High Elven, Atheist over here too, you're not alone in what you believe.
HE, do you reckon if we went far back enough, we'd all be swinging off trees?
I know this question wasn't addressed to me but my answer for that question is No,
WE didn't swing from trees, a ancient ancestor of ours did that you wouldn't look at and think of it as
WE, but as a deferent type of animal, distantly related to human beings.
As for "Especially if our existence was 'incidental'" as an Atheist, I'd have to say yes, it was incidental. But you can look at Evolution as a Theist and see that it as a very elaborate and deliberate master plan by a Creator or Creators.
The Big Bang, same as the Evolution topic, I believe it was just incidental and you could see this as part of a plan by a Creator or Creators.
My personal reason for doubting the existence of a God or Gods, is because There is No proof for the existence of such a thing beyond Religious Dogma or Texts, which I do believe was invented by Human Beings as some type of explanation for life and existence, not a creation of God or Gods of any sort.
And my reason for not believing any Religion, is because they are filled with so many inconstancies, mistakes, contradictions, and things that are uniquely human in nature, That a God has nothing to do with them. They also conflict with science which is our understanding of our existence. If a God did exist, the religious Dogma/texts would conflict with our understanding of God's creation, which to me shows that religions are created solely by people with limited knowledge, not by a God with infinite knowledge. This is again, just my opinion.
But regardless of all I've said, it's pointless to debate religion/God, as no one knows the answers to these questions.
Very true, these beliefs (including Atheism) are opinions not facts, I would prefer we not to talk about religious concerns on this site. It could always start a fight or argument that is unanswerable and pointless. The saying that the two things you don't talk about at work are Politics and Religion apply here as well in my opinion.