Atheism and Theism are equally absurd.

If there were no belief in god, everyone would be atheist by definition.

Well, I agree, however the concept of atheist would not exist either. For instance, we are all a-alchemists. However, I do not believe that there exists a word that means people that do not believe in alchemy. So from that perspective nobody is a (whatever word may mean "one who does not believe in alchemy"). From our definition, yes, everyone would have been an atheist. However, from the perspective of a world with no concept of God, the term atheist would simply not exist.

I still say that to deny or disbelieve [verbs] implies action by an individual, which means it is not passive and thus not innate (as babies are not athiests). To deny it, the concept of God has to exist, which it wouldn't if there was no belief. Basically, 150,000 years ago there were no atheists.

To deny something you must actively reject the idea. Disbelief, however, is simply a lack of a belief. "A baby disbelieves" requires no more thought from the baby than "A baby exists," both sentences using action verbs.

I think most atheists would say that atheism is better defined as the lack of belief in a God or Gods. I would also like to point out that contrary to popular belief, atheism does not imply certainty of the nonexistence of God, but simply a lack of belief in God. It is easy to fall into the trap of thinking atheism requires some faith. If it were the 100% absolute rejection of the idea of God, I would agree (although I would say it only requires as much faith as the assumption that you will never see the statue of liberty as it exists today wave at you...technically possible). But that is not what atheism is.

Finally, Grav I agree that there were no atheists 150,000 years ago, but not for the reasons you do. By our definition (assuming that there were no primitive animist religions at the time), yes, everyone at the time was an atheist. However, since the concept of atheism did not exist 150,000 years ago, nobody would have been called an atheist.