AI-based nanosurveillance, which became the basis of an exact historical record-taking, and later of simulation of conditions for which there were not any direct records.
Cliology is the study of history taken to the next level. With the coming of nanotechnology, it became possible for the first time to truly monitor an entire populace and to store all the data gathered permanently. Public reaction and fears kept this from coming to pass, until the nanoswarms came. Constant surveillance was necessary to survive in the space colonies, where a tiny leak or nanotech outbreak could spell disaster. Even after the crisis passed, the fear remained and no one ever really disbanded the constant surveillance, which humanity carried with it throughout the First Federation Era and eventually to the stars.
With the coming of transapient AIs, it became possible to correlate all that stored data and to analyze it for trends, and cliology was born. Whereas history derives social trends and events from primary source documents and such things as diaries, cliology has a record of every single interaction that ever took place and can therefore make incredibly detailed reconstructions of events. The records do not always exist--many primitive worlds lost the technology, some religions such as the Universal Church disapprove of it, and it is not possible to surveil the higher order AIs. All the same, cliology has transformed how people view their pasts.