It is not an excuse, it is a reason, an explanation of the state of things. I am not saying there is NO god if we use the term God to describe the being(s) that formulated our existence, all I am saying is that we are still investigating, searching. And to accept this god as it was described by the bible is not questioning a lot of things/faults with this "creator" (eg. why are some bloke born defective, albino, etc.) And if this God is compassionate, there sure is a lot of evil on this planet!
Evolutionary morality, the way I see it is that humanity is at least growing in our species. We have culture (or at least some of us do) is because we are more enlightened. We can still be savage and violent in our ways, but at least when times are good, we do share. We are discovering more and more with the passage of time, understanding world we live in. Through this knowledge, we overcome famine, disease, and shelter ourselves from calamities, God doesn't provide these, we have to depend on ourselves.
Our conscience, it's evolving. Naturally, we had less compassion/care during our days as wanderer gatherers, today as city folk, we have lost a lot of that brutishness. If God put it in us, it was this brutishness he put, not morality.
Cheers!
But one can also be using "we will never know in our lifetime" as an EXCUSE not to consider what has been presented to him as arguments/case for God, isn't it? Since we only have this lifetime, it makes good sense to consider based on what we know rather than to delay or postpone belief based on what we don't know. In fact, that's what Pascal's Wager is all about too.
Evolutionary morality is what you are suggesting. But what you do not have is a mechanism for that. Not only that, you have also shown what is the logical outcome of such a theory of morality (killing others in order to survive), it is really no morality at all. To say that culture is what separates us from animals is to beg the question. How come we have cultural mores, but not animals? Once more evolutionism cannot account for such things at all. Thus the best answer, and the right answer, is that only humans are created in God's image. That's where we got our conscience from, God put it there. Unfortunately sin has affected our conscience too, but it is there, telling us that something is really wrong when we have done wrong.