probably the following: larger brain case, hands, social-by-nature, losing fur/excess hair, change of diet to omnivore.
Sometime in our evolution, our DNA mutated that gave us a weaker bite. There was a powerful muscle that wrapped around the bottom of the jaw that stretched over our head and that strong muscle gave us a powerful bite but it also restricted our brain case to be smaller. When the mutation happened our jaw became weaker and the braincase expanded allowing our brain to grow.
We have hands to grip things and with our expanded brain, we learned to use/create our own tools to use on a daily basis. We aren't the only animals to do this however, many animals use tools on a regular basis. Crows, dolphins, monkeys, otters, etc. Tool use isn't special to humans but the complex usage of tools is ie. cars to travel, heavy machinery, etc. We will get to that shortly.
We are social creatures. This isn't a rare thing at all. Ants, bees, dolphins, whales, wolves, chimps, et cetera are all social. However, it is the culmination of our evolution that brought us about. We are social creatures which allowed us to share our discoveries. Word of mouth helped our societies grow by teaching each other. When we'd interact with another community be it peacefully or through war we'd often steal their ideas as well. We can see this all throughout history and I don't feel the need to elaborate further.
Something made us climb out of the trees at one point and begin hunting. Lack of food more than likely drew us out of the trees and while we ate insects we eventually evolved to be able to run long distances and hunt. Our ability to run long distances only happened because we lost our fur. If we didn't lose our fur we would overheat and be unable to cover the vast distances we are capable of. No other animal is capable of the long distance humans are able to run so while most animals are faster than us, we can outrun them in distance so we just chase our prey.
Our ability to run that distance and hunt allowed us an excess of calories that then went to our brain. Our ability to outsmart other animals and even other humans helped us to capture/kill/progress, etc. Competition went a long way in humans evolving considering we can view war as a very old thing amongst humans. The first recorded war was 2700BCE but we can safely assume that since human stupidity continues on today that the gene to murder each other over power/land/etc. has been with us for longer than that. At least according to what I have read in evolutionary biopsychology. We hunted, trapped, and ate animals which gave us plenty of calories. We learned how to do it better and better through communication with each other and the sharing of knowledge. As our brains continued to develop and become enriched with the excess calories the intelligent/creative became the primarily selected mate as smarter mates were able to bring home more food, build a better house, etc. These lead to communities and farming and so on.
Back to our hands, we eventually learned to communicate through writing which allowed us to record information and knowledge to pass on to future generations which started around 3000-3500BCE according to a quick wiki search. Hands have let us evolve our intelligence and be able to spread ideas to create more ideas which lead to the device you're reading this on. The tools we have developed is because we began to write. When a civilization is wiped out their knowledge would be lost otherwise. Anything from plagues to war could have done this. Through writing we have managed to create this thing we call science. A study of our world. We honestly aren't that special. Animal behavior is nearly the same as ours. Animals have justice systems, a sense of fairness, morality, love, grief, fears, etc. The only intelligent difference I see is our ability to have a deep and meaningful conversation outside of survival. We're only able to do these things because we are apex predators who have dominated the earth with our intellectual abilities to farm and hunt. We really aren't so different other than that. That's not such a big difference and I don't see how people can't make the connection that we've gotten here through evolution.
To address the other questions: many animals over-hunt and over-breed destroying their own habitat but nature has a way to establish equilibrium when the food source is gone. Humans only pollute because we have the ability to make complex tools and I've already discussed the why above. Many animals depend on humans for survival because we have an abundance of food. Mainly pests, but of course pests can always find another food source but since humans are everywhere we are definitely in the ecosystem and play a large factor on animals and their survival.
