 MacLean's three-part theory explores how the human brain has evolved from ancestors over millions of years, consisting of the reptilian, paleomammalian and neomammalian complexes.MacLean proposes that the neomammalian complex is only found in higher order mammals for example, the human brain, accounting for increased cognitive ability such as motor control, memory, improved reasoning and complex decision making.MacLean's theory explores how in higher order mammals, the Neomammalian brain works interdependently with the Reptilian and Paleomammalian complexes to allow sophisticated thought processes to occur.