Unit 9.2: The Evolution of Multicellular Life

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1 Unit 9.2: The Evolution of Multicellular 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 single cells. Why did multicellular organisms evolve? What led up to this major step in the evolution of life? To put the evolution of multicellularity in context, let s return to what was happening on planet Earth during this part of its history. 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, Earth experienced 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. 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, and the climate became warmer. When the levels were low, less heat was trapped and the planet cooled. Several times, cooling was severe enough to plunge Earth into an ice age. One ice age was so cold that snow and ice completely covered the planet (see Figure below).

2 Snowball Earth: During the late Precambrian, Earth grew so cold that it was covered with snow and ice. Earth during this ice age has been called 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 not adapt died out. They were replaced by organisms that evolved new adaptations. These adaptations included sexual reproduction, specialization of cells, and multicellularity. Sexual reproduction created much more variety among offspring. This increased the chances that at least some of them would survive when the environment changed. It also increased the speed at which evolution could occur. Some cells started to live together in colonies. In some colonies, cells started to specialize in doing different jobs. This made the cells more efficient as a colony than as individual cells.

3 By 1 billion years ago, the first multicellular organisms had evolved. They may have developed from colonies of 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 much more 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 or even most species abruptly disappear from Earth. There have been five mass extinctions in Earth s history. Many scientists think we are currently going through a sixth mass extinction. What caused the Precambrian mass extinction? A combination of climatic and geologic events was probably responsible. No matter what the cause, the extinction paved the way for a burst of new life during the following Paleozoic Era. 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 into six periods. Major events in each period of the Paleozoic Era are described in Figure below. The era began with a spectacular burst of new life. This is called the Cambrian explosion. The era ended with the biggest mass extinction the world had ever seen. This is known as the Permian extinction.

4 The Paleozoic Era includes the six periods described here. Paleozoic Era The Cambrian Period: Following the Precambrian mass extinction, there was an explosion of new kinds of organisms in the Cambrian Period ( million years ago). Many types of primitive animals called sponges evolved. Small ocean invertebrates called Trilobites became abundant.

5 Two representatives of more than fifty modern animal phyla from the Cambrian explosion are reef-building sponges (left) and early arthropods known as trilobites (right). Both were abundant during the Cambrian and later became extinct; however, the phyla they represent persist to this day. The Ordovician Period: During the next period, the Ordovician Period ( million years ago), the oceans became filled with invertebrates of many types. Also during this period, the first fish evolved and plants colonized the land for the first time. But animals still remained in the water. The Silurian Period: During the Silurian Period ( million years ago), corals appeared in the oceans, and fish continued to evolve. On land, vascular plants appeared. With special tissues to circulate water and other materials, these plants could grow larger than the earlier nonvascular plants.

6 A branching vascular plant with sporangia at the tips of each branch. The Devonian Period: During the Devonian Period ( million years ago), the first seed plants evolved. Seeds have a protective coat and stored food to help them survive. Seed plants eventually became the most common type of land plants. In the oceans, fish with lobefins evolved. They could breathe air when they raised their heads above water. Breathing would be necessary for animals to eventually colonize the land.

7 On land, clubmosses, horsetails, and ferns joined primitive seed plants and early trees to form the first forests. The Carboniferous Period: Next, during the Carboniferous Period ( million years ago), widespread forests of huge plants left massive deposits of carbon that eventually turned to coal. The first amphibians evolved to move out of the water and colonize land, but they had to return to the water to reproduce. Soon after amphibians arose, the first reptiles evolved. They were the first animals that could reproduce on dry land. The Permian Period: During the Permian Period ( million years ago), all the major land masses collided to form a supercontinent called Pangaea. Temperatures were extreme, and the climate was dry. Plants and animals evolved adaptations to dryness, such as waxy leaves or leathery skin to prevent water loss. The Permian Period ended with a mass extinction.

8 The supercontinent Pangaea encompassed all of today In the mass extinction that ended the Permian, the majority of species went extinct. Many hypotheses have been offered to explain why this mass extinction occurred. These include huge meteorites striking Earth and enormous volcanoes spewing ashes and gases into the atmosphere. Both could have darkened the skies with dust for many months. 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 for another 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 245 to 65 million years ago and is divided into the three periods described in Figure below. The Mesozoic began with the supercontinent Pangaea. Then, during the era, Pangaea broke up and the continents drifted apart. The movement of continents 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 for a dinosaur takeover. In the second extinction, the dinosaurs finally disappeared.

9 Mesozoic Era: The Mesozoic Era consists of the three periods described here. The Triassic Period: During the Triassic Period ( million years ago), the first dinosaurs branched off from the reptiles and colonized the land, air, and water. Huge seed ferns and conifers dominated the forests, and modern corals, fish, and insects evolved. The supercontinent Pangaea started to separate into Laurasia (today s Northern Hemisphere continents) and Gondwanaland (today s Southern Hemisphere continents). The Triassic Period ended with a mass extinction. The Jurassic Period: The next period, the Jurassic Period ( million years ago), began after the mass extinction 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 plants appeared for the first time, and new insects also evolved to pollinate the flowers. The continents continued to move apart, and volcanic activity was especially intense.

10 The Cretaceous Period: During the Cretaceous Period ( million years ago), dinosaurs reached their peak in size and distribution. Tyrannosaurus Rex, weighed at least 7 tons. By the end of the Cretaceous, the continents were close to their present locations. Earth s overall climate was warm; even the poles lacked ice. The period ended with the dramatic extinction of the dinosaurs. What happened to the dinosaurs? Why did they go extinct at the end of the Cretaceous Period? Some scientists think a comet or asteroid may have collided with Earth, causing skies to darken, photosynthesis to shut down, and climates to change. A collision was probably at least a contributing factor. Without the dinosaurs, there were many opportunities for new organisms to exploit in the next era, the Cenozoic. Which living things do you think took over where the dinosaurs left off? What happened to the dinosaurs? Why did they go extinct at the end of the Cretaceous Period? Some scientists think a comet or asteroid may have collided with Earth, causing skies to darken, photosynthesis to shut down, and climates to change. A collision was probably at least a contributing factor. Without the dinosaurs, there were many opportunities for new organisms to exploit in the next era, the Cenozoic. Which living things do you think took over where the dinosaurs left off? Cenozoic Era: Age of Mammals The Cenozoic Era literally means the era of modern life. It is also called the age of mammals. Mammals took advantage of the extinction of the dinosaurs. They flourished and soon became the dominant animals on Earth. You can learn more about the evolution of mammals during the Cenozoic at the link below. The Cenozoic began 65 million years ago and continues to the present. It may be divided into the two periods described in Figure below.

11 Cenozoic Era: One way of dividing the Cenozoic Era is into the two periods described here. The Tertiary Period: During the Tertiary Period ( million years ago), Earth s climate was generally warm and 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, and flowering 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 land bridges between continents, allowing land animals to move to new areas. Some mammals, like the woolly

12 mammoths adapted to the cold by evolving very large size and thick fur. Other animals moved closer to the equator or went extinct, along with many plants. The last ice age ended about 12,000 years ago. By that time, our own species, Homo sapiens, had evolved. After that, we were witnesses to the unfolding of life s story. Although we don t know all the details of the recent past, it is 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 can see. Africa? Maybe. But this also describes Northern California at the end of the last Ice Age. What happened to all this wildlife? Were they over hunted and killed off? Did global warming destroy their populations? Scientists are not sure, but this relatively recent loss of life does raise many interesting questions. Lesson Summary During the late Precambrian, continents drifted, carbon dioxide levels fluctuated, and climates changed. Many organisms could not survive the changes and died out. Others evolved important new adaptations. These include sexual reproduction, cell specialization, and multicellularity. The Precambrian ended with a mass extinction. 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 the water, and in the air. Mammals also evolved but were small in size. Flowering plants appeared for the first time. 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 12,000 years 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.

13 5. Describe how the continents shifted during the Mesozoic Era. 6. What explains why mammals were able to flourish during Cenozoic Era? Apply Concepts 7. Create a timeline of major evolutionary events during the Mesozoic and Cenozoic Eras. Include approximate dates 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?

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