Unit E Lesson E17 Geologic Time: The Four Eras
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Lesson E17: Geologic Time: The Four Eras
Video Lesson
Earth has a long and eventful history. In fact, more than 4 billion years of history are available for study by geologists, paleontologists, archeologists, and other scientists. Without a time machine, however, everything in the past has to be learned
from clues and patterns. Science has developed a record of Earth’s past called the geologic time scale based on what has been discovered about the past.
Lesson E17: Geologic Time: The Four Eras
Geologic Time: The Four Eras
Earth formed 4.6 billion years ago from the swirling gases and particles throughout the young universe. The first billion years of Earth’s history were spent without any forms of life anywhere on the planet, and the next billion had only very simple, single-celled organisms.
Four billion years after Earth formed, the very first living things we can call ‘animals’ appeared on Earth. Since then, Earth has experienced extinctions and ice ages, massive volcanic eruptions and earthquakes, and an incredible number of forms of life.
Earth formed 4.6 billion years ago from the swirling gases and particles throughout the young universe. The first billion years of Earth’s history were spent without any forms of life anywhere on the planet, and the next billion had only very simple, single-celled organisms.
Four billion years after Earth formed, the very first living things we can call ‘animals’ appeared on Earth. Since then, Earth has experienced extinctions and ice ages, massive volcanic eruptions and earthquakes, and an incredible number of forms of life.
Reading and Materials for This Lesson
Science in Action 7
Materials:
Science in Action 7
Reading: Pages 419–423
Materials:
1000-sheet roll of toilet paper, scotch tape, rocks or other paperweights, large labels, pen
Science has produced a geologic time scale to try to make sense of all this. Science has developed this scale by comparing sedimentary rock layers, called strata, and the fossils found in these rocks. Imagine the geologic time scale as a many-layered
sedimentary rock. The oldest rocks at the bottom contain ancient materials and the oldest fossils. Layers near the top are younger and contain fossils from the recent past.
The geologic time scale has four large segments called eras. The four eras, from oldest to most recent, are the Precambrian Era, the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. Earth formed at the beginning of Precambrian time; we are living in the Cenozoic Era.
How did scientists decide when one era ends and the next begins? The end of an era on the geologic time scale represents a major extinction event. This means that a very large number of plant and animal species disappeared from the Earth at the boundaries between the eras. Scientists have used these extinctions to help define the eras.
The geologic time scale has four large segments called eras. The four eras, from oldest to most recent, are the Precambrian Era, the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. Earth formed at the beginning of Precambrian time; we are living in the Cenozoic Era.
How did scientists decide when one era ends and the next begins? The end of an era on the geologic time scale represents a major extinction event. This means that a very large number of plant and animal species disappeared from the Earth at the boundaries between the eras. Scientists have used these extinctions to help define the eras.
Figure E.4.17.1 – The geologic time scale shows the development of life throughout Earth’s history.
Studying the Geologic Time Scale
Earth’s history is mind-boggling in many ways. The amount of change is difficult to imagine as the oceans and continents formed, mountain ranges and inland seas came and went, and earthquakes and volcanoes changed the surface. However, comprehending the amount of time this occurred in might be more difficult. The human brain cannot understand fully how big one million of anything is; 4.6 billion years (Yes, that’s 4 600 000 000 years!) is almost impossible to even start to think about.
Science models situations that are too large, too small, or too difficult to observe directly. This is similar to the model used in the video that compared Earth’s history to one 24-hour day. If we put the eras into that day as well, we arrive at the following:
- Precambrian Era: Midnight to 8:55 p.m.
- Paleozoic Era: 8:55 p.m. to 10:50 p.m.
- Mesozoic Era: 10:50 p.m. to 11:40 p.m.
- Cenozoic Era: 11:40 p.m. to midnight
In this model, every second in the day is equivalent to 50 000 years. Try blinking your eyes right now. That takes about ⅓ of a second or, in our model, 17 000 years. You know the Earth is old when the blink of an eye represents 17 000 years!
Figure E.4.17.2 shows that the Precambrian Era makes up most of Earth’s past and that the long history of living things is actually a small percentage of Earth’s existence. If you think of Earth’s history compressed into one day and the dinosaurs became extinct only 20 minutes before the end of the day, you have a sense of the length of history.
Figure E.4.17.2 – When the geologic time scale is shown to represent the amount of time in each era, we can see how much of Earth’s history occurred in Precambrian times.
When One Era Ends and the Next Era Begins
Remember that the eras on the geologic time scale are separated by major extinctions or other important changes in life on Earth. About 99% of all living species throughout Earth’s history are extinct at present. That means only 1% of all species that have ever existed are alive on earth right now.
Remember that the eras on the geologic time scale are separated by major extinctions or other important changes in life on Earth. About 99% of all living species throughout Earth’s history are extinct at present. That means only 1% of all species that have ever existed are alive on earth right now.
Major extinctions occurred for various reasons, such as meteorite or asteroid impacts, massive volcanic eruptions, ice ages, changing ocean levels, or combinations of these and other catastrophes. To discover the causes of these extinctions is very difficult,
but when paleontologists notice drastic changes in the fossil record at a certain time, they are fairly certain a massive extinction event occurred.
About 25 major extinctions have occurred, some that were so large that 70% or more of all living species of plants and animals disappeared. This is a huge reduction in life, and many species were lost forever. However, this allows other species to thrive when they might not have been able to do so before. Perhaps, this is why humans and other mammals have evolved so successfully during the Cenozoic Era after the dinosaurs and other reptiles ruled the Mesozoic era. The one exception is the end of the Precambrian and the beginning of the Paleozoic. Very little life developed on Earth during the long Precambrian era, but then conditions on Earth changed.
About 25 major extinctions have occurred, some that were so large that 70% or more of all living species of plants and animals disappeared. This is a huge reduction in life, and many species were lost forever. However, this allows other species to thrive when they might not have been able to do so before. Perhaps, this is why humans and other mammals have evolved so successfully during the Cenozoic Era after the dinosaurs and other reptiles ruled the Mesozoic era. The one exception is the end of the Precambrian and the beginning of the Paleozoic. Very little life developed on Earth during the long Precambrian era, but then conditions on Earth changed.
Figure E.4.17.3 – Major extinctions wipe out more than 75% of all living species on the planet. Some extinctions are thought to have occurred when asteroids or meteors collided with Earth.
More oxygen became available, and suddenly life began to develop rapidly. The end of the Precambrian era is marked by an explosion of life in the Paleozoic rather than an extinction. In fact, the best answer for the question “What separates the eras?”
is that the beginning of each new era is marked by the opportunity for many new life forms to appear and develop.
As you learn more about the geologic time scale, think about the following questions:
As you learn more about the geologic time scale, think about the following questions:
- Where were the continents as they drifted slowly around earth’s surface?
- What were the dominant forms of life, and on what types of terrain were they living at that time?
- What volcanic eruptions, mountain building, or other major earth events were occurring at that time?
- Was an asteroid just about to fall from the sky and destroy life as it had existed for the past thousand years or more?
Figure E.4.17.4 – Cyanobacteria are very special living things. This is the longest surviving species on Earth. Most people call it ‘pond scum’!
Figure E.4.17.5 – The Blue Whale, the largest animal ever to exist, appeared in the Cenozoic era. We live in the Cenozoic Era, the era that began after the great extinction that wiped out most dinosaurs.
Cenozoic Era
Our era, the Cenozoic Era, began 65 million years ago, just after the great extinction that killed 75% of Earth’s plant and animal species and almost all large dinosaurs. Because of the loss of the large dinosaurs, the Cenozoic Era has allowed the smaller mammal species to flourish, including the ancestors of human beings as well as modern humans. The Cenozoic Era is sometimes called the ‘Age of the Mammals’ because of this.
Many of the animal species that we know are from the Cenozoic Era, including birds, which are the surviving branch of the dinosaur family tree. The Cenozoic is known as the Age of Mammals, but other very important lifeforms of this era are the birds, grasses, and pollinating insects found throughout the world. The earliest of our ancestors appeared about 2 million years ago.
When will the Cenozoic Era end? Eras end when mass extinctions occur, and many scientists believe that we are experiencing another mass extinction due at least partially to human actions. Climate change, habitat loss, overhunting, and overfishing have resulted in a stunning loss of biodiversity and the elimination of many species. The rate of extinction is estimated by some scientists to be so high that they consider our current world to be in the middle of the sixth mass extinction in Earth’s history.
Our era, the Cenozoic Era, began 65 million years ago, just after the great extinction that killed 75% of Earth’s plant and animal species and almost all large dinosaurs. Because of the loss of the large dinosaurs, the Cenozoic Era has allowed the smaller mammal species to flourish, including the ancestors of human beings as well as modern humans. The Cenozoic Era is sometimes called the ‘Age of the Mammals’ because of this.
Many of the animal species that we know are from the Cenozoic Era, including birds, which are the surviving branch of the dinosaur family tree. The Cenozoic is known as the Age of Mammals, but other very important lifeforms of this era are the birds, grasses, and pollinating insects found throughout the world. The earliest of our ancestors appeared about 2 million years ago.
When will the Cenozoic Era end? Eras end when mass extinctions occur, and many scientists believe that we are experiencing another mass extinction due at least partially to human actions. Climate change, habitat loss, overhunting, and overfishing have resulted in a stunning loss of biodiversity and the elimination of many species. The rate of extinction is estimated by some scientists to be so high that they consider our current world to be in the middle of the sixth mass extinction in Earth’s history.
Figure E.4.17.6 – The largest dinosaur to have ever lived is thought to be Argentinosaurus. It lived in South America about 95 million years ago. The Mesozoic Era, also known as “The Age of Dinosaurs”, was the era when most familiar land animal
species first appeared.
Mesozoic Era
The Mesozoic Era is referred to accurately as the ‘Age of the Reptiles’ or ‘Age of the Dinosaurs’. The Mesozoic era began 225 million years ago, just after the largest mass extinction in Earth’s history. It lasted for 160 million years, and it ended 65 million years ago with the extinction that killed all large dinosaur species.
Earth underwent many changes during the Mesozoic Era. The Earth was generally warmer than it is today. In the middle of the Mesozoic Era, the air became warmer and more humid, and life began to flourish. This time is known as the Jurassic Period. Many dinosaur species appeared at this time. Earth experienced many volcanic eruptions and huge earthquakes as the continents continued to break apart. These conditions caused many extinctions, but they also produced conditions that allowed many animals to thrive. This was true especially of dominant animal and plant species during the Mesozoic, the dinosaurs, and plants such as evergreen trees that reproduce using cones. Birds, mammals, and flowering plants appeared during the Mesozoic Era, but none of these organisms could replace the dominant species.
The Mesozoic Era ended with a mass extinction 65 million years ago due to a large asteroid crashing into what is now the Gulf of Mexico. No matter what happened to cause the extinctions, this asteroid crash left a thin layer of sediment all over the world. This sediment contains iridium, which is very rare on Earth, but it is found on asteroids and meteors in space.
The Mesozoic Era is referred to accurately as the ‘Age of the Reptiles’ or ‘Age of the Dinosaurs’. The Mesozoic era began 225 million years ago, just after the largest mass extinction in Earth’s history. It lasted for 160 million years, and it ended 65 million years ago with the extinction that killed all large dinosaur species.
Earth underwent many changes during the Mesozoic Era. The Earth was generally warmer than it is today. In the middle of the Mesozoic Era, the air became warmer and more humid, and life began to flourish. This time is known as the Jurassic Period. Many dinosaur species appeared at this time. Earth experienced many volcanic eruptions and huge earthquakes as the continents continued to break apart. These conditions caused many extinctions, but they also produced conditions that allowed many animals to thrive. This was true especially of dominant animal and plant species during the Mesozoic, the dinosaurs, and plants such as evergreen trees that reproduce using cones. Birds, mammals, and flowering plants appeared during the Mesozoic Era, but none of these organisms could replace the dominant species.
The Mesozoic Era ended with a mass extinction 65 million years ago due to a large asteroid crashing into what is now the Gulf of Mexico. No matter what happened to cause the extinctions, this asteroid crash left a thin layer of sediment all over the world. This sediment contains iridium, which is very rare on Earth, but it is found on asteroids and meteors in space.
Figure E.4.17.7 – The Paleozoic Era was full of new life. It was the first time that lifeforms had flourished on Earth. The Carboniferous portion of this time gave us much of the fossil fuels we use today.
Paleozoic Era
The Paleozoic Era began 600 million years ago and lasted for 375 million years until the largest extinction in Earth’s history occurred 225 million years ago. Earth experienced many changes during the Paleozoic Era, but life flourished more than in any other era. Almost every type of plant and animal species that we know today can trace its ancestry to sometime in the Paleozoic Era.
Plants grew very well during the Paleozoic Era. Many huge swamps were thick with plants. In fact, most fossil fuels that we now use to power our machines were plants that grew during the Paleozoic and later were buried in sedimentary layers and became coal, oil, and gas.
The Paleozoic Era began 600 million years ago and lasted for 375 million years until the largest extinction in Earth’s history occurred 225 million years ago. Earth experienced many changes during the Paleozoic Era, but life flourished more than in any other era. Almost every type of plant and animal species that we know today can trace its ancestry to sometime in the Paleozoic Era.
Plants grew very well during the Paleozoic Era. Many huge swamps were thick with plants. In fact, most fossil fuels that we now use to power our machines were plants that grew during the Paleozoic and later were buried in sedimentary layers and became coal, oil, and gas.
Conditions were so good for life in the water at the beginning of the Paleozoic Era, specifically the Cambrian Period, that it has been nicknamed ‘The Cambrian explosion’ because of the huge number of new forms of life that appeared. Almost every modern
marine species appeared during this time.
Life moved from the water onto the land as the continents broke apart over millions of years. Vast forests provided food and shelter for new animal species. The end of the Paleozoic is marked by the largest extinction in Earth’s history.
Life moved from the water onto the land as the continents broke apart over millions of years. Vast forests provided food and shelter for new animal species. The end of the Paleozoic is marked by the largest extinction in Earth’s history.
Figure E.4.17.8 – The Canadian Shield contains some of the oldest rocks ever discovered on Earth. On this map, the pink, red, and orange colours in the Canadian Shield represent rocks that formed in Precambrian times.
Precambrian Era
The time before the Paleozoic Era is referred to as Precambrian or the Precambrian Era. The Precambrian is a huge period in Earth’s history. It begins with Earth’s formation 4.6 billion years ago and lasts for 4 billion years until the start of the Paleozoic Era 600 million years ago. Precambrian time accounts for nearly 87% (about seven-eighths) of Earth’s entire history.
The very first forms of life appeared on Earth when our planet was one billion years old. These life forms were single-celled and very simple. Two billion years later, multicellular organisms appeared at the end of the Precambrian. Almost all forms of life during the Precambrian were in the water.
The time before the Paleozoic Era is referred to as Precambrian or the Precambrian Era. The Precambrian is a huge period in Earth’s history. It begins with Earth’s formation 4.6 billion years ago and lasts for 4 billion years until the start of the Paleozoic Era 600 million years ago. Precambrian time accounts for nearly 87% (about seven-eighths) of Earth’s entire history.
The very first forms of life appeared on Earth when our planet was one billion years old. These life forms were single-celled and very simple. Two billion years later, multicellular organisms appeared at the end of the Precambrian. Almost all forms of life during the Precambrian were in the water.
Not much is known about Precambrian conditions because it occurred so long ago. The clues that scientists have found are rare, and arranging them to support solid ideas is difficult. Various ideas include a huge collision with a small planet that formed
the Moon and a lack of oxygen in the atmosphere for billions of years.
Unlike the other eras discussed in this section, the Precambrian era did not end with a major extinction. In fact, it ended in the opposite way -- with the Cambrian explosion. Until 600 million years ago, life struggled to do well. The changing conditions that led to the Cambrian explosion of life marks the end of the Precambrian Era.
Unlike the other eras discussed in this section, the Precambrian era did not end with a major extinction. In fact, it ended in the opposite way -- with the Cambrian explosion. Until 600 million years ago, life struggled to do well. The changing conditions that led to the Cambrian explosion of life marks the end of the Precambrian Era.
Try It!
Geologic Time Scale
Models of the geologic time scale are very important for understanding the order of events in Earth’s past and the immense amount of time Earth has been changing.
Models of the geologic time scale are very important for understanding the order of events in Earth’s past and the immense amount of time Earth has been changing.
Try this simple experiment at home, or develop your own model of the geologic time scale. Other ideas you might use to separate Earth’s history into 1000 pieces include taking 1000 steps, measuring 1000 centimetres, or counting to 1000.
Materials:
Materials:
- 1000-sheet roll of toilet paper
- scotch tape
- rocks or other paperweights
- large labels (Post-It notes should work well.)
-
pen
Instructions:
- Tape one end of the toilet paper to a sturdy object. Choose somewhere the entire roll can be unspooled -- try the garage and the driveway on a calm day! Then, unroll the paper gently without tearing it.
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Use paperweights (or a few small stones) to hold the paper in place if there is any wind where you are
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Label one end the toilet paper as 0, and the other end as 1000. The 0 end is the formation of Earth about 4.6 billion years ago. The 1000 end is today. Each square in the roll represents 4.6 million years of time. Wow
- Make the following labels and stick them at the appropriate positions on the paper roll.
Questions:
Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.
Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.
The geologic time scale has 1000 sheets to represent 4.6 billion years.
4.6 billion years ÷ 1000 sheets = 4.6 million years per sheet
4.6 billion years ÷ 1000 sheets = 4.6 million years per sheet
The closest event to the middle of the geologic time scale is the appearance of the first complex cells at 2 billion years ago.
Honeybees gather their food from flowering plants, but such plants appear about 130 million years ago. Therefore, honeybees must have appeared more recently than 130 million years ago. In fact, the earliest fossils of honeybees are about 34 million years
old.
Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to
guide your learning.
Unit E Lesson 17 Self-Check
Instructions
Complete the following 6 steps.
Don't skip steps – if you do them in order, you will confirm your
understanding of this lesson and create a study bank for the future.
- DOWNLOAD the self-check quiz by clicking here.
- ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.
- COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.
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REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct
any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.
- SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.
- CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!
Self-Check Time!
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Self-Check Quiz Answers
Click each of the suggested answers below, and carefully compare your answers to the suggested answers.
If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.
The main strength of the geologic time scale is probably that it shows Earth’s entire history of life in one graphic. Its main weakness is probably that it represents such a large amount of time that understanding the length of these changes is difficult.
A possible nickname for the Paleozoic is “A New Age of Life”. The beginning of the Precambrian Era might be “The Age of Beginning” or “The Struggle for Life”.
A mass extinction is really not a good thing! The species lost in a mass extinction cannot be replaced. However, an argument can be made that a mass extinction “makes room” for the appearance and development of new, different species.
Archaeopteryx is the connection between dinosaurs and birds. Because Archaeopteryx is considered to be a dinosaur and an ancestor to all birds, all birds are considered to come from dinosaurs.
All other eras end with major extinctions, but the Precambrian Era ends with the rapid explosion in forms of life that signals the start of the Paleozoic Era.