According to the He-Man production bible (yes there is such a thing), Adam's mother, Evil-lyn, Beastman and Tri-Klops were actually earth astronauts that got caught in a rip in space and crash landed their shuttle on an unfamiliar world... The latter three were found and enslaved by Skeletor, the magics of his homeland warping them from normal humans into evil versions of their inner nature, while the former woman was found by Randor, king of Eternia. He made her his queen, and their son, Adam, later became He-Man. The Sorceress left her daughter, Teela, for Man-At-Arms to find, and telepathically commanded him to raise her as if she were his own.