12.3 History of Earth’s Life Forms

12.3 History of Earth’s Life Forms

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.

Vocabulary

  • adaptation
  • evolution
  • paleontologist
  • tropical
  • variation

Introduction

In the summer of 1909, an American paleontologist named Charles Doolittle Walcott (Figure below) was searching for fossils. His location was the Rocky Mountains of British Columbia, Canada. Riding on horseback, he noticed a fossil on the ground. He dug around and found even more fossils. These were some of the most bizarre organisms anyone had ever seen! One of the organisms had a soft body like a worm, five eyes, and a long nose like a vacuum cleaner hose. Nothing of the kind is alive today.

Charles Doolittle Walcott, an American paleontologist, discovered the fossils of numerous bizarre organisms.

These organisms lived during the Cambrian Period. The Cambrian marked the beginning of the Phanerozoic Eon. This time began about 540 million years ago. Many new and complex life forms began appearing on Earth. We still live in the Phanerozoic Eon. But life on Earth is very different today than it was 540 million years ago.

Biological Diversity

There are over 1 million species of plants and animals living on Earth today. Scientists think that there are millions more that have not yet been discovered.

Ways to Live in the Environment

Each organism has the ability to survive in a specific environment. Dry desert environments are difficult to live in. Desert plants have special stems and leaves to conserve water. Animals have other ways to live in the desert. The Namib Desert receives only 1.5 inches of rainfall each year. The Namib Desert beetle lives there. How do the beetles get enough water to survive? Early morning fog deposits water droplets. The droplets collect on a beetle's wings and back. The beetle tilts its rear end up. When the droplet is heavy enough, it slides forward. It lands in the beetle’s mouth. There are many other environments that need unique approaches for survival (Figure below).

(A) Peacocks have tremendous feather displays to attract mates. (B) The Namib Desert Beetle has bumps on its back for collecting water. (C) Octopuses use their eight arms to hold on to the ground, hold on to prey and to escape predators. (D) Saguaro cacti are adapted for conserving water in the desert. (E) A mountain hare is well camouflaged in snow in winter.

Getting Food and Being Food (Or Not)

Organisms must be able to get food and avoid being food. Hummingbirds have long, thin beaks that help them drink nectar from flowers. Some flowers are tubular to fit hummingbird beaks. The battle between needing food and being food plays out in the drama between lions and zebras. When a herd of zebras senses a lion, the animals run away. The zebras’ dark stripes confuse the lions. It becomes hard for them to focus on just one zebra. The zebras may get away. But lions are swift and agile. A lion may be able to get a zebra, maybe one that's old or sick.

Variation and Adaptation

Every organism is different from every other organism. Every organism's genes are different, too.

Variations

There are variations in the traits of a population. For example, there are lots of variations in the color of human hair. Hair can be blonde, brown, black, or even red. Hair color is a trait determined by genes.

Mutations

At some point, the variation probably came from a mutation. A mutation is a random change in an organism's genes. Mutations are natural. Some are harmful, but many are neutral. If the trait from the mutation is beneficial, that organism may have a better chance to survive. An organism that survives is likely to have offspring. If it does, it may pass the mutation on to its offspring. The offspring may be more likely to survive.

Genetic mutation is central to the creation of biological diversity.

Adaptations

Some of the characteristics an organism has may help it survive. These characteristics are called adaptations. Some adaptations are better than others.

Adaptations develop this way. Think about a population of oak trees. Imagine that a fungus has arrived from Asia to North America. Most of the North American are killed by the fungus. But a few oak trees have a mutation that allows them to survive the fungus. Those oak trees are better adapted to the new environment than the others. Those trees have a better chance of surviving. They will probably reproduce. The trees may pass on the favorable mutation to their offspring. The other trees will die. Eventually, the population of oak trees will change. Most of the trees will have the trait to survive the fungus. This is an adaptation. Over time, traits that help an organism survive become more common. Traits that hinder survival eventually disappear.

Biological Evolution

Adaptations in a species add up. If the environment is stable, the species won't change. But if the environment is changing, the species will need to adapt. Many adaptations may be necessary. In time, the species may change a lot. The descendants will be very different from their ancestors. They may even become a new species. This process is called evolution. Evolution happens as a species changes over time.

Organisms alive today evolved from earlier life forms. We can learn about this from fossils. For example, horse fossils from 60 million years ago are very different from modern horses. Ancient horses were much smaller than they are today (Figure below). The horses’ teeth and hooves have also changed. The horses evolved because of changes in their environment.

Ancient horses were quite different from present-day horses.

Studying the Fossil Record

Most of the organisms that once lived on Earth are now extinct. Earth’s environment has changed many times. Many organisms could not adapt to the changes. They died out. The organisms that did survive passed traits on to their offspring. The changes added up, eventually producing the species we see today.

We study fossils to see the organisms that lived at certain times. We can see how those organisms changed with time. We can see how they evolved.

Phanerozoic Eon

The Phanerozoic Eon is divided into three eras — the Paleozoic, the Mesozoic, and the Cenozoic (Table below). They span from about 540 million years ago to the present. We live now in the Cenozoic Era.

Earth’s climate changed numerous times during the Phanerozoic Eon. Just before the beginning of the Phanerozoic Eon, much of the Earth was covered with glaciers. As the Phanerozoic Eon began, the climate became a warm and humid tropical climate. 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.

The Cambrian Explosion

The warm, humid climate of the early Cambrian allowed life to expand and diversify. This brought the Cambrian Explosion. Life exploded both in diversity and in quantity!

By the beginning of the Paleozoic, organisms had developed shells. Shells could hold their soft tissues together. They could protect the organisms from predators and from drying out. Some organisms evolved external skeletons, called exoskeletons. Organisms with hard parts also make good fossils. Fossils from the Cambrian are much more abundant than fossils from the Precambrian.

There was much more diversity, so complex ecosystems could develop (Figure below). All of this was in the seas.

Paleozoic Era

Paleozoic life was most diverse in the oceans. Paleozoic seas were full of worms, snails, clams, trilobites, sponges, and brachiopods. Organisms with shells were common.

The first fish were simple, armored, jawless fish. Fish have internal skeletons. Some, like sharks, skates, and rays, have skeletons of cartilage. More advanced fish have skeletons of bones. Fish evolved jaws and many other adaptations for ocean life. Figure below shows some of the diversity of Earth's oceans.

Mudskippers are fish that are able to walk short distances.

Moving onto Land

An organism that lives in water is supported by the water. It does not need strong support structures. It also does not need to be protected against drying out. This is not true of land. Moving from the seas to land required many adaptations.

Algae had covered moist land areas for millions of years. By about 450 million years ago, plants began to appear on land. Once there were land plants, animals had a source of food and shelter. To move to land, animals needed strong skeletons. They needed protection from drying out. They needed to be able to breathe air. Eventually they had skeletons, lungs and the other the adaptations they needed moved onto the land.

Halkieria, or scale worms, are an example of a fossil life from the Cambrian.

One group of fish evolved into amphibians. Insects and spiders were already land dwellers by the time amphibians appeared.

The Mesozoic Era

The Mesozoic Era is the age of reptiles. Mostly we think of it as the age of dinosaurs. Earth was populated by an enormous diversity of reptiles. Some were small and some were tremendously large. Some were peaceful plant eaters. Some were extremely frightening meat eaters. Some dinosaurs developed protection, such as horns, spikes, tail clubs, and shielding plates. These adaptations were defense against active predators.

Most dinosaurs lived on land. Still, pterosaurs flew the skies. Plesiosaurs and ichthyosaurs swam in the oceans (Figure below). Feathered dinosaurs gave rise to birds.

Plesiosaurs were swimming dinosaurs.

The Cenozoic Era

The Cenozoic Era is the age of mammals. The Cenozoic began with the extinction of every land creature larger than a dog. The most famous victims were the dinosaurs.

Mammals have the ability to regulate body temperature. This is an advantage, as Earth’s climate went through sudden and dramatic changes. Mastodons, saber tooth tigers, hoofed mammals, whales, primates and eventually humans all lived during the Cenozoic Era (Figure below).

(A) A sea dragon is a type of fish. (B) African maribou. (C) Elephant shrew. (D) A mountain gorilla mother holding her baby. (E) A dolphin pod.

Table below shows some of the life forms that developed during the Phanerozoic Eon. Life gradually became more diverse and new species appeared. Most modern organisms evolved from species that are now extinct.

Development of Life During the Phanerozoic Eon
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

Mass Extinctions

The eras of the Phanerozoic Eon are separated by mass extinctions. During these events, large numbers of organisms became extinct very rapidly. There have been several extinctions in the Phanerozoic but two stand out more than the others.

Permian Extinction

Between the Paleozoic Era and the Mesozoic Era was the largest mass extinction known. At the end of the Permian, nearly 95% of all marine species died off. In addition, 70% of land species became extinct. No one knows the cause of this extinction. Some scientists blame an asteroid impact. Other scientists think it was a gigantic volcanic eruption.

Cretaceous Extinction

The most famous mass extinction was 65 million years ago. Between the Mesozoic Era and the Cenozoic Era, about 50% of all animal species died off. This mass extinction is when the dinosaurs became extinct. Most scientists think that the extinction was caused by a giant meteorite that struck Earth. The impact heated the atmosphere until it became as hot as a kitchen oven. Animals roasted. Dust flew into the atmosphere and blocked sunlight for a year or more. This caused a deep freeze and ended photosynthesis. Sulfur from the impact mixed with water in the atmosphere. The result was acid rain. The rain dissolved the shells of the tiny marine plankton that form the base of the food chain. With little food being produced, animals starved.

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. Earth’s overall environment, especially the climate, has changed many times. As organisms adapt to changing environmental conditions, new species appear and many become extinct.

Lesson Review Questions

Recall

1. Describe what is meant by adaptation.

Apply Concepts

2. Explain why unfavorable traits do not usually get passed to offspring.

3. List the order in which the major types of animals appeared on Earth.

Think Critically

4. 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?

5. How might climate have affected the ability of plants to grow over large areas during a given time?

6. One cause of mass extinctions is meteorite or comet impacts. What might be some additional causes of mass extinctions?

Points to Consider

  • The processes of evolution are fundamental to much of biology. Why do people have such a hard time understanding them?
  • A lot of organisms are dying out now due to changes in climate and effects of human activities. How does what's happening now resemble a mass extinction?
  • The amount of biodiversity on Earth is staggering. Why are there so many different types of organisms?

For Table above, from top to bottom:

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