1. A preservation system, comprising: a preservation computer coupled to a collaborative content server over a network, the preservation computer comprising: a processor, a data store, and a preservation module configured to: determine a first set of objects of a collaborative content object on the collaborative content server at a first time, wherein the objects of the collaborative content object are arranged in a hierarchy and related by a hierarchical relationship; create a first archive comprising the first set of objects of the collaborative content object and store the first archive in the data store, wherein the data store includes a plurality of archives corresponding to a plurality of different collaborative content objects; determine a second set of objects of the collaborative content object on the collaborative content server at a second time; determine that the second set of objects includes one or more changed objects that have changed relative to the first set of objects between the first time and the second time; and create a second archive comprising the one or more changed objects that have changed between the first time and the second time and a structure descriptor that includes data indicating the hierarchical relationship between the one or more objects that have changed in the second archive and the first set of objects in the first archive and store the second archive on the data store, wherein the second archive lacks objects from the first set of objects that remain unchanged at the second time; and a collection module configured to: identify the first archive and second archive among the plurality of archives in the data store as relating to the same collaborative content object based on the structure descriptor and group the first archive and the second archive; and create a review archive for the collaborative content object from the first archive and the second archive, wherein the review archive comprises the first set of objects and the one or more changed objects structured according to the hierarchical relationship indicated by the structure descriptor.