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It was revealed in the Star Trek: the Next Generation episode "The Chase" that all humanoid life in the galaxy, including human beings; Vulcans; Klingons; Cardassians; and others, had evolved on their individual planets because of a common genetic seeding that took place 4.5 billion years ago.
In The Urantia Book, the creation of life on all planets, including life on Earth (aka "Urantia"), was the result of seeding by the "Life Carriers."
The Prime Directive
According to The Urantia Book, once life on Earth had reached a point where the early proto-Humans were capable of free will and of making "decisions of mind," the "Life Carriers" report that "...we were directed not to intervene in the affairs of human progress."
Starships of Star Trek's United Federation of Planets are similarly prohibited from performing any act that could alter or affect the development of a civilization. This regulation is known as the "Prime Directive."
In the Star Trek episode "Bread and Circuses," (article in progress)
In both Star Trek and the Urantia book, many planets are inhabited, and all members of these disparate civilizations are of humanoid form.