I think purpose is how you look at it. Many people get a cat in their house for the purpose of catching mice. But to a cat, the purpose of catching a mouse is for fun. We as humans are in a unique position though, we are able to analyze things which means we can make our own purposes. If you want your purpose in life to be making sandwiches at a subway, then you can actually accomplish that.
Can you define nature? Everything part of the Earth that isn't man?
Well, to be able to look at our true purpose would be to look at the next level of intelligence that is above us. The cat cannot understand exactly why we took it into the house, sure it can draw its own conclusions but it will never know for sure unless we could somehow communicate with it. And communication with a cat is impossible, there is an intelligence barrier. So I don't think true purpose can ever be known, but like the cat, we should just do what we think is fun.