From Dragonball Online Wikipedia
Humans are natives of Earth, and are considered to be the most populous race.
As of the time the game takes place in, fighting and martial arts have become incredibly popular among the humans of Earth.They are strong due to the Saiyan blood line being passed through the entire population.
With the blood of the Saiyans flowing through the Human ancestral genes they can, with enough training or maybe a wish from the dragonballs they can become the powerful 'Super Saiyan' of legend to beat their rivals, however due to the bloodline of the Saiyans they do not have the power available to them to turn into an Accended Super Saiyan or Super Saiyan 2,3,or 4. Some Characters from earth include Bulma, Krillin, and Yamcha.
Edited By: Son MB