I do think atheists base their lack of faith on evolution, because what would they base it on?
My lack of belief is based on the lack of evidence for proposed deities. It isn't based on having scientific explanations for how things happen. Rather, it is based on the lack of any evidence for the supernatural. Right now we lack an explanation for the origin of the universe and the origin of life. This has no effect on my atheism.
REMEMBER - objective logic REQUIRES that see thing unbiasedly. So it should concern you that 100% organisms come from other organisms.
No one has ever shown that 100% of all organisms that have ever lived came from other life.