I think the purpose of the existence of any living thing is simply to live out and fulfil the potential of its inherent nature. To be itself. Which I guess it already is, but for humans it's more complicated. E.g. A tree strives to be a tree. Wherever the seed drops, it will strive to live out the potential in it, whether it has to overcome less than suitable soil, bend this way or that to get the best light, etc. It simply is itself. And human beings strive to be who they are as much as possible. Just it is much more complicated than being a plant or animal because part of our nature is to have conscious awareness of a level of existence that plants or animals are not bothered about. Plus we have choice between what is good for our nature and what is not good for it. Whereas a tree cannot consciously do something against its own nature. It can only be a victim, like if its environment is polluted and it sucks up chemicals from the ground that are bad for it.