When I met my wife, she had worked at a studio for 6yrs, she went in the door with no training or certifications as a "runner" but as time progressed (and small commercial studios declined...), she was required to be certified to advance. They certified her on-the-job. She eventually got promoted to "assistant" but after 2yrs. the job that I was working at closed, we moved and she changed careers. So yes it's possible, but not probable nowadays without training...with all the Schools and Programs available, getting past the lobby would take a lot of luck, unless you know somebody or you have some training.