Below is the geological timescale adapted from the International Commission on Stratigraphy (2009) accessed on February 27, 2011, available here.


(Click to enlarge)

It’s a busy chart, isn’t? The kind of chart that makes your eyes glaze over and invokes such thought-provoking questions as, “I wonder what’s on TV?” and, “do the dishes need washing?”. Indeed, charts like these are the bane of undergraduate geology students everywhere. But to be fair, the geological timescale is a fantastic piece of work.

Comprising 4 eons, the Hadean, Archean, Proterozoic, and Phanerozoic, the timescale is generally applied as the Phaneroizoic (542 million years ago to the present day) and the Precambrian (the previous ~4 billion years). The division seems unbalanced and arbitrary; for what could the last 542 million years from the Cambrian period onward represent that the previous, much longer interval of time does not?

The answer? Animal life.

Now while the the appearence of animal life has traditionally been associated with the Cambrian explosion, and the ensuing, various mass extinctions and evolutionary radiations throughout the remainder of the Phanerozoic (including the extinction of the dinosaurs ca. 65 Ma and the largest known mass extinction ca. 250 Ma), research over the past couple of decades indicates that signatures of animal life are observed as far back as 630 million years ago. Macroscopic body fossils of animals are well documented ca. 580 Ma; clearly within the Precambrian context!

The division appears to be a remnant of an earlier time when paleontologists recognized the abrupt “explosion” of animal life in the geological record at the Precambrian/Cambrian boundary. Today, the distinction stands for any number of reasons, most likely (in my opinion) as equal parts an artifact of tradition and the massive headache that would come with timescale revision. Thus, it’s business as usual and no appears to be too troubled by it.

My own research as an undergrad evaluating the structure of a mid-crustal shear zone (~10-15 km deep) within the Canadian Shield focuses in on the Mesoarchean/Neoarchean divide at approximately 2.8 Ga, well before the appearence of animal life. Given the great age of this terrane, it is hard to discern the exact character of the Earth at this time, though a few key features are convincing: an atmosphere devoid of oxygen, a biosphere devoid of life, and the onset of plate tectonics.

Crazy stuff, eh?

It’s within this framework that the importance of a geological timescale becomes clear: the chart isn’t just a set of random divisions designed to complicate your life. Rather, each division signifies something unique and interesting about the history of the Earth, and as geologists, our primary goal is to figure out exactly what that is, and how it set the stage for future events.