Scientists have developed precise and accurate dating methods based on work in the fields of physics and chemistry. Using these methods, scientists are able to establish the age of Earth as well as approximate ages of the organisms that have lived here. Earth is roughly 4.6 billion years old, give or take a few hundred million years. The first evidence for a living organism appeared around 3.5 billion years ago (bya). The scale of geologic time can seem downright overwhelming. In order to organize and make sense of Earth’s past, geologists break up that time into subunits, which are human-made divisions along Earth’s timeline. The largest subunit is the eon. An eon is further divided into eras, and eras are divided into periods. Finally, periods are divided into epochs (see Figure 7.6; Williams 2004, 37). Currently, we are living in the Phanerozoic eon, Cenozoic era, Quaternary period, and probably the Holocene epoch—though there is academic debate about the current epoch (see below).
These divisions are based on major changes and events recorded in the geologic record. Events like significant shifts in climate or mass extinctions can be used to mark the end of one geologic time unit and the beginning of another. However, it is important to remember that these borders are not real in a physical sense; they are helpful organizational guidelines for scientific research. There can be debate regarding how the boundaries are defined. Additionally, the methods we use to establish these dates are refined over time, occasionally leading to shifts in established chronology (see the discussion on calibration in the radiocarbon dating section below). For instance, the current epoch has been traditionally known as the Holocene. It began almost twelve thousand years ago (kya) during the warming period after that last major ice age. Today, there is evidence to indicate human-driven climate change is warming the world and changing the environmental patterns faster than the natural cyclical processes. This has led some scientists within the stratigraphic community to argue for a new epoch beginning around 1950 with the Nuclear Age called the Anthropocene (Monastersky 2015; Waters et al. 2016). Nobel Laureate Paul Crutzen places the beginning of the Anthropocene much earlier—at the dawn of the Industrial Revolution, with its polluting effects of burning coal (Crutzen and Stoermer 2000, 17–18). Geologist William Ruddiman argues that the epoch began 5,000–8,000 years ago with the advent of agriculture and the buildup of early methane gasses (Ruddiman et al. 2008). Regardless of when the Anthropocene started, the major event that marks the boundary is the warming temperatures and mass extinction of nonhuman species caused by human activity (Figure 7.7). Researchers now declare that “human activity now rivals geologic forces in influencing the trajectory of the Earth System” (Steffen et al. 2018, 1).