Making Sense of Evolution

The Geologic Time Scale

As this unit pertains to the concepts of Earth’s history, geologic time, and evolution, it makes sense to begin with a brief amount of background information on the geologic time scale. The time scale, for student and standards purposes, does not need to be memorized; however, having a rough sense of where these time periods lie can help students visualize when major events – such as extinctions and climatic/geologic events – occurred throughout Earth’s very long history. The development of the now ubiquitous geologic time scale is an endeavor that has spanned over 200 years. When first developed, scientists used the tools they had available to them, including observations of the thickness of sedimentary strata and the salinity of oceans, as well as using their understanding of the input from streams into oceans to figure out how old the oceans were, generally. What these methods yielded was simply the understanding that Earth is much older than was thought at the time.⁴ Thanks to the discovery of radioactivity, scientists were able to more accurately organize and subdivide geologic time, beginning to develop the time scale as we are familiar with it today. Scientists now had numerical (absolute) ages for the rock layers they studied.⁵ Traditionally, boundaries between geologic time units were made when there was some major shift in observed life forms, a widespread geologic event (e.g., ice age), or a large-scale structural deformation of Earth’s crust. Most divisions in geologic time have been refined to major evolutionary events, indicated by the initial emergence of a distinguishable guide fossil.⁶ Today, there are six methods commonly used to more precisely determine geologic time – astrochronology, chemostratigraphy, geochronology, magnetic polarity stratigraphy, rock magnetic stratigraphy, and stratigraphy. For the purposes of this unit, it would be most useful to understand three of these methods – chemostratigraphy, geochronology, and stratigraphy. Beginning with the simplest, most well-known of the three, stratigraphy is looking at the physical arrangement of rock layers and sequences (lithostratigraphy) as well as the fossil content (biostratigraphy). This method will result in obtaining the relative ages of rock layers compared to those around them (as opposed to numerical or absolute ages). Geochronology is more commonly known as radiometric dating. Chemostratigraphy can be seen as a sort of extension of geochronology in that is also uses radiometric dating but tells us more about catastrophic events that have a profound effect on the rock record. These two methods provide us with the numerical ages of rock layers.⁷

From longest to shortest, in terms of geologic time, the scale is broken down into eons, eras, periods, epochs, and ages. Precambrian time (approximately 4,500 Ma to 541 Ma) is not technically an era, as it comprises three eons. However, this massive chunk of time is often juxtaposed next to the three most well known eras - the Paleozoic Era (541 Ma – 252 Ma), the Mesozoic Era (252 Ma – 66 Ma), and Cenozoic Era (66 Ma – present). These three eras are then divided into 12 periods.⁸