I consider to be very convincing the theory that most of our capacity for reasoning actually revolves around the modeling of other humans' thought processes. Together with language (which allows us to 'groom' many people at once, where individuals from other ape species have to groom each other one by one. Grooming as in 'interact to bond and build cooperative associations') it has allowed for incredible cooperative achievements.

So; monkey systems -> other humans, albeit flippantly.