@Yazata --
What does being incapable of believing in gods have to do with beliefs that gods don't exist, that belief in gods is irrational and not supported by credible evidence, or that religion is socially harmful and intellectually obscurantist?
Nothing, but what does that list have to do with whether or not someone is an atheist? Oh that's right, nothing.
Many self-avowed 'atheists' attach a great deal of additional content to the word that isn't suggested by simple lack of belief.
And they're free to do so if they want to, but that doesn't change the
fact that a person who lacks a belief in one or more gods(or who can't believe and thus lacks belief by default) is still an atheist by definition. You may not like it, but that's reality.
That introduces another one of those 'semantic' points that Sciforums hates so much. Namely, what's 'theism'?
And there's a good reason why we dislike pointless semantics like that, the only reason to bring them up is to obfuscate the topic under discussion and make seeing the issues impossible. It also tends to close down communication completely as those advancing semantic arguments invariably follow them with their own personal definitions which they then refuse to budge even an inch from(see Wynn's thread about religious violence for many wonderful examples of this behavior).
So, rather than engage in pointless semantics that waste everyone's time and won't add anything meaningful to the debate(or what's left of it) I will instead go to the dictionary and choose the definition in there that best fits the topic we're discussing. The best fitting definition in this case would be the broadest definition, that of "belief in at least one deity".
Of course, no matter how you slice it, no matter which definition you want to choose(from the dictionary, of course, as personal definitions are virtually always useless or worse), humans predate theism. Since all humans are born atheists we can then conclude that atheism predates theism. In fact, such a conclusion is logically the only valid one we can come to given what the facts are.
I'm inclined to think that any belief in invisible conscious and willful personal agencies is 'theist' in some basic sense.
See, the problem with your definition is that it really doesn't mean anything because it's too inclusive. According to how you define theism we could easily show that most(if not all) species of mammal as well as many species of birds would be considered theists as they all engage in agency detection and all of them respond to false positives(assuming an agent when there is none). So, according to you, all pet dogs and cats are theists. Should we "respect" their freedom of religion then? LOL!
See, this is the problem with designing your own definitions and then basing your arguments on those personal definitions.
Seen that way, I'd speculate that this kind of belief might be very old.
True, it would be quite old. Billions of years old, in fact. However there would still be organisms who didn't/don't possess agency detection software(anything without a nervous system of some kind) who would, thus, be unable to believe in a deity, who predate even your definition of theism. And since your definition can't be limited just to humans, we must also take those organisms into account. And since we must do that we must still conclude that atheism predates theism.
Of course, that's all only relevant if we grant your definition of theism, which I haven't because it's effectively useless.
I sense that there are deep and interesting questions here, unexamined issues.
Perhaps, but unless these issues can magically make infants believe in god as well as completely rewrite history, the outcome of this debate will remain the same.