[pluteoid]

Well that's definitely not true. Many of the most potent toxins known to us are produced by living organisms – for an example, actually read the comment you replied to. You can find examples of substances that are spectacularly lethal to us from all of the five major kingdoms of life. Merely saying something is natural does not imply we co-evolved with it at all, and even if we had, it doesn't automatically follow that selection always gives us the means to "deal with it" in the way you suppose.