The vast majority of atheists aren't atheists because of evolution. Science, as of yet, hasn't figured out abiogenesis (how life started, minus a god). It's still speculation at this point. Yet, that doesn't mean that atheists will start believing in a god. If evolution were disproved, atheists wouldn't spontaneously start believing in God either.There's only point that atheism is contingent upon - the fact theists have not demonstrated their god claims. If evolution is disproved, they still haven't demonstrated their claim, so we won't believe them.In addition, many theists do fully accept that evolution is a real thing. They just think that God made evolution.For many atheists, accepting evolution didn't lead to atheism. They accepted evolution after they became atheists, because religious dogma was no longer interfering and scattering the knowledge.In the end, evolution is incidental to atheism.