How Well Do You Know the Eras of Earth History?

Scientists theorize that during the Hadean eon, a large planetesimal body (about the size of Mars) impacted the surface of the Earth. The resulting ejection of matter from that collision eventually formed Earth’s only natural satellite, the moon. It’d take another 4 billion years for humans to start sending up artificial ones.

Earth’s earliest years were spent in the Hadean eon, which began 4.6 billion years ago and lasted about 600 million years. During this eon, Earth was basically a giant fireball and these intense temperatures kept the planet’s crust from solidifying. This hellish landscape is why the eon is named after Hades, the Greek god of the underworld.

Around 252 million years ago, the Permian Extinction, likely caused by global warming, spelled the end of 95% of marine life and 70% of terrestrial life on Earth. This extinction event, known as “The Great Dying,” was the largest in Earth’s history and paved the way for the eventual rise of the dinosaurs.

Before the Cambrian explosion, the world was populated by single-celled or simple multi-celled organisms. But around 541 million years ago, an explosion of life occurred and organisms began forming hard body parts, which now fill today’s fossil record. The leading theory for this explosion is that a rise of oxygen levels made it possible to support complex life.

While very simple organisms appeared during the Archean eon, it was during the Proterozoic (between 2.5 billion years ago and 554 million years ago) that simple ocean-bound organisms reigned supreme. This life flourished in the shallow waters near continents until higher oxygen levels gave rise to more complex organisms and ushered in our current eon, the Phanerozoic.

The human story begins 6 million years ago, making Homo sapiens and our legion of ancestors relative newcomers on the planet. While the Pliocene epoch, which immediately followed the Miocene, saw a great diversification of hominids (aka our earliest ancestors), the human evolutionary tree begins in the late Miocene epoch.

About 720 millions years ago during the Neoproterozoic Era, the Earth entered a chilly chapter of its history called the Cryogenian period, otherwise known as “Snowball Earth.” Global temperatures at this time hovered around 10 degrees Fahrenheit. Scientists believe the Earth experienced a runaway effect where ice sheets reflected sunlight, cooling surface temperatures and creating even more ice.

Before the continents were arranged into today’s dispersed geography, plate tectonics caused the Earth’s ancient land masses to form a supercontinent known as Pangea. Pangea isn’t the only supercontinent in Earth’s history — only the most recent. Other supercontinents include Rodinia, Pannotia, and Columbia, which all predated Pangea.

While eras encompass tens of millions of years (the length of time varies era by era), eons are even longer — the longest eon was more than a billion years. In fact, eons are so long that humans and dinosaurs occupy the same one, known as the phanerozoic eon. Eras describe long stretches of time within an eon, and are further divided by periods, epochs, and finally ages.

Commonly known as the "Age of Dinosaurs," the Mesozoic Era (derived from the Greek word for “middle life”) began around 252 million years ago at the dawn of the Triassic period, which was then followed by the Jurassic and Cretaceous periods. The Paleogene period, which saw the rise of mammals, was next in the geological line but was part of the Cenozoic Era, meaning “recent life.”