There were prominent atheist philosophers in both Greece and India as far back as 600 B.C., but modern atheist thought kicked off during the Renaissance and gained steam after the industrial revolution (hurr... gained steam... industrial revolution...).
I don't think that's possible to know.
Probably some guy that didn't give a damn about religion many thousands or perhaps millions of years ago. Maybe he did his art on some yet undiscovered cavern. Perhaps a middle finger imposed on some representation of what ever god was worshiped at the time.
The first human, presumably. Before anyone had a chance to think "Hey, what's that big bright hot thing in the sky?"