I wonder if it is possible to "explain" various science fiction franchises by placing them in the same fiction universe as other franchises?

So in this fashion let's start with Battlestar Galactica and why Humans seem to have evolved on another planet besides Earth. This can be explained if the (Donald D. Moore) series is actually set in the Stargate SG-1 universe. Doing this we can also introduce the Firefly universe and Culture series of novels and short stories into the same universe.

The SG universe posits that Humans evolved twice on Earth. The first "version" are called the "Ancients" and they apparently emerged about fifty million years ago. The eventual fate of the Ancients is mixed. Some "Ascended" to a higher plane of existence, some turned into a vampiric race known as "Wraiths" and some lived on Earth among the Humans that later evolved.

It would be relatively easy to imagine that the planet "Kobol" in Battlestar Galactica is either some kind of lost Ancient colony or outpost or that Humans managed to evolve on many planets on many different occasions. In the last case Kobol is just one planet on which a Humanity evolved on, in a similar fashion to the various Human species in the Culture series. We can perhaps explain the "Firefly" universe as being a future Human society that wished to escape from the eternal cycle of man-machine violence.

The spin-off series "Stargate: Atlantis" posits a potential conflict between intelligent machines and Humans. It is revealed that while the Ancients and other sentient beings are able to Ascend, intelligent machines are not. This in turn could be one cause of the eternal conflicts between Humans and their creations. It also suggests that the Angelic Messengers in Battlestar Galactica are a group of Ancients who are allowed to interfere with Humanity, or at least colonial society. This actually introduces a further connection with the Stargate franchise because it eventually transpires that the Ancients were divided, just like the colonials and cylons, on religious grounds. A rouge order of Ancients founded a religious society in another galaxy in which they were worshipped as gods. This society was set up in direct opposition to the main Ancient society resident in the Milky Way galaxy which was focused on science, self-determination and non-intervention.

The finale of Battlestar Galactica is therefore a prelude to the Stargate franchise. Presumably the Ancients wished for Humans to replace them as the dominant civilisation in the Milky Way galaxy and they manipulated the Colonials and Cylons in order for this to happen. In other words "humans" did evolve on Earth but they went extinct or experienced their own apotheosis. Our primitive ancestors who arrived later would never have developed language or civilisation so this had to be introduced by the alliance between the Galactica fleet and the rebel Cylons. The new culture would have to be tempered by that existing so therefore both colonial and cylon society would have to undergo the holocaust.  

Edit: ought to point out the difference between "Ascension" and "Subliming". The first is on an individual level, the second is usually a whole society going to another plane of existence. The exception to this is an advanced AI Subliming on its own. I think we can assume that the Dra'azon and other Elder civilisations ascended a la Stargate over going "full Sublimation".

There is also the possibility that the founder species of the Culture evolved from the various animal types indigenous to Kobol, "Earth" and the Twelve Colonies. An example could be bird/avian life developing sapience and reverse-engineering Colonial/Kobolian technology.