I have come to the conclusion that Saiyans and humans are the same species based on the happenings of tthe manga and anime series.
In order for two animals (opposite genders) to have offspring, they must either be of the same species or (if not the same species) have the same number of chromosomes in each cell. This is the case with a donkey and a horse can create a mule. The problem is this offspring is sterile (cannot reproduce). The only way two animals can have fertile (capable of reproducing) offspring is for those two animals to be of the same species. That is, in fact, a good definition of species. This is why all domesticated dogs are the same species but merely different breeds.
I'm thinking that saiyans and humans are a bit like that: different breeds but same species.
We know that humans and saiyans have the same number of chromosomes as soon as we know of Gohan (as with Trunks and Goten later on), but we don't really know they're the same species 'til we have knowledge of Pan (and that is at the very end of Z (it's even in the manga), not the beginning of GT as has been claimed in this forum). Since Pan is known to be the (genetic) daughter of Videl (a human) and Gohan (a human-saiyan hybrid), we know Gohan is fertile, demonstrating that saiyans and humans are of the same species.
WHAT CONSPIRACY LEAD TO THIS COMPATIBILITY?
I'm posting here to fish for answers to even greater questions that stem from these observations.
How could there be populations of the same species on different planets without knowledge of each other? How did they come to be separated? Why didn't Gohan, Bulma, Dr. Briefs or Dr. Gero, the resident smarties of the Dragonball world, ever think of this?
I eagerly await your hypotheses...