Geologic Time Scale – Major Eras

Scientific evidence indicates that Earth is about 4.6 billion years old – a huge number that is difficult for our human brains to visualize! Over this long period (called deep time), Earth has changed in many ways. The Geologic Time Scale (GTS) is a system that connects rock layers and fossil evidence to time. Scientists use the geologic time scale to describe the timing of events that occurred throughout Earth’s history.

The GTS is divided into many different “units” of time. For example, the Geologic Time Scale is divided into four eras: Precambrian, Paleozoic, Mesozoic, and Cenozoic. These eras – which represent very large periods of time – are further divided into smaller units called periods. Periods are divided into smaller epochs, and epochs are divided into even smaller ages.

For this lesson we will concentrate on the four eras. Each era can be described by the geological (Earth activities) and the biological (living organisms) events that were happening at that time. In other words, the eras are NOT divided into equal units of time. Instead, the eras are divided by major changes occurring in Earth systems and the development of life.

1. Precambrian Era: (4.6 billion to 600 million years ago)

Geology: Beginning with the formation of Earth, the Precambrian Era lasted for most of Earth’s geologic history. During this long period of time, many changes to Earth’s geology were occurring. The hot, molten surface cooled and hardened into crust; oceans and continents formed, and an atmosphere (with a lot of carbon dioxide, but almost no oxygen) developed.

Biology: The Precambrian Era also included the very first life forms. These extremely simple bacteria appeared in the oceans about 3.6 billions years ago. These organisms were photosynthetic, which means that they used energy from the Sun and carbon dioxide to make food. As a result of photosynthesis, the bacteria released oxygen into the atmosphere. Slowly – over a period of hundreds of millions of years – oxygen levels in the atmosphere increased enough to support animal life. Now the oceans had both plant and animal life, although animal life forms were very simple – such as shelled marine organisms.

Boundary Event: However, once conditions on Earth were sufficient to support both plants and animals, new life forms developed very quickly. In a geological “blink of an eye” (meaning a few million years), an astonishing number of new organisms appeared. This event – called the Cambrian Explosion – marks the end of the Precambrian Era and the beginning of the next era – the Paleozoic.

2. Paleozoic Era: (600 to 225 million years ago) “Ancient Life”

Geology: Rock layers from this time period show that the southern continents (Africa, Australia, South America, and India) had similar fossils. This leads scientists to believe that these four southern continents were all joined together in one single, super land mass called Gondwana Land. Toward the end of the Paleozoic Era, shifting tectonic plates brought the rest of the continents together to form the “supercontinent” known as Pangaea.

Biology: During the Paleozoic Era, simple life forms that existed during the Precambrian Era evolved into more complex life forms, including fish and sharks. The ocean was filled with many different forms of life.

The first land plants appeared during this time, followed by animals including insects, spiders, amphibians, and simple reptiles. Small land plants evolved into larger plants such as ferns and early conifers. In fact, it was the massive fern forests of this era that would eventually turn into the vast coal deposits that we use today in the form of fossil fuels!

Boundary Event: Although the Paleozoic Era began with an explosion of life, it ended with an avalanche of death. The largest mass extinction in Earth’s history killed off 96% of all marine life, and 70% of all terrestrial life. This extinction event (called the End Permian Mass Extinction) is believed to have been caused by global warming as the result of massive, ongoing volcanic eruptions. The End Permian extinction event marks the end of the Paleozoic Era and the beginning of the Mesozoic Era.

3. Mesozoic Era: (225 to 65 million years ago) “Middle Life”

Geology: During the Mesozoic Era, the continents that made up Pangaea began drifting apart. As a result, the Atlantic Ocean began to form. The continents slowly began moving closer to the positions they are in today.

Biology: Plants became better adapted to dry land conditions and to seasonal weather changes. The first flowering plants developed during this time.

The Mesozoic Era is called the “Age of the Dinosaurs” because it covers the reign of dinosaurs. These reptiles were the dominant species’ of the time, but other life forms were evolving as well. Turtles, crocodiles, snakes, lizards, and bees all developed during the Mesozoic Era. Primitive birds and small mammals appeared as well.

Boundary Event: The Cretaceous Mass Extinction Event brought the Mesozoic Era to a close. The event – believed to have been caused by an asteroid striking Earth – also brought the reign of dinosaurs to an end.

4. Cenozoic Era: (65 million years ago to today) “New Life”

Geology: During the Cenozoic Era (which continues today), the continents moved to their current positions, and the Atlantic Ocean continued to grow. India and Asia collided with so much force that the Himalayan Mountain Range began to form. Antarctica moved to the South Pole. Cooling climate conditions led to the most recent Ice Age, which occurred about 100,000 years ago.

Biology: This era is known as the “Age of the Mammals.” Following the Cretaceous Mass Extinction, Earth was a very different place. The extinction of the dinosaurs led to the rise of mammals as the dominant life form. Bats, whales, and primates (including humans) evolved during the Cenozoic Era. Birds and fishes continued to evolve as well.