As
gitm mentioned above, the
Argument from Design has historically been one of the most compelling elements of "proof" for the
existence of God. This argument basically goes along the lines of "Look around at all the
order, all the
design in this world. How likely is it that all of this happened by
chance?" Since it seems impossible for everything so purposed to have come into the world without
purpose and without guidance, people assume that there must be a
mind behind it all, designing everything.
Within this argument, the best proof we have of God's Existence is ourselves. We are the most complicated, extraordinary beings we know of--how in the world could we have happened by chance? Cells are arranged in tissues, tissues form things such as lips which are used for language, which is used to communicate. Veins and arteries run throughout the body, supplying blood which is at the perfect pressure, enriched by food that is digested in the stomach, etc. The level of complexity is mind boggling, and we aren't even looking at the brain! Okay, maybe forests could have developed on their own, but certainly not humans! We were the best proof that God existed; that is until Darwin came along. Once he provided a plausible way for humans to have developed without the aid of God, we no longer had our best proof for His existence. This is why Darwin's theory has been so controversial, and why so many people have spent so much time trying to refute it.
Once you have removed the most complicated piece of evidence from the Argument from Design, it becomes that much weaker, and our position from which to prove God becomes weaker as well.