I'll start by assuming a fundamental base of science and logic, in things like evolution, modern cosmology, etc. Recent work that I've seen by Stephen Hawking and other string theorists suggest that universes themselves have the capacity to spontaneously spring into being. I won't go into the details of how this work, mostly because I dont understand them myself (though i encourage you to google around for it yourself), but what I will focus on is how this affects human purpose. This base-principle immediately cancels out a need for god(s) as we commonly understand the term, although if you want to call the elusive theory of everything god, then go right ahead.
This essentially rules out objective purpose to human existence, leaving only subjective purpose or that which we give ourselves. On first glance, this seems to be very dark and sobering, and this is the wrap that existentialists have been dealing with since the philosophy's inception. I disagree. Liberated from the shackles of someone or something else giving purpose and meaning to us, we are free to choose our path for ourselves and make our lives what we want them to be. This freedom can be a burden for the indecisive and the submissive, but for others it is empowering to an extraordinary extent. It may be corny to quote Palahniuk, but it think it applies here: "It's only after we've lost everything that we're free to do anything."
Ruling out objective purpose also has ramifications for human morality. The lack of something objective to give us morals again implies that they're inherently subjective. I basically covered this in "Why Every Moral Philosopher Ever to Philosophize Was Wrong", so I won't go into great detail here. This too has a bright side however, in that we are free to act with or against our societally agreed-upon morals because we choose to. Not because we face divine retribution if we don't. This is why people who use "atheist" with a negative connotation implying 'immoral' realllly piss me off. I have faith (somewhat ironically) in humanity in that we can act morally towards each other because we choose to, not because we face eternity in a pit of fire if we don't. Ghost stories like those keep immature minds in subjugation. Mature minds know better then to fear the boogeyman.
Life itself appears to be the product of the energy, matter, and fundamental dimensions of our universe. Meaning that given the prerequisite proteins and certain other conditions, life will inevitably be produced. Besides precluding the existence of a god, I think this impacts us in that it pierces the concepts of uniqueness, supremacy, and importance that we associate with ourselves and Earth. The probability of those proteins and energy and other matter mixing in a way to form life may be astronomical, but the size and timeline of the universe are even more astronomical still. A gambling man would bet heavily on the existence of intelligent life elsewhere in the universe, but he would not bet on the chance we will ever meet in the vast cosmos.
I guess the last bit to explore involves the future of our universe (as it was explored in "The Last Three Minutes" by Paul Davies). Many of us, myself included, have entertained notions of some part of us living forever, whether it be in our biological descendants, our accomplishments, etc. but the possibility of this is inevitably tied up in the ultimate fate of the universe (which remains uncertain). Regardless of whether the universe is steady-state or dynamic, it seems ridiculous to suggest that any part of humanity can live forever. This fact may again appear dark and gloomy, but its really no different then confronting your own death. Maybe not the most cheerful of comparisons, but it is something that you can come to terms with and accept.
This is a very post-modern-style post in that it disenchants a lot of the Marxist superstructure we've constructed in our lives. I guess the lesson in this is that we may not have an inherent purpose in our existence, but that in of itself is no reason to despair. For instead of a world built by others for us, we build our world as we see fit. A burdensome freedom, but a freedom nonetheless.