High School Earth Science/History of Earth's Life Forms
In the summer of 1909, an American scientist named Charles Doolittle Walcott (Figure 12.15) was in the Rocky Mountains of British Columbia, Canada. He was a paleontologist, which is a scientist who studies past life on Earth. He was searching for fossils. Riding on horseback, he was making his way down a mountain trail when he noticed something on the ground. He stopped to pick it up. It was a fossil! He began to dig around the area and found even more fossils. The fossils that Walcott found were of some of the most bizarre organisms anyone had ever seen. One of the organisms preserved in the fossils had a soft body like a worm, five eyes, and a long nose like a vacuum cleaner hose (Figure 12.16). Most of the fossils were the remains of animals that do not live today. They are now extinct, which means that nothing of their kind lives and that they are gone forever.
The organisms in Walcott's fossils lived during a time of geologic history known as the Cambrian. The Cambrian period began about 540 million years ago. It marked the beginning of the Phanerozoic Eon. It also marked the beginning of many new and complex life forms appearing on Earth. In fact, the term Phanerozoic means "time of well-displayed life". We still live today in the Phanerozoic Eon. However, life on Earth is very different today than it was 540 million years ago. This lesson covers some of the history of life on Earth. It will show you how living things have developed and changed over the last 540 million years of the Phanerozoic Eon. You will learn about how species adapt and evolve over time.
Lesson Objectives 
- Describe how adaptations develop.
- Explain how the fossil record shows us that species evolve over time.
- Describe the general development of Earth's life forms over the last 540 million years.
Evaluating Prior Knowledge 
Be sure that you can answer the following questions before you begin this lesson.
- What is a fossil?
- How is geologic time divided?
- How do organisms depend on their environment to live?
Earth's Diversity 
There are over 1 million species of plants and animals known to be currently alive on Earth (Figure 12.17). Scientists believe there are millions more that have not been discovered yet. Look around you and you notice that the organisms on this planet have incredible variation. One of the most remarkable features of Earth's organisms is their ability to survive in their specific environments. For example, polar bears have thick fur coats that help them stay warm in the icy waters that they hunt in (Figure 12.18). Reindeer have sponge-like hoofs that help them walk on snowy ground without slipping and falling. Plants that live in dry desert environments have special stems and leaves that help them conserve water.
Other organisms have special features that help them hunt for food or avoid being the food of another organism. For example, when zebras in a herd run away from lions, the zebras' dark stripes confuse the lions and make it hard for them to focus on just one zebra during the chase. Hummingbirds have long thin beaks that help them drink nectar from flowers. Some plants have poisonous or foul-tasting substances in them that keep animals from eating them.
Adaptations and Evolution 
The characteristics of an organism that help it survive in a given environment are called adaptations. Adaptations develop when certain variations in a population help some members survive better than others (Figure 12.19). Often the variation comes from a mutation, or a random change in an organism's genes. The ones that survive pass favorable traits on to their offspring.
To help you understand adaptation, think about a population of oak trees. Imagine that most of the trees are easily killed by a certain fungus but that every now and then, there is one tree that has a natural ability to survive the fungus. That one tree displays a variation that gives it a better chance of surviving its environment. It also has a better chance of living to produce seeds and have offspring. It will reproduce and carry on the species, while the other trees will die off. The tree with the natural ability to resist the fungus will pass that trait on to its offspring. The other trees will not live and have offspring. Eventually the population will change so that most of the individual trees have the trait to survive the fungus. This is an adaptation. Adaptations are inherited traits that an organism gets from its parents. Over time traits that help an organism survive become more common. Traits that hinder survival eventually disappear.
Changes and adaptations in a species accumulate over time. Eventually the descendants are very different from their ancestors and may become a whole new species. Changes in a species over time are called evolution. We learn about evolution from the fossil record. It shows us that many of the life forms that live today developed from earlier, different life forms. For example, horse fossils show us that about 60 million years ago horses were much smaller than they are today (Figure 12.20). Fossils also show us that horses' teeth and hooves have changed several times as horses have adapted to changes in the environment.
Studying the Fossil Record 
Like the organisms that were represented in Walcott's fossils, many of the organisms that once lived on Earth are now extinct. Earth's overall environmental conditions have changed many times since the Cambrian, and many organisms did not have the traits to survive the changes. Those that did survive the changes passed traits on to their offspring. They gave rise to the species that live today.
We study fossils to learn about how species responded to change over the Earth's long history. Fossils show us that simple organisms dominated life on Earth for its first 3 billion years. Then, between 1 and 2 billion years ago, the first multi-cellular organisms appeared on Earth. Life forms gradually evolved and became more complex. During the Cambrian period, animals became more diverse and complex. We sometimes refer to this part of the Phanerozoic Eon as the Cambrian Explosion—meaning a time when the Earth "exploded" with incredible numbers of new complex life forms.
Phanerozoic Eon 
The Phanerozoic Eon is divided into three chunks of time called eras—the Paleozoic, the Mesozoic, and the Cenozoic (Table (12.1). They span from about 540 million years ago to the present. We live now in the Cenozoic Era. The table below shows how life has changed during the long span of the Phanerozoic Eon. Notice that different types of organisms developed at different times. However, all organisms evolved from a common ancestor. Life gradually became more diverse and new species branched out from that common ancestor. Most modern organisms evolved from species that are now extinct. To get an idea of how an organism can change from a single common ancestor to many different types, think about all the different types of dogs. All dogs evolved from a common wolf ancestor. Today there are hundreds of varieties of dogs that all look very different.
|Era||Millions of Years Ago||Major Forms of Life|
|Cenozoic||0.2 (200,000 years ago)||First humans|
|35||First grasses; grasslands begin to dominate the land|
|Mesozoic||130||First plants with flowers|
|150||First birds on Earth|
|200||First mammals on Earth|
|251||Age of dinosaurs begins|
|Paleozoic||300||First reptiles on Earth|
|360||First amphibians on Earth|
|400||First insects on Earth|
|475||First plants and fungi begin growing on land|
|500||First fish on Earth|
The eras of the Phanerozoic Eon are separated by events called mass extinctions. A mass extinction occurs when large numbers of organisms become extinct in a short amount of time. Between the Paleozoic and the Mesozoic, nearly 95% of all species on Earth died off. The cause or causes of this extinction are still being debated.
Between the Mesozoic and the Cenozoic, about 50% of all animal species on Earth died off. This mass extinction, 65 million years ago, is the one in which the dinosaurs became extinct. Although there are other hypotheses, most scientists think that this mass extinction took place when a giant meteorite struck Earth with the energy of the most powerful nuclear weapon. The impact kicked up a massive dust cloud. When the particles rained back onto the surface they heated the atmosphere until it became as hot as a kitchen oven, roasting animals. Dust that remained in the atmosphere blocked sunlight for a year or more, causing a deep freeze and ending photosynthesis. Sulfur from the impact mixed with water in the atmosphere to form acid rain, which dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced by land plants and plankton, animals starved. Carbon dioxide was also released from the impact and eventually caused global warming. Life forms could not survive the dramatic temperature swings.
Earth's climate changed numerous times during the Phanerozoic Eon. Just before the beginning of the Phanerozoic, much of the Earth was cold and covered with glaciers (Figure 12.21). As the Phanerozoic began, however, the climate was changing to a warm and tropical one (Figure 12.22). The glaciers were replaced with tropical seas. This allowed the Cambrian Explosion of many new life forms on Earth. During the Phanerozoic, Earth's climate has gone through at least 4 major cycles between times of cold glaciers and times of warm tropical seas. Some organisms survived environmental changes in the climate; others became extinct when the climate changed beyond their capacity to cope with it.
Lesson Summary 
- Adaptations are favorable traits that organisms inherit. Adaptations develop from variations within a population and help organisms to survive in their given environment.
- Changes in populations accumulate over time; this is called evolution.
- The fossil record shows us that present day life forms evolved from earlier different life forms. It shows us that the first organisms on Earth were simple bacteria that dominated the Earth for several billion years.
- Beginning about 540 million years ago more complex organisms developed on Earth. During the Phanerozoic Eon all of the plant and animal types we know today have evolved.
- Many types of organisms that once lived are now extinct. Eart's overall environment, especially the climate, has changed many times, and organisms change too over time.
Review Questions 
- Describe what is meant by adaptation.
- The first animals on Earth had soft bodies. Gradually many animal species evolved that had hard outer parts called exoskeletons covering their bodies. How might an exoskeleton be a favorable adaptation?
- Explain why unfavorable traits do not usually get passed to offspring.
- List the order in which the major types of animals appeared on Earth.
- How might climate have affected the ability of plants to grow over large areas during a given time?
- One cause of mass extinctions is meteorite or comet impacts. What might be some additional causes of mass extinctions?
- A trait that an organism inherits that helps it survive in its natural environment.
- The change in an organism's traits over time such that a new species is often the result.
- Large sheets of flowing ice.
- A scientist who studies Earth’s past life forms.
- A climate that is warm and humid.
- Having many differences.