They all came together to experience this particular universe, the similarity they had is that they are all evil. However, they all believe they are good,hence the Good versus Evil Motto. There's been a war in the universes for eons and eons and it is still being played out on earth to this present day.
The different alien races are Serpent/Dragon/Reptilian/Hu-manoid. It's got so messed up people started to incarnate into different alien races that they hated, this is where so much confusion comes from and the mixing with other alien species that would never of mixed.
Now every hu-man being was an alien, and they have become hu-man beings, their cells started to degenerate for being evil for so long by going against nature.
Now the Aliens came up with a plan,make the hu-mans slaves they have forgotten who they are, and they believe us to be their God(s).
So in effect their own alien family turned on each and everyone of them, this is the biggest secret that is being kept from hu-man beings.