Earth’s history is organized into eras and periods within the geologic time scale. This system helps scientists place important events in Earth’s past, such as the appearance and disappearance of new and existing organisms. However, these events did not occur at regular intervals. Because of this, the boundaries between units of time on the geologic time scale are not all the same length.

The Paleozoic Era began about 544 million years ago and lasted until about 245 million years ago. At the beginning of this era, scientists recognized a major event known as the Cambrian explosion. During this time, many new life-forms appeared relatively quickly in the oceans.

The early Paleozoic Era, which includes the Cambrian and Ordovician Periods, is often called the Age of Invertebrates because animals without backbones dominated the seas. At this time, most life lived in the oceans, and little to no life existed on land. Large portions of the continents were covered by shallow inland seas where many marine organisms lived.

The middle Paleozoic Era includes the Silurian and Devonian Periods and is often referred to as the Age of Fishes. During this time, every major group of fish appeared in the oceans. Jawless fish and other early vertebrates, or animals with backbones, evolved. Meanwhile, life began to slowly move onto land. Some of the first land animals included insects and spiders. At the same time, the movement of Earth’s tectonic plates caused continents to collide, forming mountain ranges.

The late Paleozoic Era includes the Carboniferous and Permian Periods and is often called the Age of Amphibians. During this time, amphibians were widespread, and reptiles evolved from amphibian ancestors and began to dominate life on land.  Winged insects, including early forms of cockroaches and dragonflies, also appeared. As the continents continued to collide, they eventually joined together to form a supercontinent called Pangaea. However, the Paleozoic Era ended with a mass extinction that wiped out many marine invertebrates. Scientists continue to study the possible causes of this extinction.

After the Paleozoic Era came the Mesozoic Era, which lasted from about 245 million years ago to 65 million years ago. This era is often called the Age of Reptiles because reptiles, especially dinosaurs, dominated life on land. The Mesozoic Era includes the Triassic, Jurassic, and Cretaceous Periods. During this time, the first birds and small mammals appeared, and new plant types, including flowering plants (angiosperms), began to spread. Meanwhile, the supercontinent Pangaea slowly broke apart, forming the continents and oceans we recognize today. The Mesozoic Era ended with another mass extinction, which many scientists believe was caused by the impact of a large meteorite.

The most recent era is the Cenozoic Era, which began about 65 million years ago and continues today. This era is often called the Age of Mammals because mammals became the dominant animals on Earth. During this time, mammals adapted to many different environments, living on land, in water, and even in the air.

During the early part of the Tertiary Period, major mountain ranges formed as continents continued to move. For example, India collided with Asia to form the Himalayas, and Africa collided with Europe to form the Alps. In North America, the Cascade Range and the Sierra Nevada Mountains also began to form. At the same time, grasses and flowering plants spread across the land, providing new habitats for many animals.

As the Cenozoic Era progressed, mammals continued to evolve and diversify. Eventually, Homo sapiens, or modern humans, appeared about 400,000 years ago. Today, we live in the Holocene Epoch of the Quaternary Period, which represents the most recent part of Earth’s geologic history.

Review:

1. List three characteristics of the Paleozoic Era.
2. List three characteristics of the Mesozoic Era.
3. List three characteristics of the Cenozoic Era.