Uhm... the simple fact that God created earth?
So you could argue, I suppose, that God "could've" created out of dust something that's not in HIS image. But how likely is that when God regards only his own image as Godly? One would have to be an unbeliever to deny the image of God.
Besides, that something could not be man since... man has to be, well, an image of God, created. AND on top of it, Adam in Hebrew means man. Thus, Adam only could be the first man. Period. So again, one would have to be an unbeliever to reject this.