tired: humans discover an ancient progenitor species seeded the galaxy with DNA, explaining the strange predominance of humanoid aliens
wired: humans make first contact with aliens, all strangely humanoid. over time they gradually discover that they are the only humanoid species whose fossil record includes even remotely comprehensive evidence that they evolved on their planet; every other species appears suddenly, with their scientists having collectively concluded that they were seeded by a mysterious progenitor species. awareness of humans starts to spread throughout the galaxy and things start to become Interesting for humanity
Stargate uses a mix of the two. In the Milky Way, ordinary Earth humans vastly outnumber humanoid aliens due to transplantation by the Goa'uld. After Earth's gate was buried, information about the planet where humans first evolved faded into legend.
Other humanoid aliens, as well as the humans of the Pegasus galaxy, use the typical seeded-by-progenator explination.
(This is off the top of my head after not being into Stargate in ten years so bear with me if this is incorrect.)
