6.1.4 Life in Earth's Past

Learners can:

• identify conditions for life to exist on Earth

• examine the development of life through significant portions of Earth's geologic past

• epoch

• Geologic Time Scale

• period

Prerequisite Vocab

• Precambrian time

• Paleozoic era

• Mesozoic era

• Cenozoic era

New Vocab

the earliest eon of Earth's history

​​Precambrian Time

a major interval of geologic time when primitive life was abundant

Paleozoic Era

the second of Earth's major era characterized by the presence of dinosaurs

Mesozoic Era

the present era noteworthy for the rise of mammals

Cenozoic Era

The Geologic Time Scale is divided into two significant time spans based on the fossil record. The two divisions are:

• Precambrian time; and

• Phanerozoic eon.

The phanerozoic eon is further divided into three distinct eras known as the Paleozoic, Mesozoic, and Cenozoic eras. While Precambrian time is not divided into eras, scientists will often divide this long span of time into sections known as the Proterozoic, Archean, and Hadean.

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Geologic Time Scale

Precambrian Time

Precambrian time is the earliest eon of Earth's history. This time begins with the formation of Earth about 4.6 billion years ago. It lasts until life becomes more abundant during the beginning of the Phanerozoic eon, known as the Cambrian period. It is during the Precambrian time that Earth transitions from an inhospitable planet to one that can begin to sustain life.

​During early Precambrian time, Earth could not support life. Earth's surface would have been molten, and the atmosphere would have contained toxic gases and no oxygen.

Earth's Formation

Temperatures cool, and water vapor condenses, forming liquid water on the Earth's surface. Changes in the atmosphere increase the amount of carbon dioxide, which is necessary for plants.

Changes Occur

While scientists are unsure of the exact circumstances, organic compounds eventually appear on Earth. These compounds become the foundation for life to develop.

Organic Compounds Form

The earliest fossil evidence of life comes from a rock formation in Africa. These rocks have been dated to be about 3.4 billion years old. Fossil evidence indicates that simple bacteria dominated this early life.

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Earth's First Fossil of Life

The next notable shift in life occurred about 3 billion years ago. Scientists estimated that around this time, blue-green algae appeared. These organisms would have performed photosynthesis. This would have started the slow process of introducing oxygen into the atmosphere.

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Ancient Algae

The Phanerozoic eon begins with the Paleozoic era. The Paleozoic era is a major interval of geologic time when primitive life was abundant. This era is further divided into several periods, with the earliest period known as the Cambrian period. The Cambrian period lasted for about 40 million years, spanning the time from about 540 to 500 million years ago.

Scientists believe that the first forms of life to move onto land would have been semi-aquatic algae. The transition of organisms from water to land resulted in significant changes in organisms.

• animals began interacting more with each other and their environments in more complex ways;

• animals began eating other animals, leading to many developing strong skeletons and shells for protection or burrowing into the seafloor for food and shelter; and

• early sponges and bacteria began to form reefs.

In the End...

By the end of the Paleozoic era, during the Permian period, the continents were all joined together into the supercontinent known as Pangaea. The fossil record indicates that significant forms of life existed both in the oceans and on land. Organisms included fish, insects, amphibians, and reptiles. However, around 250 million years ago, an extinction event wiped out more than 95% of Earth's life. This event marked the end of the Paleozoic era and the start of the Mesozoic era and the rise of dinosaurs.

Mesozoic Era

The Mesozoic era is the second of Earth's major eras and is characterized by the presence of dinosaurs. At the beginning of the Mesozoic era, known as the Triassic period, life was relatively scarce, but reptiles quickly became the dominant land and water dwellers. The abundance of reptiles eventually led to the appearance of dinosaurs. Many of the ancestors of present-day reptiles, such as turtles and crocodiles, also appeared during the Mesozoic era.

During the Jurassic period, enormous dinosaurs roamed the land, while aquatic reptiles, such as plesiosaurs, dominated the oceans.

The Jurassic period also included flying reptiles, such as pterosaurs. While the pterosaur is a reptile, it was also during this period that the first birds appeared in the fossil record.

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In the Beginning

Cenozoic Era

The Cenozoic era is the present era and is often referred to as the Age of Mammals. Early in the era, mammals began as small rodent-like creatures. However, over the course of millions of years, larger mammals appeared, such as the ancestors of modern-day camels, elephants, and whales. Also during the early Cenozoic era, various apes appeared in areas of Africa and Asia.

Some of these ground sloths were estimated to grow as large as an elephant. They lived in parts of North and South America.

Ground Sloth

Had thick fur to survive in the coldest regions of North America, Europe, and Asia.

Wooly Mammoth

Type of saber-toothed cat that died out almost 10,000 years ago as the climate changed and human hunting reduced the availability of its prey.

​​Smilodon

Modern Humans

Fossil evidence shows that modern humans first appeared around 200,000 years ago. Through the mid to latter part of the Cenozoic era, between 115,000 and 11,000 years ago, Earth went through various ice ages. Increased hunting of organisms such as mastodons and wooly mammoths most likely contributed to their extinction. With the loss of these food sources, predators such as saber-toothed cats and dire wolves also became extinct.