It seems to me as if Darwinism really made materialism a possibility.  Prior to natural selection and evolutionary theory, to suggest the non-existence of God would have been silly.  Where else would human life have possibly come from? Or life at all? Thoughts?

The philosophic school of 'materialism' predates Darwin by a couple of centuries (Locke would be an early example of 'materialist' doctrine).  One could say that materialist ideas have been around since Plato's famous assertion of dualism.

Indeed, all of western philosophy begins with Thales of Miletos (8th century BC) - who famously asserted that the 'world was created in a manner not unlike the silting up of the Nile delta'.  This is the first proposed explanation of the origin of the world that does not include/require the act of a supernatural agent. 

Some would argue that any asertion of the existence of God is silly and always has been - before or after Darwin matters nothing.

That is correct under the Roman Catholic Christian doctrine, where 'big bang' theory and 'evolution' are both officially compatible with Roman Catholic doctrine under the 'watchmaker anology'.

I'm not aware of any other Christian theology that can accept evolution or big bang.

All Protestants must have a problem with it because of the Protestant insistence of Biblical scripture being the authorative word of God.

before Scientific rationalism I don't think we could really apply "rationality" as we now define it.

so you don't think that the Greeks had aspects of Scientific Rationalism?

Apparently my sarcasm was too subtle.

I was mocking the notion that scientific rationalism begins with Darwin.

Chance, spontaneous assembly, some unknown material process, perhaps.

The watchmaker argument was constructed by theists in response to the challenge of the theory of evolution.

Besides, I see no logical or rational reason why any atheist should need or want or bother to argue against the watchmaker argument.  Atheism itself does not require or necessitate that act.

From the perspective of philosophy, the watchmaker argument is nothing more than a restatement of the Genesis theory in that it is asserted entirely as an article of faith.  Atheism essentially just rejects all human-assertions or human-constructions of alleged statements of God's truth.  That is to say, atheism is entirely a negative assertion, it contains no positive content.  One cannot project positive content onto atheism itself. 

For example, the theory of evolution, or theory of natural selection, may be disproven in the future - if this were to occur, this would not directly impact the rational validity of atheism itself.  Atheism is not dependent upon evolutionary theory (they are merely mutually compatible concepts).

Btw, Candide is the name of a novel written by Voltaire in 1759, based on the famous Lisbon earthquake of 1755.  This novel serves as a classic example of 'atheist' type ideas in evidence - a full century before Darwin came along.  One might say that this novel represents the introduction of the idea of atheism into popular consciousness.