The Evolution of MulticellularLife

Say Thanks to the AuthorsClick http://www.ck12.org/saythanks

(No sign in required)

To access a customizable version of this book, as well as otherinteractive content, visit www.ck12.org

CK-12 Foundation is a non-profit organization with a mission toreduce the cost of textbook materials for the K-12 market both inthe U.S. and worldwide. Using an open-source, collaborative, andweb-based compilation model, CK-12 pioneers and promotes thecreation and distribution of high-quality, adaptive online textbooksthat can be mixed, modified and printed (i.e., the FlexBook®textbooks).

Copyright © 2016 CK-12 Foundation, www.ck12.org

The names “CK-12” and “CK12” and associated logos and theterms “FlexBook®” and “FlexBook Platform®” (collectively“CK-12 Marks”) are trademarks and service marks of CK-12Foundation and are protected by federal, state, and internationallaws.

Any form of reproduction of this book in any format or medium,in whole or in sections must include the referral attribution linkhttp://www.ck12.org/saythanks (placed in a visible location) inaddition to the following terms.

Except as otherwise noted, all CK-12 Content (including CK-12Curriculum Material) is made available to Users in accordancewith the Creative Commons Attribution-Non-Commercial 3.0Unported (CC BY-NC 3.0) License (http://creativecommons.org/licenses/by-nc/3.0/), as amended and updated by Creative Com-mons from time to time (the “CC License”), which is incorporatedherein by this reference.

Complete terms can be found at http://www.ck12.org/about/terms-of-use.

Printed: May 10, 2016

www.ck12.org Chapter 1. The Evolution of Multicellular Life

CHAPTER 1 The Evolution ofMulticellular Life

Lesson Objectives

• Describe important events of the late Precambrian.• Give an overview of evolution during the Paleozoic Era.• Explain why the Mesozoic Era is called the age of the dinosaurs.• Outline the main evolutionary events of the Cenozoic Era.

Vocabulary

• Cambrian explosion• Cenozoic Era• mass extinction• Mesozoic Era• Palezoic Era• Permian extinction

Introduction

Nearly 80% of Earth’s history passed before multicellular life evolved. Up until then, all organisms existed as singlecells. Why did multicellular organisms evolve? What led up to this major step in the evolution of life? To put theevolution of multicellularity in context, let’s return to what was happening on planet Earth during this part of itshistory.

Setting the Stage: The Late Precambrian

The late Precambrian is the time from about 2 billion to half a billion years ago. During this long span of time, Earthexperienced many dramatic geologic and climatic changes.

• Continents drifted. They collided to form a gigantic supercontinent and then broke up again and moved apart.Continental drift changed climates worldwide and caused intense volcanic activity. To see an animation ofcontinental drift, go to this link: http://www.ucmp.berkeley.edu/geology/anim1.html .

• Carbon dioxide levels in the atmosphere rose and fell. This was due to volcanic activity and other factors.When the levels were high, they created a greenhouse effect. More heat was trapped on Earth’s surface, andthe climate became warmer. When the levels were low, less heat was trapped and the planet cooled. Severaltimes, cooling was severe enough to plunge Earth into an ice age. One ice age was so cold that snow and icecompletely covered the planet (see Figure 1.1).

1

www.ck12.org

FIGURE 1.1Snowball Earth. During the late Precam-brian, Earth grew so cold that it was cov-ered with snow and ice. Earth during thisice age has been called “snowball Earth.”

Life During the Late Precambrian

The dramatic changes of the late Precambrian had a major impact on Earth’s life forms. Living things that could notadapt died out. They were replaced by organisms that evolved new adaptations. These adaptations included sexualreproduction, specialization of cells, and multicellularity.

• Sexual reproduction created much more variety among offspring. This increased the chances that at least someof them would survive when the environment changed. It also increased the speed at which evolution couldoccur.

• Some cells started to live together in colonies. In some colonies, cells started to specialize in doing differentjobs. This made the cells more efficient as a colony than as individual cells.

• By 1 billion years ago, the first multicellular organisms had evolved. They may have developed from coloniesof specialized cells. Their cells were so specialized they could no longer survive independently. However,together they were mighty. They formed an organism that was bigger, more efficient, and able to do muchmore than any single-celled organism ever could.

The Precambrian Extinction

At the close of the Precambrian 544 million years ago, a mass extinction occurred. In a mass extinction, many oreven most species abruptly disappear from Earth. There have been five mass extinctions in Earth’s history. Manyscientists think we are currently going through a sixth mass extinction.

What caused the Precambrian mass extinction? A combination of climatic and geologic events was probablyresponsible. No matter what the cause, the extinction paved the way for a burst of new life during the followingPaleozoic Era.

2

www.ck12.org Chapter 1. The Evolution of Multicellular Life

Life During the Paleozoic

The Paleozoic Era is literally the era of “old life.” It lasted from 544 to 245 million years ago and is divided intosix periods. Major events in each period of the Paleozoic Era are described in Figure 1.2. The era began with aspectacular burst of new life. This is called the Cambrian explosion. The era ended with the biggest mass extinctionthe world had ever seen. This is known as the Permian extinction. At the following link, you can watch a videoabout these and other events of the Paleozoic Era: http://www.youtube.com/watch?v=Bf2rrRmconU .

FIGURE 1.2The Paleozoic Era includes the six peri-ods described here.

3

www.ck12.org

Paleozoic Era

The Cambrian Period: Following the Precambrian mass extinction, there was an explosion of new kinds oforganisms in the Cambrian Period (544-505 million years ago). Many types of primitive animals called spongesevolved. Small ocean invertebrates called Trilobites became abundant.

FIGURE 1.3Two representatives of more than fiftymodern animal phyla from the Cambrianexplosion are reef-building sponges (left)and early arthropods known as trilobites(right). Both were abundant during theCambrian and later became extinct; how-ever, the phyla they represent persist tothis day.

The Ordovician Period: During the next period, the Ordovician Period (505-440 million years ago), the oceansbecame filled with invertebrates of many types. Also during this period, the first fish evolved and plants colonizedthe land for the first time. But animals still remained in the water.

The Silurian Period: During the Silurian Period (440-410 million years ago), corals appeared in the oceans, andfish continued to evolve. On land, vascular plants appeared (Figure 1.4). With special tissues to circulate water andother materials, these plants could grow larger than the earlier nonvascular plants.

FIGURE 1.4Cooksonia, a branching vascular plant with sporangia at the tips of eachbranch. Cooksonia fossils measure just centimeters in height and datefrom the Silurian period.

The Devonian Period: During the Devonian Period (410-360 million years ago), the first seed plants evolved(Figure 1.5). Seeds have a protective coat and stored food to help them survive. Seed plants eventually became themost common type of land plants. In the oceans, fish with lobe fins evolved. They could breathe air when they raisedtheir heads above water. Breathing would be necessary for animals to eventually colonize the land.

The Carboniferous Period: Next, during the Carboniferous Period (360-290 million years ago), widespread forestsof huge plants left massive deposits of carbon that eventually turned to coal. The first amphibians evolved to move

4

www.ck12.org Chapter 1. The Evolution of Multicellular Life

FIGURE 1.5On land, clubmosses, horsetails, andferns joined primitive seed plants andearly trees to form the first forests.

out of the water and colonize land, but they had to return to the water to reproduce. Soon after amphibians arose, thefirst reptiles evolved. They were the first animals that could reproduce on dry land.

The Permian Period: During the Permian Period (290-245 million years ago), all the major land masses collidedto form a supercontinent called Pangaea (Figure 1.6). Temperatures were extreme, and the climate was dry. Plantsand animals evolved adaptations to dryness, such as waxy leaves or leathery skin to prevent water loss. The PermianPeriod ended with a mass extinction.

FIGURE 1.6The supercontinent Pangaea encom-passed all of today’s continents in a sin-gle land mass. This configuration limitedshallow coastal areas which harbor ma-rine species, and may have contributedto the dramatic event which ended thePermian - the most massive extinctionever recorded.

In the mass extinction that ended the Permian, the majority of species went extinct. Many hypotheses have been

5

www.ck12.org

offered to explain why this mass extinction occurred. These include huge meteorites striking Earth and enormousvolcanoes spewing ashes and gases into the atmosphere. Both could have darkened the skies with dust for manymonths. This, in turn, would have shut down photosynthesis and cooled the planet.

Despite the great loss of life, there was light at the end of the tunnel. The Permian extinction paved the way foranother burst of new life at the start of the following Mesozoic Era. This included the evolution of the dinosaurs.

Mesozoic Era: Age of Dinosaurs

The Mesozoic Era is literally the era of “middle life.” It is also known as the age of dinosaurs. It lasted from 245to 65 million years ago and is divided into the three periods described in Figure 1.7. The Mesozoic began with thesupercontinent Pangaea. Then, during the era, Pangaea broke up and the continents drifted apart. The movement ofcontinents changed climates. It also caused a lot of volcanic activity.

Mass extinctions occurred at the end of the Triassic and Cretaceous Periods. The first extinction paved the way fora dinosaur takeover. In the second extinction, the dinosaurs finally disappeared. At the link below, you can watcha video about these and other exciting events during the age of dinosaurs. http://www.youtube.com/watch?v=watgb11LOHE

The Triassic Period: During the Triassic Period (245-200 million years ago), the first dinosaurs branched off fromthe reptiles and colonized the land, air, and water. Huge seed ferns and conifers dominated the forests, and moderncorals, fish, and insects evolved. The supercontinent Pangaea started to separate into Laurasia (today’s NorthernHemisphere continents) and Gondwanaland (today’s Southern Hemisphere continents). The Triassic Period endedwith a mass extinction.

The Jurassic Period: The next period, the Jurassic Period (200-145 million years ago), began after the massextinction that ended the Triassic Period. This mass extinction allowed dinosaurs to flourish in the Jurassic Period.This was the golden age of dinosaurs. Also during the Jurassic, the earliest birds evolved from reptile ancestors,and all the major groups of mammals evolved, but individual mammals were still small in size. Flowering plantsappeared for the first time, and new insects also evolved to pollinate the flowers. The continents continued to moveapart, and volcanic activity was especially intense.

The Cretaceous Period: During the Cretaceous Period (145-65 million years ago), dinosaurs reached their peak insize and distribution. Tyrannosaurus Rex, weighed at least 7 tons. By the end of the Cretaceous, the continents wereclose to their present locations. Earth’s overall climate was warm; even the poles lacked ice. The period ended withthe dramatic extinction of the dinosaurs.

What happened to the dinosaurs? Why did they go extinct at the end of the Cretaceous Period? Some scientiststhink a comet or asteroid may have collided with Earth, causing skies to darken, photosynthesis to shut down, andclimates to change. A collision was probably at least a contributing factor. Without the dinosaurs, there were manyopportunities for new organisms to exploit in the next era, the Cenozoic. Which living things do you think took overwhere the dinosaurs left off?

What happened to the dinosaurs? Why did they go extinct at the end of the Cretaceous Period? Some scientiststhink a comet or asteroid may have collided with Earth, causing skies to darken, photosynthesis to shut down, andclimates to change. A collision was probably at least a contributing factor. Without the dinosaurs, there were manyopportunities for new organisms to exploit in the next era, the Cenozoic. Which living things do you think took overwhere the dinosaurs left off?

6

www.ck12.org Chapter 1. The Evolution of Multicellular Life

FIGURE 1.7Mesozoic Era. The Mesozoic Era consists of the three periods described here.

Cenozoic Era: Age of Mammals

The Cenozoic Era literally means the era of “modern life.” It is also called the age of mammals. Mammals tookadvantage of the extinction of the dinosaurs. They flourished and soon became the dominant animals on Earth. Youcan learn more about the evolution of mammals during the Cenozoic at the link below. The Cenozoic began 65million years ago and continues to the present. It may be divided into the two periods described in Figure 1.8. http://www.youtube.com/watch?v=H0uTGkCWXwQ

7

www.ck12.org

FIGURE 1.8Cenozoic Era. One way of dividing the Cenozoic Era is into the two periods described here.

The Tertiary Period: During the Tertiary Period (65-1.8 million years ago), Earth’s climate was generally warmand humid. Mammals evolved to fill virtually all niches vacated by dinosaurs. Many mammals increased in size.Mammals called primates evolved, including human ancestors. Modern rain forests and grasslands appeared, andflowering plants and insects were numerous and widespread.

The Quaternary Period: During the Quaternary Period (1.8 million years ago-present), Earth’s climate cooled,leading to a series of ice ages. Sea levels fell because so much water was frozen in glaciers. This created landbridges between continents, allowing land animals to move to new areas. Some mammals, like the woolly mammothsadapted to the cold by evolving very large size and thick fur. Other animals moved closer to the equator or wentextinct, along with many plants.

8

www.ck12.org Chapter 1. The Evolution of Multicellular Life

The last ice age ended about 12,000 years ago. By that time, our own species, Homo sapiens, had evolved. Afterthat, we were witnesses to the unfolding of life’s story. Although we don’t know all the details of the recent past, itis far less of a mystery than the billions of years that preceded it.

KQED: The Last Ice Age

Imagine a vast grassy ecosystem covered with herds of elephants, bison and camels stretching as far as the eye cansee. Africa? Maybe. But this also describes Northern California at the end of the last Ice Age. What happened to allthis wildlife? Were they over hunted and killed off? Did global warming destroy their populations? Scientists arenot sure, but this relatively recent loss of life does raise many interesting questions.

MEDIAClick image to the left or use the URL below.URL: https://www.ck12.org/flx/render/embeddedobject/524

Lesson Summary

• During the late Precambrian, continents drifted, carbon dioxide levels fluctuated, and climates changed. Manyorganisms could not survive the changes and died out. Others evolved important new adaptations. Theseinclude sexual reproduction, cell specialization, and multicellularity. The Precambrian ended with a massextinction. It paved the way for the Cambrian explosion.

• The Paleozoic Era began with the Cambrian explosion. It ended with the Permian extinction. During the era,invertebrate animals diversified in the oceans. Plants, amphibians, and reptiles also moved to the land.

• The Mesozoic Era is the age of dinosaurs. They evolved from earlier reptiles to fill niches on land, in thewater, and in the air. Mammals also evolved but were small in size. Flowering plants appeared for the firsttime. Dinosaurs went extinct at the end of the Mesozoic.

• The Cenozoic Era is the age of mammals. They evolved to fill virtually all the niches vacated by dinosaurs.The ice ages of the Quaternary Period of the Cenozoic led to many extinctions. The last ice age ended 12,000years ago. By that time, Homo sapiens had evolved.

Lesson Review Questions

Recall

1. Describe geologic and climatic changes that occurred during the late Precambrian.

2. What is a mass extinction?

3. What is the relationship between the Precambrian extinction and the Cambrian explosion?

4. List several important evolutionary events that occurred during the Paleozoic Era.

5. Describe how the continents shifted during the Mesozoic Era.

6. What explains why mammals were able to flourish during Cenozoic Era?

9

www.ck12.org

Apply Concepts

7. Create a timeline of major evolutionary events during the Mesozoic and Cenozoic Eras. Include approximatedates in your timeline.

Think Critically

8. Explain the evolutionary advantages of sexual reproduction and multicellularity.

9. Relate the Permian, Triassic, and Cretaceous extinctions to the evolution and extinction of the dinosaurs.

10. Compare and contrast the Tertiary and Quaternary Periods of the Cenozoic Era.

Points to Consider

The human species evolved during the Cenozoic Era. The scientific name of the human species is Homo sapiens.

• Do you know what this name means? Do you know why species are given scientific names?• What is a species? What determines whether a group of organisms is considered a species?

References

1. Neethis. http://commons.wikimedia.org/wiki/File:Fictional_Snowball_Earth_1_Neethis.jpg . The copyrightholder of this work allows anyone to use it for any purpose including unrestricted redistribution, commercialuse, and modification

2. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.03. Left: Stanton F. Fink; Right: User:TheoricienQuantique/Wikimedia Commons. Left: http://commons.wik

imedia.org/wiki/File:Archaeocyatha.jpg; Right: http://commons.wikimedia.org/wiki/File:Asaphiscus_Wheeleri_3.jpg . Left: CC BY 2.5; Right: Public Domain

4. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.05. Eduard Riou. http://commons.wikimedia.org/wiki/File:Devonianscene.jpg . Public Domain6. Laura Guerin. CK-12 Foundation . CC BY-NC 3.07. Dinosaur images from top to bottom: Nobu Tamura; Image copyright Computer Earth, 2014; Charles R.

Knight. Dinosaur images from top to bottom: http://commons.wikimedia.org/wiki/File:Staurikosaurus_BW.jpg; http://www.shutterstock.com; http://commons.wikimedia.org/wiki/File:T._rex_old_posture.jpg . Di-nosaur images from top to bottom: CC BY 3.0; Used under license from Shutterstock.com; Public Domain

8. Top image: Gustav Mützel; Bottom image: Mauricio Antón. Top image: http://commons.wikimedia.org/wiki/File:Gelber_Babuin-drawing.jpg; Bottom image: http://commons.wikimedia.org/wiki/File:Woolly_mammoth_siberian_tundra.jpg . Top image: Public Domain; Bottom image: CC BY 2.5

10