Surprised to see this thread surge back to life, as its initiator I feel the need to make a quick contribution.
The title of the thread is in a form of a question, and this question can be approached in two ways:
1) as an empirical (or as gandful just said, scientific) question.
2) as -- what I call -- a "logical question."
Questions of the first kind are answered by amassing evidence (but this can only be done if the terms are already defined, and in science Nature is a totalizing concept). As I have pointed out previously (and gandful has again stated in the last post), our understanding of evolution cannot justify any delineation of human beings from the rest of nature. The answer is clear; even the worst kinds of neuroses, psychoses, etc. have their basis in the natural world (i.e. the brain), EVEN IF we have reason to believe they could be avoided if we organized our societies differently.
Questions of the second kind don't require any evidence to answer. All we have to do is look at how we use the language. If human beings (and our products) are defined as 'unnatural' then, by definition, we are unnatural. We can only answer the question in the empirical sense if we already have defined it. So the confusion here is due to multiple uses of the terms, and so multiple meanings. However, answering the question as an empirical question demands that we think scientifically, and therefore presupposes a definition for us.
So we must confront the fact that -- even while knowing the answer in the empirical sense -- our standard (non scientific, but also within science) uses of the words 'natural' and 'unnatural' repeat to us again and again our tendency to want (need) to separate human beings from the rest of nature, either for our aggrandizement or for our condemnation. The second sense of the question prompts us to another kind of investigation, which I have called 'metaphysical,' in which we look at the presuppositions that our regular use of language relies upon.
I have pointed out how the same views (the same presuppositions), which either elevate or condemn human beings in relation to the rest of nature, are woven into our mythological/religious narratives. Of course there are practical needs, as the terms 'natural' and 'unnatural' (or 'cultural') can be -- and are -- used effectively all of the time.
As environmentalists, I think it is important for us to avoid these traps within our thinking, and avoid the simple answers that come with either elevating or condemning human beings.
Finally, what nanning -- not to single you out, but you are the most persistent -- and some others in this thread have done repeatedly (in spite of my efforts to point it out) is attempt to answer the question "yes," in an empirical way (i.e. by appealing to evidence), but this is only possible by changing the definition. However, no change in the definition can address the tension caused by the scientific understanding of evolution... And so we go round and round, as those who think they are answering the question are actually begging it: if we set up this new distinction ('living nature' for example), in order to establish how it is that we are separate, we can then we can see how it is that we are really 'unnatural' by looking at some evidence.
The point is not that the terms can be used in different ways, and therefore to come up with some new creative ways of using them (of course, language is flexible). The point is that within the scientific, and monistic paradigms, we cannot overcome the arbitrariness of where we draw the line between what is natural and unnatural. This point -- and more importantly the insights that can come from meditating on it -- cannot be refuted by redrawing the line!