And the 'marble'. I would assume the word came about long after we started making things that could be described by it - tracking down the 'first' one might be really tricky. It could be as bad as trying to find the time when there was only one human.

Possibly harder, given the possibility that objects more closely resembling an archetypal marble than the first marbles actively created probably existed elsewhere by chance. In fact, given the simplicity of the item and the material, marble-like objects probably existed a long time ago in a galaxy far far away. Humans on the other hand are sufficiently complex, arbitrary and anthropically selected that we can with reasonable confidence narrow 'first human' down one of the direct ancestors of the surviving humans (or maybe the cousin of one of those ancestors if we are being cautious).

ie. In addition to the 'where do you draw the line' question you also have the 'if a marble-equivalent-object falls in forest and there is nobody there to hear it or ascribe it it purpose is it really a marble'? Then, unless you decide that spheres made out of marble aren't 'marbles' unless proximate intelligent agents intend them to be you are left with an extremely complex and abstract application of theoretical physics, cosmology, geology and statistics.

I would probably start making an estimate by looking at when second generation planets first formed.