History of Earth’s Life Forms

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AUTHORCK12 Editor

www.ck12.org Concept 1. History of Earth’s Life Forms

CONCEPT 1 History of Earth’s LifeForms

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 1.1) was searchingfor fossils. His location was the Rocky Mountains of British Columbia, Canada. Riding on horseback, he noticed afossil on the ground. He dug around and found even more fossils. These were some of the most bizarre organismsanyone had ever seen! One of the organisms had a soft body like a worm, five eyes, and a long nose like a vacuumcleaner hose. Nothing of the kind is alive today.

FIGURE 1.1Charles Doolittle Walcott, an American paleontologist, discovered thefossils of numerous bizarre organisms.

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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 stilllive 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 millionsmore 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. TheNamib Desert receives only 1.5 inches of rainfall each year. The Namib Desert beetle lives there. How do the beetlesget enough water to survive? Early morning fog deposits water droplets. The droplets collect on a beetle’s wingsand back. The beetle tilts its rear end up. When the droplet is heavy enough, it slides forward. It lands in the beetle’smouth. There are many other environments that need unique approaches for survival (Figure 1.2).

FIGURE 1.2(A) Peacocks have tremendous featherdisplays to attract mates. (B) The NamibDesert Beetle has bumps on its back forcollecting water. (C) Octopuses use theireight arms to hold on to the ground, holdon to prey and to escape predators. (D)Saguaro cacti are adapted for conservingwater in the desert. (E) A mountain hareis 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 drinknectar from flowers. Some flowers are tubular to fit hummingbird beaks. The battle between needing food and beingfood 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 getaway. 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.

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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’sgenes. 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.

FIGURE 1.3Genetic mutation is central to the creationof biological diversity.

Adaptations

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

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

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Biological Evolution

Adaptations in a species add up. If the environment is stable, the species won’t change. But if the environment ischanging, 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 iscalled 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, horsefossils from 60 million years ago are very different from modern horses. Ancient horses were much smaller thanthey are today (Figure 1.4). The horses’ teeth and hooves have also changed. The horses evolved because of changesin their environment.

FIGURE 1.4Ancient 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. Manyorganisms could not adapt to the changes. They died out. The organisms that did survive passed traits on to theiroffspring. 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 PhanerozoicEon, much of the Earth was covered with glaciers. As the Phanerozoic Eon began, the climate became a warm andhumid tropical climate. During the Phanerozoic, Earth’s climate has gone through at least 4 major cycles betweentimes of cold glaciers and times of warm tropical seas. Some organisms survived environmental changes in theclimate; others became extinct when the climate changed beyond their capacity to cope with it.

[Insert table of eras in Phanerozoic Eon]

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The Cambrian Explosion

The warm, humid climate of the early Cambrian allowed life to expand and diversify. This brought the CambrianExplosion. 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 moreabundant than fossils from the Precambrian.

There was much more diversity, so complex ecosystems could develop (Figure 1.6). 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, andrays, have skeletons of cartilage. More advanced fish have skeletons of bones. Fish evolved jaws and many otheradaptations for ocean life. Figure 1.5 shows some of the diversity of Earth’s oceans.

FIGURE 1.5Mudskippers are fish that are able to walkshort distances.

Moving onto Land

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

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

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

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FIGURE 1.6Halkieria, or scale worms, are an example of a fossil life from theCambrian.

The Mesozoic Era

The Mesozoic Era is the age of reptiles. Mostly we think of it as the age of dinosaurs. Earth was populated byan enormous diversity of reptiles. Some were small and some were tremendously large. Some were peaceful planteaters. 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.

FIGURE 1.7Plesiosaurs 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 thana dog. The most famous victims were the dinosaurs.

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www.ck12.org Concept 1. History of Earth’s Life Forms

Mammals have the ability to regulate body temperature. This is an advantage, as Earth’s climate went throughsudden and dramatic changes. Mastodons, saber tooth tigers, hoofed mammals, whales, primates and eventuallyhumans all lived during the Cenozoic Era (Figure 1.8).

FIGURE 1.8(A) A sea dragon is a type of fish. (B)African maribou. (C) Elephant shrew.(D) A mountain gorilla mother holding herbaby. (E) A dolphin pod.

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

[Insert table showing evolution of organisms during Phanerozoic Eon.]

Mass Extinctions

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

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 thecause of this extinction. Some scientists blame an asteroid impact. Other scientists think it was a gigantic volcaniceruption.

Cretaceous Extinction

The most famous mass extinction was 65 million years ago. Between the Mesozoic Era and the Cenozoic Era, about50% of all animal species died off. This mass extinction is when the dinosaurs became extinct. Most scientiststhink that the extinction was caused by a giant meteorite that struck Earth. The impact heated the atmosphere untilit became as hot as a kitchen oven. Animals roasted. Dust flew into the atmosphere and blocked sunlight for ayear or more. This caused a deep freeze and ended photosynthesis. Sulfur from the impact mixed with water in theatmosphere. The result was acid rain. The rain dissolved the shells of the tiny marine plankton that form the base ofthe food chain. With little food being produced, animals starved.

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Lesson Summary

• Adaptations are favorable traits that organisms inherit. Adaptations develop from variations within a popula-tion 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 andmany 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 calledexoskeletons 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 massextinctions?

Points to Consider

• The processes of evolution are fundamental to much of biology. Why do people have such a hard timeunderstanding them?

• A lot of organisms are dying out now due to changes in climate and effects of human activities. How doeswhat’s happening now resemble a mass extinction?

• The amount of biodiversity on Earth is staggering. Why are there so many different types of organisms?

Opening image courtesy of Miles Orchinik, http://miles-home.smugmug.com/Nature/Yellowstone-journal/9367656_bN4r9#626957014_UdT2X, used with permission.

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