The Ancona Penrose Conference of October 2007 dealt with the current state of understanding of the late Eocene and the Eocene-Oligocene boundary, ~34 million years ago, a critical but very brief interval in Earth history. In this paper, we place that brief interval and the lessons from the conference in the broadest possible context by viewing them in the light of “Big History.” This new intellectual concept maintains that there may be value in considering the entire past, from the big bang until today, as a single unit of study. At this very early stage in the study of Big History, not even the most fundamental questions have been well formulated, let alone answered. As a first cut, Big History can be divided into regimes by considering the disciplines that study it: cosmic history (studied by cosmology and astronomy), Earth history (studied by geology), life history (studied by paleontology and evolutionary biology), and human history (studied by archaeology and historiography). These disciplines differ in terms of problems, techniques, and intellectual traditions. If we seek a common basis for a finer subdivision, the changes in utilization of the energy that have driven historical changes would seem like a good candidate. In two thought-provoking papers in 2007, Robert Aunger proposed “periodizing” all of history by placing divisions between periods when new methods of utilizing concentrated energy came into being, e.g., cellular metabolism or human agriculture. Aunger draws a parallel between this periodization of Big History and the establishment of the geological time scale. In this paper, we carefully consider this parallel and conclude that periodizing history on the basis of energy use or any other conceptual scheme is quite different from the division of Earth history into the intervals that yielded the geological time scale. Both are important but they have different purposes. Time-scale construction is a procedure that ties history to the rocks that record the history. It is a necessary step in reconstructing Earth and life history but is neither necessary nor possible in studying the history of cosmos or humanity. In contrast, periodizing history into intervals provides a conceptual framework on which to hang a growing understanding of history. We conclude that it is important to differentiate between (1) time-scale construction, (2) correlating events, (3) dating events, and (4) periodizing history. In this light, Aunger’s focus on changes in energy use remains an instructive way of periodizing history, but it must be clearly differentiated from time-scale construction.