First, we aren't just random molecules. That is an extremely simplistic theist way of looking at ourselves just without a supernatural cause. We are one of very many evolved species and evolution is not random. People have cared and done good for each other for thousands of years before religion was created. Ethics have guided people for much longer than religion has existed and has evolved over time in each society on our planet. Ethics are best when they are determined with logic, consideration, analyzing each situation, what causes the least damage and best outcome, etc... It is absolutely ridiculous for anyone to suggest that religion is the pinnacle of morality and it's insulting for anyone to say that it's the only source of morality.
Humans are a social species and like any other, traits that benefit the pack are promoted and passed on through natural selection. Humans that helped others, assisted in hunting, treated others with kindness, cared for the sick, etc. had a much better chance of breeding and passing along these traits to their offspring. Conversely those that were selfish, unusually brutal, independent, or basically tried to go it alone, often dead or had difficulty finding mates. What you believe was given by a god were actually hard earned traits that hundreds of thousands of years of selective breeding honed into the morals we have today.
Saying that religion spawned morality is putting the cart before the horse. Morality and the Golden Rule were part of our evolution as a social animal. We learned early on that we are treated better if we treated those around us with kindness. We observe this even in chimpanzees. This attribute borne of evolution has been with us for so long that it had become a genetic trait long before humans had the intellectual sophistication to conceive of the supernatural and subsequently create the first religions. This innate understanding of right and wrong and the Golden Rule were simply incorporated into religious dogma. Morality came first, then religious belief.
We are by FAR not a random arrangement of molecules. Everything, from diamonds to laundry detergent has a VERY specific molecular arrangement. Already you are starting with an incorrect assumption. The Egyptians and Greeks (as well as the native Americans) had a moral code as part of their belief system and both existed before the modern religions. Unfortunately, we don't have the same level of documentation for the Aztecs or Incas because the Christians burned their libraries during the Conquest. Thus ATHEISTS do not have a moral code but PEOPLE have a moral code. It developed because, as a species, it was advantageous for us to work together rather than as an individual. Those who were "immoral" were a detriment to the group and gotten rid off. Those that were "moral" were beneficial to the group and were kept. Thus morality NEVER had anything to do with religion but ALWAYS had to do with the survival of our species.
Morality is evolutionary, not religious. We see the same core morality in other advanced societal species, like gorillas, bonobos, and chimpanzees. That behavior which kept society cohesive and functioning provided a reproductive and survival advantage, and this was carried forward. Codes of ethics and morality long predate religion. In fact, one could argue that true morality is doing what is right just because it is right, rather than out of fear of punishment or hope of reward.
As for an atheist's motivations in life- you don't honestly think we are incapable of emotion, do you? We live for our loved ones, just like anyone else. Since this life is the only one we have, we are more likely to take it seriously and live it to the fullest.
The society I was born into. If you were raised into a society that did not care about others or the rest of the world, you would most likely be the same way. But because caring about others and the rest of the world enables us to live prosperously and have better control over other species of animals, we choose this direction in order to become a better species.