The boy emerged and introduced himself as Marty McFly, and asked if she was "The Harriet Tubman". Harriet did not say a word. he also knew there was a bounty on her head. This boy looked like he was intending to collect.
The official line on Back to the Future's origin states that co-writer Bob Gale was visiting his native Missouri, found his father's high school yearbook, and wondered if he'd have befriended him as a teen.