Where do atheists get their morals from?

One of the oddest arguments I have ever heard is "where do atheists get their morals from?" This argument seems almost exclusive to the United States, as I have seldom heard this being presented anywhere else.

The question seems very odd because the answer is so simple: We are a social, intelligent, thinking species, and we can deduce what's good and bad for the society. As a society we can agree which rules we should follow that benefit everybody, and how to deter members of the society from doing harm to others.

I don't understand what's so hard in understanding that.

If we delve more into the origins of morals, they come directly from evolution. Humans have been a social species for a long, long time, and it's the reason why humans have flourished and succeeded in survival. There are certain behaviors within a society that naturally maintain this beneficial status, while other behaviors disrupt it. The beneficial behaviors help in survival, while the harmful behaviors impede survival. Therefore it's only natural that the beneficial behaviors get favored by the societies within the species, while the harmful ones are abhorred. Instincts like empathy and altruism have been naturally selected because they benefit the society and therefore the survival of the species.

Imagine two hypothetical, isolated human societies. One of them behaves like what these American Christians imagine atheists to behave: They are immoral and selfish, they have no qualms in harming others for their own benefit, and they have no concept of empathy. They are all savages and ravaging criminals who steal and kill as they like. The other society is a civilized one, with an agreed code of proper behavior, where everybody helps those in need, where the society cares for its own, and where criminals are punished and deterred.

Which one of those two societies has a larger chance of survival, especially in harsh conditions? I don't think it could be clearer where morals come from: Evolution.