^do you believe that humanity descends to survival of the fittest? or,
do you believe that humanity ascends to survival of the fittest?^
I think this is a fundamental misunderstanding of what survival of the fittest actually means. It's not a normative ideal to be aspired to; being 'fit' is not the same as being 'good', and it's also not the same as being the last man standing in some sort of gladiatorial circus of life. It's simply a description of the criteria of natural selection in the evolution of species: if an organism is better equipped to survive in its environment than another, it is likely to live longer and therefore has a better chance to reproduce before dying.
Reading some kind of a moral meaning into natural selection is really missing the point of what the phrase actually describes.
Incidentally, there are good arguments in favour of altruism actually being an aspect of 'fitness' - so even if you happen to think that people are completely dominated by their biology (and FWIW that's not a unanimous atheist position) this still doesn't necessarily imply any kind of Social Darwinist dystopia.
if you believe that the story ends at one's death and that there is no Hereafter then the focus is more on enjoying oneself with the time one has left.
whereas the religious view is to store up as many good deeds as one can before death because all actions, good or bad, shall be accounted by God.
Frankly, I completely fail to see the inherent 'goodness' of acting in a certain way in order to rack up a few more points on some heavenly scoreboard. if that is why a person acts in a certain way I would say that their actions are inherently selfish, even if in the best case scenario you could say that the effects of their behaviour happen to be beneficial.
I also don't think you give humans quite enough credit: as a social species, we're capable of empathy and generally don't enjoy seeing others in pain or distress. IME even religious people usually act in an altruistic way because they feel compelled to - not in order to get an extra gold star on their record.