t's a matter of convenience. People (should) like a baseline from which to start placing burdens of proof. The baseline is Earth without people. I hear people came into being about 200k years ago. But some folks, quite reasonably I think, push the baseline up to when we started making fire. Others push it up further, quite reasonably I think, to domestication of species (plants, animals). At that point we've kind of gone off the rails; relatively speaking, of course. It's all natural, yes. But a good baseline to help us in deciding how far off the rails we can go, without ruining the gifts that we were given, is the Earth with space, clean air, clean water, clean food, and without a parasite killing the host. — James Riley

This is assuming quite a bit - like that what you have described ISN'T the way large-brained species with opposable thumbs normally evolve and manipulate their environment. You're still singling out humans as special in some way, when we still don't know the number of other species in the universe that have done the same on much larger scales.

This is assuming quite a bit — Harry Hindu

It's not assuming anything. The distinction between us and nature is and always has been a matter of convenience. We like to distinguish between us and nature that we are part of. We could do the same with giraffes and the rest of nature. But that is not convenient. It doesn't make us feel better or special. And they aren't fucking up the Earth. So there's that.

You're still singling out humans as special in some way, — Harry Hindu

I'm not. We are. I don't think we're special at all. Different, yes. But so is a giraffe. "Special" denotes "better" (in my mind, anyway). And we clearly are not better.

why people say, man made things are unnatural ? — Nothing

Who says that? The common expression for man made things is ... simply "man-made"! And this is to distinguish them from those found in nature, without having been processed by man.

Since his origins, Man has been creating tools from elements of nature, which we call man-made. For example, I can use a piece of wood, which is cut or fallen from a tree, as a weapon. This would be called a "natural" object. But if sculpt it to make it sharp, straight, better to grasp, etc., this would be called a "tool". It will be a man-made object. The difference lies in the processing of the material.

For example product from plastic, metal,.. computer keyboard: where did you get material ? from nature. — Nothing

All these are called man-made objects, not unnatural.
BTW, plastic does not come from nature! It's a synthetic material made from a wide range of organic polymers. It does not even belong to objects that man gets from nature and process them, like metal rings, which are man-made, because they cannot be found in nature as such. (A metal needs a lot of processing my man to become a ring.) A diamond, on the other hand, if it is processed lightly (cut, trimmed, polished) to fit a man-made ring, can be still called "natural". This, to differentiate it e.g. from an "imitation", a fake diamond, which is created by man.

Likewise with synthetic vs natural vitamins and minerals. The first are processed in a laboratory, while the second are derived from plants.

It all has to do with the amount and kind of processing ...