Earth’s history is divided into a hierarchical series of smaller chunks of time, referred to as the geologic time scale. These divisions, in descending length of time, are called eons, eras, periods, epochs, and ages.

These units are classified based on Earth’s rock layers, or strata, and the fossils found within them. From examining these fossils, scientists know that certain organisms are characteristic of certain parts of the geologic record. The study of this correlation is called stratigraphy.

Officially, the current epoch is called the Holocene, which began 11,700 years ago after the last major ice age. However, the Anthropocene Epoch is an unofficial unit of geologic time, used to describe the most recent period in Earth’s history when human activity started to have a significant impact on the planet’s climate and ecosystems. The word Anthropocene is derived from the Greek words anthropo, for “man,” and cene for “new,” coined and made popular by biologist Eugene Stormer and chemist Paul Crutzen in 2000.

Scientists still debate whether the Anthropocene is different from the Holocene, and the term has not been formally adopted by the International Union of Geological Sciences (IUGS), the international organization that names and defines epochs. The primary question that the IUGS needs to answer before declaring the Anthropocene an epoch is if humans have changed the Earth system to the point that it is reflected in the rock strata.

To those scientists who do think the Anthropocene describes a new geological time period, the next question is, when did it begin, which also has been widely debated. A popular theory is that it began at the start of the Industrial Revolution of the 1800s, when human activity had a great impact on carbon and methane in Earth’s atmosphere. Others think that the beginning of the Anthropocene should be 1945. This is when humans tested the first atomic bomb, and then dropped atomic bombs on Hiroshima and Nagasaki, Japan. The resulting radioactive particles were detected in soil samples globally.

In 2016, the Anthropocene Working Group agreed that the Anthropocene is different from the Holocene, and began in the year 1950 when the Great Acceleration, a dramatic increase in human activity affecting the planet, took off.