Where do atheists get their morality from?

[Undeceived]

In Romans 2:14, the Apostle Paul said that the Gentiles, who had not received the law [of Moses] or, in other words, a revelation from God, had nevertheless sometimes done "by nature the things of the law" and were therefore "a law unto themselves."

If this doesn't mean that Paul believed that the Gentiles who had no divine revelation had discovered morality on their own, then pray tell what does it mean?

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Romans 2:14-15
Indeed, when Gentiles, who do not have the law, do by nature things required by the law, they are a law for themselves, even though they do not have the law. They show that the requirements of the law are written on their hearts, their consciences also bearing witness.
Judging by the context, the Gentiles did stumble upon God's predetermined morality. When they were given the scriptures, they were pleasantly surprised to find that their consciences already agreed with its ethics.

Morality is not something that is inherently innate as some would like to argue, but is in fact nurtured which is why you don't see the same type of morality preserved in every culture.

And from whence did the nurturers obtain their morality? There must be a reason for my ancestors to adopt an altruistic lifestyle before they can pass it to me. The same criteria apply. Either God gave selfless tendencies or evolution did... or some obscure alternative. Of course, this is assuming the Christian definition of morality--I'm still waiting for someone to argue how programmed responses and selfish motivations qualify as morals. Then again, if programming/selfishness and morals are one and the same, a moral action becomes identical to any other action. Thus, everything is moral (a tough sell for the dictionary) or morals do not exist in an evolved world. To rephrase, morality without altruism is nothing special... it's everyday life. So the word 'moral' becomes defunct. Can anyone see flaws in my reasoning?