Penn A&S masthead

Giegengack photo

Geological Time Compared to a Roll of Toilet Paper
by Professor Robert Giegengack

Robert Giegengack is professor of geology, chair of the Department of Earth and Environmental Sciences, and director of the Center for Environmental Studies. The following is a 60-Second Lecture delivered on campus last summer as part of a program offered by CGS Summer Sessions.

We know the earth is 4.5 billion years old. We seek a metaphor to help us grasp the immensity of that sweep of time.

Consider a standard roll of toilet paper: 1000 sheets, each 4.5 by 4.5; let each inch represent one million years of earth history.

(Since we donít have time to unroll it) imagine we have rolled up the toilet paper, and we are looking at the last square, the last 4.5 million years, the time of our bipedal primate ancestors. The last 50,000 years, or 1/20th of an inch, represents the history of our species, Homo sapiens, and the last 10,000 years, or 1/100th of an inch, is the span of recorded human history. (Snips it off with a pair of scissors.)

We know that our behavior today is influenced as much by our biological heritage as by our cultural legacy. If we hope to make sense out of these troubled times, if we hope to project present trends into an uncertain future, if we hope to understand the last 10,000 years of human history, we must study the 4 billion 499 million 990 thousand years that came before.

This (holding up the 1/100th inch) may indeed become the best of times. To make it so, we must learn what we can about the rest of time (holding up the rest of the roll of toilet paper).

Return to Table of Contents