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1 The Eras of the Earth In this lesson, we will study the changes that have taken place on the earth since it was formed 4.5 billion years ago. There have been big changes in the temperature of the land, the air, and the sea. These changes have caused major differences in the kinds of plants and animals that live on the earth. If we could travel backward in time for millions and billions of years, we would see the surface of the earth has changed very much. Geologists divide the history of the world into four different eras or ages to study those changes. The eras are: Precambrian Era Paleozoic Era Mesozoic Era Cenozoic Era An era is not an exact number of years. It means a long time. The first era lasted four billion years. The other eras have lasted many millions of years. We are still in the fourth era. Era Name Began (years ago) Ended (years ago) Duration (years) Precambrian 4600 million 600 million 4000 million Paleozoic 570 million 345 million 125 million Mesozoic 225 million 65 million 160 million Cenozoic 65 million Present 65 million Other charts place the age of the earth and eras as follows: Era Name Began (years ago) Ended (years ago) Duration (years) Hadean Eon Formation of Earth 3,800 Million 800 Million Eoarchean 3,800 Million 3,600 Million 200 Million Paleoarchean 3,600 Million 3,200 Million 400 Million Mesoarchean 3,200 Million 2,800 Million 400 Million Neoarchean 2,800 Million 2,500 Million 300 Million Paleoproterozoic 2,500 Million 1,600 Million 900 Million Mesoproterozoic 1,600 Million 1,000 Million 600 Million Neoproterozoic 1,000 Million 5,42 Million 458 Million Paleozoic 542 Million 252 Million 290 Million Mesozoic 252 Million 66 Million 196 Million Cenozoic 66 Million Present 66 Million +

2 Precambrian Era The first and longest geologic era was called the Precambrian Era. It was composed of the Periods prior to the Paleozoic Era. At the beginning of he Precambrian, the earth had no solid crust. The first oceans and large bodies of land began to form. There were volcanoes and earthquakes, and the earth was much warmer than it is now. It began to cool slowly over four billion years. Geologists think that during the Precambrian, simple plants lived in the waters of the oceans. In fact, green algae may have been the most common plant found. There may have been some worms or jellyfish in the seas toward the end of the Precambrian, but there was no life on land. There were no animals or people for many millions of years. Here are three important facts about the Precambrian Era: 1. The crust was slowly forming. 2. The earth was slowly cooling. 3. There were only simple plants and animals in the oceans and seas Paleozoic Era The second geologic era is called the Paleozoic Era. It lasted for approximately 290 million to 345 million years. 345 million years is only one-tenth as long as the Precambrian Era, even though 345 million years is still a long time. During this second age in the earth s history, all of the land that had formed began to move together to become one large continent. Some of the first mountains formed when chunks of land bumped into each other. At the same time, the rain washed large amounts of dirt into the sea, and that dirt was changed into rock by the pressure of more dirt on top of it. Much of the rock that is the earth today was formed during the Paleozoic Era Fossils New kinds of plants developed, and some even began to grow on land. Gradually, very simple, very small animals with shells developed in the seas. Much of what geologists know about life on earth from long ago comes from studying fossils. Fossils are the remains of plants and animals that have been preserved like pictures in rocks in the earth. As the plants and animals were preserved in the rock, they created impressions in the rock. Those impressions, called fossils, show the shape and size of those old plants and animals. All the fossils together are called the fossil record of the earth. Geologists can perform tests on fossils to see how old they are. One of these tests is called carbon-14 dating, For this test, scientist measure the number of carbon-14 atoms present in the animal or plant, and they can tell how long ago the plants or animals were alive. Carbon-14 is an isotope of the more stable carbon-12 atom.

3 The fossil record shows geologists that fish developed during the second era, the Paleozoic Era. Fish were the first animals to have bones inside their bodies. Such animals are called vertebrates. Vertebrates refer to any animal that has a backbone. The bones of the first fish in the Paleozoic Era made their bodies strong enough to let some kinds of fish to begin to walk on land. At first, they used their fins as feet, and they pushed themselves along the ground. The first animals to leave the water also had to learn to breathe out of water. These first land animals spent only part of their lives out of water. They are called amphibians. We still have amphibians today, such as frogs and toads. Over many more millions of years, some amphibians developed legs and stayed out of the water all of their lives. They became small, lizard-like animals, which we call reptiles. They lived in wet, swampy areas that covered the earth during the Paleozoic. Insects, such as cockroaches and dragonflies, also developed during the second geologic age. We remember the Paleozoic Era for five things: 1. The land was moving together to form one large mass. 2. The first mountains were pushed up. 3. Rock was forming under the seas. 4. New kinds of plants began to grow on land. 5. New animals were developing in the seas and on land; fish, amphibians, reptiles, and insects. Mesozoic Era The third geologic age began between 225 and 252 million years ago and is called the Mesozoic Era. It lasted between 160 and 196 million years. By the beginning of the third age in the earth s history, the land had come together to make one large continent. Geologists call that continent Pangaea. Slowly the movement of the magma in the earth s mantle began to cause Pangaea to break apart. Different pieces of land began to move in different directions. Geologists call each piece of moving land a plate. Sometimes more than one plate makes up one continent. Sometimes a whole plate is underwater and moves the seafloor with it. The waters of the seas kept flowing into the spaces between the new continents as they spread. And the seafloor itself was dividing where two plates were moving away from each other. We call this movement sea-floor spreading. Many of the animals that had lived in the second age became extinct in the third age. Extinct means they died out, and no more of them lived on the earth. Geologists say that new kinds of plants and animals developed because of the temperature changes. Some of the old forms of life could not live in the cooler temperatures. There were many changes in life forms during the Mesozoic Era. One kind of animal in the third geologic age was the dinosaur. Dinosaur comes from two Greek words, deinos, which means fierce or terrible, and sauros, which means lizard, Dinosaurs looked, like terrible, big lizards. Even though dinosaurs died out long before people lived, we know what they look like from the fossil record. Dinosaurs developed over millions of years, starting with the small lizards of the earlier age. There were dinosaurs of all sizes during the Mesozoic Era. Some grew to be many times larger than the elephants of today. Others were as small as the tiniest birds.

4 In the third age, tropical forests covered the land, so there was plenty of food for the dinosaurs. Some ate plants. Others ate animals, and some even ate other dinosaurs. In every age, many new plants and animals developed. And at the same time all of the continents were slowly moving and drifting away from each other The Dinosaurs Die Out The Mesozoic Era ended about 65 million years ago. The dinosaurs began to disappear. No one can explain why these giant animals died out. No one knows whether it took a few years or a few hundred or thousand years. But the fossil record shows the dinosaurs became extinct very rapidly. The lower Triassic boundary is set by the Permian-Triassic extinction, during which approximately 90% to 96% of marine species and 70% of terrestrial vertebrates became extinct. This period was at the beginning of the Mesozoic Era and before the Jurassic Period. It is also known as the "Great Dying" because it is considered the largest mass extinction in history. The upper Cretaceous boundary is set at the Cretaceous- Tertiary (KT) extinction, which may have been caused by the meteor that created the Chicxulub Crater on the Yucatán Peninsula. Approximately 50% of all genera became extinct, including all of the non-avian dinosaurs. This extinction occurred just before the Cenozoic Era. Some geologists thaink that a tremendous volcano erupted and threw so much dust into the air that a dust cloud covered the whole earth. If that were the case, not enough sunlight could have shone through the clouds to warm the earth. The dinosaurs and many other animals and plants would have died. Other scientists feel that the plate movements cused changes in air and sea temperatur. As the plates moved toward the north and south poles, the weather must have gotten colder on that land. Even the temperature of the seas changed. If ice began to form on the land, the dinosaurs might not have been able to live in such cold temperatures. Still other scientists believe that a very large asteroid hit the earth and stirred up a dust cloud that blocked the sunlight and changed the earth s temperature. Asteroids are large, rock-like bodies in space between Mars and Jupiter. If a large asteroid left its orbit and hit the earth, it could have caused great changes in the earth. For whatever reason, dinosaurs disappeared from the earth. Only some of the smaller reptiles that we still see today survived. Animals such as lizards, snakes, turtles, and crocodiles are examples of the possible dinosaur survivors. We remember the Mesozoic Era for three things: 1. Pangaea broke up and moved the new continents drifted apart. 2. The dinosaurs lived and died in this era. 3. There were great temperature changes as the era ended.

5 Cenozoic Era Finally the fourth era of the earth s history began 65 million years ago. That is the age we live in. The continents had drifted apart to what we know as the world today. We know that the continents are still moving, very slowly, and that some day our current world map will change. For many millions of years in the Cenozoic Era, there were still no human beings on earth. Plants and animals continued to develop on land and in the seas. The number of birds and mammals increased. Mammals are animals like cats, dogs, horses, and elephants, all of which breathe air, bear their young alive instead of laying eggs, and nurse their offspring. Mammals usually have fur or hair on their bodies. All mammals have bones in their bodies (skeletons) instead of shells on the outside. There are mammals in the oceans too. Whales, dolphins, and porpoises all breathe air, nurse their young, and have skeletons of bone. Yet they live in the water all of their lives. Finally human beings developed. We have fossil records, which show that women and men first lived on the earth about 2 million years ago. You can see that people have developed rather recently in earth s history Ice Ages During the Cenozoic Era, there have been four Ice Ages. Four times in our history, great sheets of ice moved south from the North Pole to cover the land in the northern continents of North America, Europe, and Asia. Each Ice Age lasted for thousands of years. Each time, some kinds of animals became extinct because of the temperature changes. And each time, new animals developed. People were living on the earth during those Ice Ages. We know from the fossil record that they lived in many parts of the world. During the Ice Ages, the air was much cooler that it is now. It is likely that there will be another Ice Age in the future, perhaps in several thousand years. However, such changes in the temperatures on the earth take place very slowly. Geologists do not know what causes an Ice Age. We remember the Cenozoic Era for three things: 1. Mammals and birds became more numerous. 2. People developed about 2 million years ago. 3. Four great Ice Ages occurred so far in the Cenozoic Era

6 Lesson Summary Precambrian Era The longest era Lasted for about 4 billion years Accounts for 87% of earth s history Began 4.6 million years ago Ended 570 million years ago The earth s crust begins to cool. Earth s history begins Seas start to form Mountains begin to grow Oxygen builds up in the atmosphere First life forms lived in the sea Then as time passes bacteria, algae, jellyfish, corals, and clams develop. Paleozoic Era Lasted about 345 million years Began 570 million years ago Ended 225 million years ago Consisted of 6 periods Cambrian began 570 million years ago Ended 500 million years ago Lasted for 70 million years Had shallow seas cover parts of the continents Many trilobites, brachiopods, sponges, and other living invertebrates are present Ordovician began 500 million years ago Ended 430 million years ago Lasted 70 million years Many volcanoes and mountains form North America is flooded First fish (jawless) appear Invertebrates flourish in the sea Silurian began 430 million years ago Ended 395 million years ago Lasted for 35 million years Caledonian mountains of Scandinavia rise Coral reefs form First land plants, air breathing animals, and jawed fish develop Devonian Began 395 million years ago Ended 345 million years ago Lasted 50 million years Acadian mountains of New York rise Erosion of mountains deposits much sediment in seas

7 First forests grow in swampy areas First amphibians, sharks, and insects develop Carboniferous began 345 million years ago Ended 280 million years ago Lasted for 65 million years Appalachian Mountains of North America form Ice covers large areas of the earth Swamps cover lowlands First mosses, reptiles, and winged insects appear Great coal forming forest form Seed bearing ferns grow Permian began 280 million years ago Ended 225 million years ago Lasted for 55 million years Ural Mountains of Russia rise First cone bearing plants appear Ferns, fish, amphibians, and reptiles flourish Many sea living invertebrates including trilobites; die out Mesozoic Era Began about 225 million years ago Ended 65 million years ago Lasted for 160 million years Consisted of 3 periods Triassic began 225 million years ago Ended 190 million years ago Lasted 35 million years Palisades of New Jersey and Caucasus mountains of Russia form First dinosaurs and first mammals appear Modern corals, modern fish, and modern insect types develop Jurassic began 190 million years ago Ended 136 million years ago Lasted for 54 million years The Rocky Mountains rise Volcanoes of North America west are active First birds appear Palms and cone bearing trees flourish Largest dinosaurs thrive; primitive mammals develop Cretaceous began 136 million years ago Ended 65 million years ago Lasted for 71 million years First flowering plants appear Placental mammals develop

8 Dinosaurs died out, as do many sea living reptiles Cenozoic Era Began 65 million years ago Still continuing today Consisting of 2 periods Tertiary began 65 million years ago Ended 1.8 million years ago Lasted for 63.2 million years Andes, Alps, and Himalayan mountains rise First horses, primates, and humanlike creatures develop Flowering plants thrive Mammals take on present day features Quaternary began 1.8 million years ago Still continuing today Ice covers large parts of North America and Europe Great lakes form as ice melts First modern human beings appear Wooly mammoths die out Civilization begins. This is the era that we live in.