Even if it is only due to repetition almost everyone is familiar with a few geological dates. That the non-avian dinosaurs became extinct about 65 million years ago and the earth is around 4.5 billion years old are figures that are at least familiar to many. (There are a few folks who would prefer to jam the entirety of geologic time into just a few thousand years, of course, but I will not worry over them here.) It truly is wonderful that we have been able to lay out such a detailed map of Deep Time but this was not always so. Today’s standard geologic time scale, with all its time delineations on the side, is a relatively new thing.*
[If you happen to be in the market for a poster-sized chart of Deep Time I recommend the Correlated History of the Earth wall chart.]
For much of the early history of geological science understanding the history of the earth required a knowledge of names and places. There was a nested hierarchy of names that marked off different parts of earth history, but determining just how old those strata were seemed an insurmountable task. This problem persisted into the early 20th century.
Take Amadeus Grabau’s 1913 textbook Principles of Stratigraphy, for instance. On page 1108 Grabau presents the geologic time scale as then known, from the Cambrian to the “Psychozoic” (aka Quaternary). There are no time estimates, only names. Presented below is a shorter version that goes from the Azoic (“without life”, or the Archaean of today’s geology) to the Psychozoic (the time of “mind life”);
Geologists knew that going without defined times for eras of earth history could not last, however. In 1902 Henry Fairfield Osborn and Henry Shaler Williams prepared a report for the Carnegie Institution of Washington in which they recommended that the organization authorize some paleontologists “to undertake the establishment of a few carefully selected time bench-marks, which shall constitute a permanent set of standards for discriminating divisions of geologic time, and which shall serve as datum planes for the correlation of geological formations and events of other continents than those in which the standards are established.” The present understanding of evolution would not only be essential to this endeavor but would also greatly benefit from the results;
The imperfection of the standards now in use arises from the fact that they were originally denned and established in pre-evolution times, before the reality of the gradual and continuous modification of the life of the globe was appreciated, and only the rocks between indefinite boundaries are defined by lists of their characteristic species, whereas it is now seen to be essential to determine the precise stage of evolution of the races whose representatives lived before as well as after any artificial time boundary which may be adopted as a standard.
What was even more problematic, however, was the lack of an accurate and reproducible method of obtaining dates for geologic strata. Computations based upon rates of sedimentation and the amount of salt dissolved in seawater had been used but these methods resembled those of older scholars like James Ussher to divine the history of the world by adding up the ages of the biblical patriarchs. As a 1917 article in the Scientific Monthly showed, widely different dates had been obtained by several different authors as to when life first appeared, and it was radiometric dating that seemed to have the best potential to give scientists a better understanding of geological time.
Admittedly I do not know as much about this part of the history of geological dating as I would like, but even so it is apparent that at about the same time as the Scientific Monthly article was published scientists started including dates in their geological time scales. A 1917 paper from the Bulletin of the New York State Museum of Natural History included this time scale;
It seemed that less then five million years separated us from the marauding terrors of the Age of Reptiles, and it had been less than 25 million since our fishy ancestors first crawled out of the Devonian muck. Just how old the world was, it was difficult to say, but at least this timescale provided some context. A similar timescale was printed in the 1919 edition of the Encyclopedia Americana and a more detailed version was included in Osborn’s 1921 The Origin and Evolution of Life;
According to Osborn’s timescale all of evolution, from the first life to the present, occurred in 60 million years. That’s just a little less than the time that separates us from the extinction of the non-avian dinosaurs. It is a long time in relation to a single lifetime but it is still just a geological moment.
I have to wonder how the development of this timescale influenced considerations of evolution, especially human evolution. If the “Age of Mammals” was only about three million years long, and human evolution occurred during only a tiny fraction of that, how could there have possibly been enough time for humans to become differentiated from other apes? Perhaps natural selection was too slow. Ideas of vital evolutionary forces, saltationism, and orthogenesis were already in place by this time but perhaps the relative “shortness” of the geologic timescale circa 1920 fed some support to these alternative evolutionary mechanisms.