The average lifespan of a human being takes up a miniscule fraction of time in comparison to Earth’s overall history. If we were to squeeze Earth’s entire 4.5 billion year history into just one year, how much time do you think it took before humans first appeared on this planet?

If Earth formed at exactly 12:01 a.m., January 1, the only signs of life present in the first half of the year would be microscopic, single-celled organisms first appearing near the end of March and living exclusively in the ocean. The development of more complex life, such as hard-shelled mollusks, would not occur until about November 17. On November 20, we would see early forms of fish.

The last week in November would mark the emergence of land plants, along with early ancestors of spiders and scorpions. Amphibians would venture out from the water and inhabit the land starting on December 1. Early reptiles would appear six days later, and dinosaurs would roam Earth from December 12 until late December 25. Early mammals would begin to develop around December 15 and flowering plants would begin to flourish after December 20.

We would need to wait until the evening of December 30—almost an entire year after the formation of the planet—for the emergence of modern humans. That would bring us to “today,” which coincides with midnight, December 31.

 

 
 

Major Subdivisions of Geologic Time

Based on the previous time scale, you should better understand the age of Earth. However, from a geologist’s point of view, it would be impractical to squeeze Earth’s entire history into just one year. Instead of looking at time based on a scale of days and months, they use another method to divide geologic time.

Scientists have divided Earth’s history into major subdivisions called eons, eras, periods and epochs. These time frames are generally based upon the geologic land formations and organisms inhabiting Earth. They are continually revised as scientists learn new things about Earth’s past. Continual improvements in the field of radiometric dating have also improved the accuracy of some of the time estimates for geologic land formations and fossil discoveries upon which these subdivisions are based.

Eons

Eons are the largest division of geologic time. Four eons divide Earth’s entire history: the Hadean, Archaean, Proterozoic, and Phanerozoic eons.

The Hadean (formation of Earth – 3,800 million years ago), Archaean (3,800 – 2,500 m.y.a.), and Proterozoic (2,500 – 542 m.y.a.) eons are collectively known as Precambrian time. These eons span from the formation of the planet, development of early oceans, the first sedimentary rocks, and the first signs of life. During the late Archaean and early Proterozoic eons, scientists speculate that one of the most important events in biological history took place—single-celled photosynthetic organisms emerged and filled the atmosphere with oxygen.

The oxygen-enriched atmosphere ushered in new forms of life on Earth. The beginning of the Phanerozoic eon coincides with the appearance of animals with external skeletons. The Phanerozoic eon dates from 542 million years ago to the present. Although it makes up less than 15 percent of Earth’s entire history, it represents the time during which the majority of life on Earth has existed.

Eras

The Phanerozoic eon is divided into the Paleozoic, Mesozoic, and Cenozoic eras. In general, each era is based upon the group of animals that dominated life on Earth during the time period. This is determined by the prevalence of fossils found dating back to those eras. To that end, the Paleozoic era is called the “Age of Fishes,” the Mesozoic era represents the “Age of Dinosaurs,” and the Cenozoic is called the “Age of Mammals.”

Periods and Epochs

Eras are further divided into periods. Periods are partly based upon rock evidence of major disturbances that occurred in Earth’s crust. There are 11 periods that divide the eras in the Phanerozoic eon. Periods are further divided into epochs.

Piecing Together Time

By applying an understanding of geologic processes and analyzing data collected from radiometric dating, scientists have calculated the age of Earth to be about 4.5 billion years old. Relative and radiometric dating of land formations have provided geologists with an estimated time scale for geologic changes on Earth’s surface. The discovery, analysis, and dating of fossils have provided us with estimated time periods for when these prehistoric plants and animals existed.

Scientists have now begun to piece together these clues to create a detailed story of Earth’s past. This story incorporates the geologic changes on Earth’s surface with the way life has developed since the formation of the planet.