Humans are found all over the galaxy, and seem to the second most dominant life form in the known universe, after the Cylons. Unlike the Cylons though, most groups and planets of humans are not allied with each other, or in some cases even know that other humans exist.
There are generally three regarded views for this, with evidence for all of them. The first view holds that humans originated on more than one planet. The theory states that with similar conditions on worlds it is logical to assume that similar beings will develop. The humans of the planet Terra support this theory.
The second theory is that a lot of these human planets are in fact just simply colonies of the Colonies themselves. This is especially true for the planets around the Colonies where there are many agro colonies. A few of these have even been separated from the Colonies for so long that they do not remember their planets of origin and thus think that their current world is their home world. Generally though, this only applies to some backward planets.
The third theory is that while the Twelve Tribes of Man came and settled the Colonies, it may be that the Thirteenth Tribe scattered on its route to Earth and elements settled on worlds, or possibly that other peoples from Kobol left to set up their own Colonies. The Borellian Nomen are examples of these, as they left Kobol long before the Colonies were founded.
Whatever the reason, mankind exists on many worlds and in many states. Some have reverted to backwater planets without much technology, some are just developing technology and space flight and some are currently just existing to survive.
Humans are the standard template and can be used as is, without any advantages or disadvantages. Or, they can be modified to take into account any special planetary characteristics that might exist.
Probably the most common advantages that will exist - especially if the humans are descended from the Colonies or Kobol - are Extended lifespan and Early Maturation. A Colonial character has 4 levels each of these. Normal humans may not have these though.
From the GURPS Prime Directive rulebook, the Alpha-Centaurans, Prellarians, Deians, Romulans and the Orions could all be used as templates for a new sub species of humanity.