Where do atheists get their morality from?

[Minimalist]

Evolution may, in fact almost certainly must, play a part in developing systems of morality

But not evolution by natural selection. The human concept of the group goes back to Homo Erectus where we have found skeletons which show signs of individuals recovering from broken leg bones. While they may have been crippled because the bone was not properly set these individuals were still given food and water by other members of the group.

Human culture interferes with evolution by natural selection.