Which was first: Atheism or Religion?

[IAmTheeWallrus]

Atheism is the non-acceptance or the rejection of theistic claims. How could an atheist reject theistic claims if they haven't been invented yet?

IMO, and I'm no expert, nor have I studied this in depth (and I don't know how you could), I think it comes down to seeing agency where there is none (Michael Shermer's hypotheses about agenticity and patternicity). Animals see an unexpected movement nearby and bolt, assuming that some agency was responsible for that movement. I think in our human ancestors there was a period of time when this agency-detector began to spin out of control to the point where the Australopithecine or Homo ancestor didn't just read agency where there was one, they began attributing specific personalities to that agency (lightening gods and storm gods are angry, harvest gods are giving, sea gods are tempestuous) and developed the foundations of polytheistic mythic figures.

Just my opinion, though.

You misinterpreting the term "rejection of theistic claims".
Most atheists believe that the default state is disbelief.
That is to say that people are born atheists and are told by others at some point that their is a God. In that context it is the "rejection of theistic claims"
But without those claims we would not have any position on the question.
I personally don't believe that I would have any position on whether or not there was an invisible being that hears my thoughts and created everything. I just would not ever even suggest it were it not for crazy religions.
I do believe in Schermer's ideas about agenticity but I think that it alone is not enough in modern man to create full fledged deism from the ground up.