For starters, this is just my theory and of course it is supported by filler so you might as well stop reading now and tell me that
.Anyway, here goes:
EXHIBIT A:The dragonball series takes place at around 700 A.D, on the Earth it exists on.
EXHIBIT B: During the Bardock TV special, Freeza sends Vegeta to one of his 'toughest assignments'. At the end of the special we see Vegeta surrounded by corpses that look like those of Vikings.
EXHIBIT C: In heaven, Goku encounters a fighter who I believe is named Hercules, it may be noted that the fighters in the Afterworld Tournament are the strongest fighters of all time and from around the galaxy
Now, what I'm getting at is that maybe either Akira Toriyama or TOEI Animation wanted to hint at the fact that the DB series takes place on another planet other than Earth, though both similiar to Earth and coincidentally, also named Earth. This can be backed up by saying that Hercules was, in fact, the Hercules of Greek Mythology and was selected to be in the afterworld tournament due to his various accomplishments. Maybe the planet Vegeta was sent to as a child to purge of life was 'our' Earth, due to the presence of humanoid corpses who were equipped with what looked like Viking weaponry and armour to me.It may also be noted that the AD in the date used on 'DB' Earth may stand for something else, such as Anno Draconi (as was speculated by VegettoEX). It must most certainly be noted that I'm probably a nerd for trying to look that deep into a fighting Anime.
