Most likely, the very first tattoo happened on accident. All it takes is a few rowdy primitives playing around the fire. It wouldn't take long for someone to fall in, get poked with a charred stick and later heal before they'd realized that they were on to something. The news probably spread as fast as the fires that prompted the invention. As a matter of fact, it is rumored that the first tattoos were of the sun or flames in honor of the sun god(s)...