Atheism has been around as long has humans have been around, it may have not been called atheism, but it has always been around, as long as there was someone who did not believe in any god(s).
In fact, here is an Atheistic tribe from the Amazon.
http://en.wikipedia.org/wiki/Pirahã_people
the Pirahã have no concept of a supreme spirit or god[11] and they lost interest in Jesus when they discovered that Everett had never seen him. They require evidence based on personal experience for every claim made. [5]
However, they do believe in spirits that can sometimes take on the shape of things in the environment. These spirits can be jaguars, trees, or other visible, tangible things including people.[12] Everett reported one incident where the Pirahã said that “Xigagaí, one of the beings that lives above the clouds, was standing on a beach yelling at us, telling us that he would kill us if we go into the jungle.” Everett and his daughter could see nothing and yet the Pirahã insisted that Xigagaí was still on the beach.[13]
Yes, they are still atheist despite the bolded.