I often hear. We atheists find our basis for morality, of course, in nature. They can't be. To be objective is to exist independently of a mind. If no minds existed, no morality would exist.
Morality is simply the avoidance of unnecessary harm.
Since harm is natural, its avoidance is a material exercise. Organisms suffer as they bump into their environment, and as rational animals, we humans have some choice about how this happens. If we minimize pain and enhance the quality of life, we are moral. If we don't, we are immoral or amoral, depending on our intentions. To be moral, atheists have access to the simple tools of reason and kindness.