In my world, I have a reptilian overlord theme, with humanoid slaves. The planet revolves around a binary star system. What are the basic requirements of the climate and the ecosystem, for the reptilians to have evolved more than the humanoids?
The setting I have in mind right now is an arid world with lack of vegetation.
EDIT: What are the star temperature and atmosphere required to create a superior reptilian race? This should be kept in mind, that the humanoid race's evolution is hindered by the same atmospheric condition.