HOW MUCH IS A MILLION? HOW BIG IS A BILLION? GETTING A HANDLE ON THE IMMENSITY OF GEOLOGIC TIME

Radiometric dates give an age of the Earth of 4.6 billion years and place the geologic eons in their proper temporal perspective: The Precambrian occupies over 80% of Earth history and humans are but a part of the most recent and briefest sliver of this history. Geologists casually toss off references to ages of Earth materials and processes in units (10⁶ and 10⁹ years) that astonish lay audiences. Human life experiences play out over 10² years, and histories/oral traditions go back to 10³ years. The 10³-10⁶ year gap between what our species can know empirically and the vast history that is beyond human experience results in a population in which most individuals cannot truly grasp the immensity of geologic time and the implications of this time scale for geologic and biologic processes.

Numerous icons are used to place the age of the Earth and major geologic events in human perspective: Condensing Earth history to a 12-month calendar or a 30-day-month, scaling to the length of a football field, etc. One of many excellent web resources for millions/billions analogies can be found at [www.ucmp.berkeley.edu/education/explorations/tours/geotime/ guide/billion.html].

Kinesthetic activities are preferred for their educational efficacy over rote memorization or passive observation. An effective exercise for both students and lay audiences of all ages is to construct a geologic timeline along a sidewalk at a scale of 1 inch = 1 million years. Construction requires only a single class session, yet the resulting timeline is large enough to impress even the most inexperienced participants with the immensity of Proterozoic time, leading to the inescapable conclusions that “simple” life appeared early in Earth history; that it took the bulk of Earth history to achieve the next, multi-cellular stage of development; and that once the metazoan threshold was crossed, all subsequent biological diversification—and the resulting fossil record—followed in rapid succession.