When I was high school and college student I had read that on "Shounen Jump (Boys Jump)" it is for boy's cartoon magazine.
At first, the Dragon Ball was a fantasy story, didn't fight to aliens.
The world of the Dragon Ball, there ware many people who likes animals, dogs, pigs, monkeys, and so on.
Son Goku who is a hero in the story grew up and became strong, no enemies in the earth gradually so Akira Toriyama, the author, was appeared the aliens coming from other planets to encroach the earth.
Akira Toriyama had drawn "Dr. Slump Arare-chan" that is a fantasy sci- fi manga brfore the Dragon Ball, and the Dragon Ball was like a sequel of the Arare-chan at first.
|The Dragon Ball official site|