Humans are used as the default because they are the most common. Other species may be weaker or more powerful, although experience would suggest players are really only interested in the more powerful variety.
The choices of species are limited by the priority level assigned to species.
0) Standard (AR up to 10, no attributes above 11, minimal bonuses)
1) Superior (AR up to 20, attributes up to 12, low bonuses)
2) Supernatural (AR up to 40, Attributes up to 13, moderate bonuses)
3) Cosmic (AR up to 60, Attributes over 13, high bonuses)
Standard covers those we are most familiar with starting with human, but also includes any other species without a radical departure from normal human abilities (standard player species in most games). Examples include Elves, Dwarves, Orcs, Vulcans, Twi’Leks, Gamoreans, Duros, and Grays.
Ability limits; standard species won’t have excessive natural armor, generally no more than AR 10, they won’t have extremely high attributes with one or two just above human range being possible, they generally don’t have a lot of supernatural powers or skill bonuses over 1 point.
These are those species that have a clear edge over humans. Examples include Gennies (Genetically modified beings), Jaffa, Clan Warriors, Vanguard Brawlers, Sword Fists, Trolls and Ogres.
Ability Limits; may have natural armor up to AR 20, May have multiple attributes with a beyond human range, possibly with caps of 12 (or up to 15 for Body). May have supernatural abilities exceeding human and/or skill bonuses of 1 or 2. May be able to fly or swim at high speed.
By this level the ability of humans to compete with the species except through technology, magic, and numbers is questionable. Examples would include the Goa’Uld, Godlings, Dragons, Splicers, and Zentraedi.
Ability Limits; may have natural armor up to AR 40, may have attributes up to 13 (20 for Body), likely has multiple abilities beyond human such as natural attacks, magic, psychic ability, flight, and possibly the ability to move through space.
Far beyond human scope these species typically rule their surroundings as gods. Examples are ancient dragons, precursor species, and beings of pure magic.
Ability limits; while some may exceed these limits player character versions won’t have a natural AR over 60, attributes won’t go over 13 (except body which might go to 30), other powers might be quite impressive, players should check with GM to make sure species is okay and what the stat translation will be.