In reality Humans evolved on the planet Earth, DNA evidence suggests that Humans originated in Africa about 200,000 Years Ago. The Human population is now spread across the entire world, inhabiting every continent with the exception of Antarctica.
In Science Fiction, Humans are not always from Earth. In Battlestar Galactica, Humans originated on a planet called Kobol; from there colonists split up into thirteen tribes, the last tribe settled on Earth. The other twelve became the Twelve Colonies of Kobol and settled on other planets. The Earth tribe became a legend, no one really new if that colony has actually exist, no one went looking for them until the Cylons attacked the Twelve Colonies. In Stargate Humans originated on Earth, but Goa'uld System Lords used the Stargate to transplant some of the Human population to other worlds to use as slaves. In Star Wars, Humans are not from Earth, they originated in a galaxy far, far away. In Firefly, Humans left Earth in search of other worlds, they found another Solar System with many inhabitable planets. In Star Trek Humans became the charter member of the United Federation of Planets, the capital is based on Earth in San Francisco, California.