It is no belief for two reasons. Not all atheists see things the same, i might think the universe came to be by a sudden expansion, another atheist thinks it just suddenly poofed to what it has been since the beginning of mankind.
As I said, Atheists have different beliefs about things, for sure. However, the reason they are "Atheists" is because they believe that there is no God.
Also, neanderthals which we are descendents from didn't even think that there was a god, it just was thought up about 2000 years ago.
Assuming the existence of Neanderthals (and when I say that, I mean assuming their existence as they are portrayed by evolutionists), I find it hard to believe that anybody could be so certain about something that occurred before the advent of recorded history.
Not believing in a god is no belief, it is a clear view on your surroundings like man already had millenia ago.
It is a belief, that is like saying Theism isn't a belief because it is disbelief that there is not a God. Also, lack of evidence (and I'm not going to go into the whole "proof is in the order" debate) does not prove lack of existence. It merely proves that it hasn't been found yet. The recent biological discovering in the rain forest if proof of this. They found a species of rat that they thought had become extinct long ago. Why? Because it hadn't been seen. Yet it still existed.
Now, that isn't to say that we are going to find some solid objective proof of the existence of any particular God, until He Himself clarifies it in person. However, just because you can't find evidence of Him, doesn't mean that He doesn't exist.