April 2024

While you’ve probably heard of the Triassic, Jurassic, and Cretaceous periods by way of their most charismatic animals, the dinosaurs, other units of geologic time are less well known. The Siderian Period, for example, rarely makes headlines because it is, by definition, old news – 2.5-billion-year-old news. However, discussions of geologic time recently sprang up in news feeds when geologists considered shaking things up and declaring the end of the Holocene Epoch, which we’ve been living in since the last Ice Age ended more than 11,000 years ago. The idea was to cap the Holocene by sectioning off a new unit of time called the Anthropocene – the age of humans. Here we’ll dig into the geologic time scale to see why it matters and what the idea of the Anthropocene Epoch is all about.

Where's the evolution?

The geologic time scale connects Earth’s rock record with events in its history, providing a framework for scientists to understand the timing of volcanic eruptions, asteroid bombardments, mass extinctions, adaptive radiations, and much more over the last 4.5 billion years. When scientists first began to develop the geologic time scale, it was purely relative – that is, we could say that the rock layers (i.e., strata) containing trilobite fossils were deposited before the rock layers with dinosaur fossils, but not how much more. As our dating techniques have advanced, we’ve been able to put more and more precise numbers on these strata and, hence, on the events they represent and correspond to. This is called absolute dating.

The geologic time scale divides Earth history into four eons: the Phanerozoic, Proterozoic, Archean, and Hadean. Eons are divided into eras, eras into periods, periods into epochs, and epochs into ages. An organization of scientists, the International Commission on Stratigraphy (ICS), determines what properties of the rocks will be used to define these divisions. For example, some strata might be defined based on the sort of fossils they contain and others on physical characteristics of the rock or even on its magnetic orientation.

For the last 15 years, ICS geologists have been researching the best way to define the Anthropocene in the rock record, and correspondingly when, exactly, it began. What best demarcates the time period when humans dominated and reshaped the planet? Should the start of the Anthropocene be signaled by the appearance in sediments of tiny bits of plastic, by pesticides, by pollution from burning fossils fuels, or by something else? The team ultimately decided the epoch-in-the-making should start when a spike of plutonium (from hydrogen-bomb testing, a clear marker of modern humanity’s impact on the world) appeared in the sediments of Crawford Lake in Canada in 1952.

The ICS committee tasked with evaluating such changes ended up rejecting the proposal, but certainly not because humans are “leaving no trace” in the rock record. In fact, one point of disagreement was that the proposal did not recognize the drastic effects humans had on the planet well before 1950. After all, we humans have powered our industry with polluting fossil fuels for more than a hundred years, we’ve been cutting down forests and broadly altering the landscape for thousands of years, and we’ve probably been causing significant extinction events for tens of thousands of years. Does the year 1952 really capture the scope of what we mean by the Anthropocene? Perhaps not. Other voices in the geological community argue that the Anthropocene should be considered an ‘event,’ an occurrence that may transform Earth systems in many ways, but which is not designated as a unit of time.

The committee’s rejection of the proposal may yet be appealed, but whether or not the Anthropocene becomes official, scientists broadly agree that human activity is reshaping the planet and its systems in dramatic and often devastating ways. The age of humans is certainly epic, just not epoch … at least not yet.