Atheism isn't a philosophy. It's merely one of many premises on which you might build your philosophy. The same applies to theism, in my opinion. It only differs in that there is only one way to be an atheist, but many many ways to be a theist.
What I don't get is why everybody seems so invested in the idea of purpose. I guess it would be nice to know that there is a reason why we are here beyond the vicissitudes of evolutionary biology, but if there isn't, well tough.
