history of life on earth

cretaceous period

During the Cretaceous Period, the dinosaurs reached their maximum size and distribution. For example, the well- known Tyrannosaurus rex weighed at least 7 tons! You can get an idea of how big it was from the T. rex skeleton in Figure 7.24. (Notice how small the person looks in the bottom left of the photo.) By the end of the Cretaceous, the continents were close to their present locations. The period ended with another mass extinction. This time, the dinosaurs went extinct. What happened to the dinosaurs? Some scientists think that a comet or asteroid may have crashed into Earth. This could darken the sky, shut down photosynthesis, and cause climate change. Other factors probably contributed to the mass extinction as well.

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cenozoic era

The extinction of the dinosaurs at the end of the Mesozoic Era paved the way for mammals to take over. Thats why the Cenozoic Era is called the age of mammals. They soon became the dominant land animals on Earth. The Cenozoic is divided into two periods.

tertiary period

During the Tertiary Period, many new kinds of mammals evolved. For example, primates and human ancestors first appeared during this period. Many mammals also increased in size. Modern rain forests and grasslands appeared. Flowering plants and insects increased in numbers.

quaternary period

During the Quaternary Period, the climate cooled. This caused a series of ice ages. Glaciers advanced southward from the North Pole. They reached as far south as Chicago and New York City. Sea levels fell because so much water was frozen in glaciers. This exposed land bridges between continents. The land bridges allowed land animals to move to new areas. Some mammals adapted to the cold by evolving very large size and thick fur. An example is the woolly mammoth, shown in Figure 7.25. Other mammals moved closer to the equator. Those that couldnt adapt or move went extinct, along with many plants. The last ice age ended about 12,000 years ago. By then, our own species, Homo sapiens, had evolved. After that, we were eyewitnesses to the story of life. As a result, the recent past is less of a mystery than the billions of years before it.

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new adaptations

The earliest cells were heterotrophs. They were unable to make food. Instead, they got energy by "eating" organic molecules in the soup around them. The earliest cells were also prokaryotes. They lacked a nucleus and other organelles. Gradually, these and other traits evolved. Photosynthesis evolved about 3 billion years ago. After that, certain cells could use sunlight to make food. These were the first autotrophs. They made food for themselves and other cells. They also added oxygen to the atmosphere. The oxygen was a waste product of photosynthesis. Oxygen was toxic to many cells. They had evolved in its absence. Many of them died out. The few that survived evolved a new way to use oxygen. They used it to get energy from food. This is the process of cellular respiration. The first eukaryotic cells probably evolved about 2 billion years ago. Thats when cells evolved organelles and a nucleus. Figure 7.20 shows one theory about the origin of organelles. According to this theory, a large cell engulfed small cells. The small cells took on special roles that helped the large cell function. In return, the small cells got nutrients from the large cell. Eventually, the large and small cells could no longer live apart. With their specialized organelles, eukaryotic cells were powerful and efficient. Eukaryotes would go on to evolve sexual reproduction. They would also evolve into multicellular organisms. The first multicellular organisms evolved about 1 billion years ago.

precambrian mass extinction

At the end of the Precambrian, a mass extinction occurred. In a mass extinction, the majority of species die out. The Precambrian mass extinction was the first of six mass extinctions that occurred on Earth. Its not certain what caused this first mass extinction. Changes in Earths geology and climate were no doubt involved.

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geologic time

Its hard to grasp the vast amounts of time since Earth formed and life first appeared. It may help to think of Earths history as a 24-hour day.

earth in a day

Figure 7.17 shows the history of Earth in a day. In this model, the planet forms at midnight. The first prokaryotes evolve around 3:00 am. Eukaryotes evolve at about 1:00 pm. Animals dont evolve until almost 8:00 pm. Humans appear only in the last minute of the day. Relating these major events in Earths history to a 24-hour day helps to put them in perspective.

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ancient earth

When Earth first formed, it was a fiery hot, barren ball. It had no oceans or atmosphere. Rivers of melted rock flowed over its surface. Gradually, the planet cooled and formed a solid crust. Gases from volcanoes formed an atmosphere, although it contained only a trace of oxygen. As the planet continued to cool, clouds formed and rain fell. Rainwater helped form oceans. The ancient atmosphere and oceans would be toxic to modern life, but they set the stage for life to begin.

first organic molecules and cells

All living things consist of organic molecules. Many scientists think that organic molecules evolved before cells, perhaps as early as 4 billion years ago. Its possible that lightning sparked chemical reactions in Earths early atmosphere. This could have created a soup of organic molecules from inorganic chemicals. Some scientists think that RNA was the first organic molecule to evolve. RNA can not only encode genetic instructions. Some RNA molecules can carry out chemical reactions. All living things are made of one or more cells. How the first cells evolved is not known for certain. Scientists speculate that lipid membranes grew around RNA molecules. The earliest cells may have consisted of little more than RNA inside a lipid membrane. You can see a model of such a cell in Figure 7.19. The first cells probably evolved between 3.8 and 4 billion years ago. Scientists think that one cell, called the Last Universal Common Ancestor (LUCA), gave rise to all of the following life on Earth. LUCA may have existed around 3.5 billion years ago.

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geologic time scale

Another tool for understanding the history of Earth and its life is the geologic time scale. You can see this time scale in Figure 7.18. It divides Earths history into eons, eras, and periods. These divisions are based on major changes in geology, climate, and the evolution of life. The geologic time scale organizes Earths history on the basis of important events instead of time alone. It also puts more focus on recent events, about which we know the most.

precambrian lifes origins

The Precambrian Supereon is the first major division of Earths history (see Figure 7.18). It covers the time from Earths formation 4.6 billion years ago to 544 million years ago. To see how life evolved during the Precambrian and beyond, watch this wonderful video. Its a good introduction to the rest of the lesson. MEDIA Click image to the left or use the URL below. URL:

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silurian period

Corals appeared in the oceans during the Silurian period. Fish continued to evolve. On land, vascular plants appeared. These are plants that have special tissues to circulate water and other substances. This allowed plants to become larger and colonize drier habitats.

ordovician period

During the Ordovician Period, the oceans became filled with many kinds of invertebrates. The first fish also evolved. Plants colonized the land for the first time. However, animals remained in the water.

cambrian period

The Precambrian mass extinction opened up many niches for new organisms to fill. As a result, the Cambrian Period began with an explosion of new kinds of living things. For example, many types of simple animals called sponges evolved. Trilobites were also very common. Sponges and trilobites were small ocean invertebrates. These are animals without a backbone. You can see examples of them in Figure 7.21.

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paleozoic era

The Paleozoic Era lasted from 544 to 245 million years ago. It is divided into six periods.

mesozoic era

The Permian mass extinction paved the way for another burst of new life at the start of the Mesozoic Era. This era is known as the age of dinosaurs. It is divided into three periods.

permian period

During the Permian Period, all the major landmasses moved together to form one supercontinent. The supercontinent has been named Pangaea. You can see how it looked in Figure 7.23. At this time, temperatures were extreme and the climate became very dry. As a result, plants and animals evolved ways to cope with dryness. For example, reptiles evolved leathery skin. This helped prevent water loss. Plants evolved waxy leaves for the same purpose. The Permian Period ended with Earths second mass extinction. During this event, most of Earths species went extinct. It was the most massive extinction ever recorded. Its not clear why it happened. One possible reason is that a very large meteorite struck Earth. Another possibility is the eruption of enormous volcanoes. Either event could create a huge amount of dust. The dust might block out sunlight for months. This would cool the planet and prevent photosynthesis.

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carboniferous period

In the Carboniferous Period, forests of huge ferns and trees were widespread. You can see how these first forests might have looked in Figure 7.22. After the ferns and trees died, their remains eventually turned to coal. The first amphibians also evolved during this period. They could live on land but had to return to the water to lay their eggs. After amphibians, the earliest reptiles appeared. They were the first animals that could reproduce on land and move away from the water.

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devonian period

During the Devonian Period, the first seed plants evolved. Seeds have a protective coat and contain stored food. This was a big advantage over other types of plant reproduction. Seed plants eventually became the most common type of plants on land. In the oceans, fish with lobe fins evolved. These fish could breathe air when they raised their head above water. This was a step in the evolution of animals that could live on land.

jurassic period

The Triassic mass extinction gave dinosaurs the opportunity to really flourish during the Jurassic Period. Thats why this period is called the golden age of dinosaurs. The earliest birds also evolved during the Jurassic from dinosaur ancestors. In addition, all the major groups of mammals appeared. Flowering plants also appeared for the first time. New insects evolved to pollinate them. The continents continued to move apart.

triassic period

During the Triassic Period, the first dinosaurs evolved from reptile ancestors. They eventually colonized the air and water in addition to the land. There were also forests of huge seed ferns and cone-bearing conifer trees in the Triassic Period. Modern corals, fish, and insects all evolved in this period as well. The supercontinent of Pangea started to break up. The Triassic Period ended in a mass extinction. The majority of species died out, but dinosaurs were spared.

instructional diagrams

description_image

The diagram is a representation of the major division of earths history. The geological timescale is a representation of time elapsed after the formation of earth, divided into slices, each differentiated by a geological event whose record is held in rock samples. Geological time is primarily divided into eons, which are divided into eras, which are further divided into periods. The periods are further divided into epochs, and epochs into ages, while eons are grouped into super-eons. The lengths of these eras are often measured by the term mya, which represents millions of years ago. The first three eons are grouped under the Precambrian super-eon. The fourth eon, called the Phanerozoic, is ongoing. Although the first three eons together account for most of Earthas history, stretching out for nearly four billion years, there was little of note in terms of biological activity or geological diversity. So, in representations such as the table above, they are usually collectively called the Precambrian. It contains the Hadeon eon, when Earth was forming and the Late Heavy Bombardment took place; the Archeon eon, when water first showed up and the first lifeforms evolved; the Proterozoic eon, when the first multicellular organisms appeared and Earthas atmosphere received oxygen for the first time as a result of the proliferation of cyanobacteria.

description_image

The diagram shows an example of geologic time scale, which is a tool that scientists and historians used to describe and understand the different time frames of the Earths existence. This geologic time scale shows a timeline of events beginning from the late Proterozoic Era, approximately 650 million years ago. It is divided into eras and periods, and lists the major events that occurred in Earths history each period. From the geologic time scale, we can tell when different creatures evolved and first appeared on Earth. We know that the first amphibians appeared during the Devonian Period in the Paleozoic Era, approximately 400 million years ago. The first dinosaurs appeared during the Triassic Period of the Mesozoic Era, about 250 million years ago. Humans like us only appeared on Earth approximately 2.6 million years ago, during the Quaternary Period of the Cenozoic Era. The human race is very young, considering the Earth is approximately 4.6 billion years old!

questions

Earths second mass extinction occurred at the end of the

a. Cambrian Period

b. Triassic Period

-->  c. Permian Period

d. Tertiary Period

Life first appeared on Earth about

a. 4.6 billion years ago

-->  b. 4.0 billion years ago

c. 4.6 million years ago

d. 4.0 million years ago

The geologic time scale is based on major changes in

a. geology

b. climate

c. organisms

-->  d. all of the above

If Earths history is represented by a 24-hour day, then life first appears at about

-->  a. 3:00 am

b. 8:00 am

c. 1:00 pm

d. 3:00 pm

Which of the following divisions of the geologic time scale is longest?

a. epoch

b. era

c. period

-->  d. eon

Which feature of modern Earth was absent when the planet first formed?

a. solid crust

b. oceans

c. atmosphere

-->  d. all of the above

The earliest living things were

a. autotrophs

-->  b. heterotrophs

c. eukaryotes

d. two of the above

The earliest cells

a. had a nucleus

b. made their own food

-->  c. got energy from organic molecules

d. two of the above

The first multicellular organisms evolved about

a. 4.0 billion years ago

b. 3.6 billion years ago

c. 3.0 billion years ago

-->  d. none of the above

Organisms that evolved at the beginning of the Paleozoic Era included

a. trilobites

b. sponges

c. reptiles

-->  d. two of the above

The first mass extinction on Earth occurred at the end of the

-->  a. Precambrian Supereon

b. Permian Period

c. Mesozoic Era

d. Cretaceous Period

Primates and human ancestors first appeared during the

a. Jurassic Period

-->  b. Tertiary Period

c. Devonian Period

d. Quaternary Period

___major division of Earths history that is known as the age of dinosaurs

a. Paleozoic Era

b. Precambrian Supereon

c. Cenozoic Era

d. LUCA

-->  e. Mesozoic Era

f. mass extinction

g. geologic time scale

___first major division of Earths history

a. Paleozoic Era

-->  b. Precambrian Supereon

c. Cenozoic Era

d. LUCA

e. Mesozoic Era

f. mass extinction

g. geologic time scale

___major division of Earths history that is called the age of mammals

a. Paleozoic Era

b. Precambrian Supereon

-->  c. Cenozoic Era

d. LUCA

e. Mesozoic Era

f. mass extinction

g. geologic time scale

___event in which the majority of Earths species die out

a. Paleozoic Era

b. Precambrian Supereon

c. Cenozoic Era

d. LUCA

e. Mesozoic Era

-->  f. mass extinction

g. geologic time scale

The Jurassic Period is called the golden age of dinosaurs.

-->  a. true

b. false

Mammals became the dominant land animals during the Mesozoic Era.

a. true

-->  b. false

___division of Earths history into eons, eras, and periods

a. Paleozoic Era

b. Precambrian Supereon

c. Cenozoic Era

d. LUCA

e. Mesozoic Era

f. mass extinction

-->  g. geologic time scale

The last ice age ended about 12 million years ago.

a. true

-->  b. false

___major division of Earths history that began with the Cambrian explosion

-->  a. Paleozoic Era

b. Precambrian Supereon

c. Cenozoic Era

d. LUCA

e. Mesozoic Era

f. mass extinction

g. geologic time scale

___single cell believed to have given rise to all of the following life on Earth

a. Paleozoic Era

b. Precambrian Supereon

c. Cenozoic Era

-->  d. LUCA

e. Mesozoic Era

f. mass extinction

g. geologic time scale

The current geological era is the Cenozoic Era.

-->  a. true

b. false

When Earth first formed, it had oceans but no atmosphere.

a. true

-->  b. false

_During Earths history, continents drifted and collided.

-->  a. true

b. false

_If Earths history is represented by a 24-hour day, humans appear at about 8:00 pm.

a. true

-->  b. false

_Earths earliest atmosphere contained more oxygen than the atmosphere does today.

a. true

-->  b. false

_A total of four mass extinctions have occurred on Earth since life began.

a. true

-->  b. false

_Fish first evolved during the Paleozoic Era.

-->  a. true

b. false

_The Jurassic Period is known as the golden age of mammals.

a. true

-->  b. false

_The extinction of the dinosaurs paved the way for reptiles to take over.

a. true

-->  b. false

diagram questions

question_image

Which period from the Paleozoic era happened most recently?

a. Triassic

b. Precambrian

-->  c. Permian

d. Cambrian

question_image

What period happened 290-245 millions of years ago?

-->  a. Permian

b. Silurian

c. Tertiary

d. Devonian

question_image

How many geologic periods fall within the Paleozoic era?

-->  a. 6

b. 1

c. 2

d. 4

question_image

How many periods were there in the Paleozoic era?

-->  a. 6

b. 5

c. 7

d. 8

question_image

How many eras have been there?

a. 3

b. 2

-->  c. 4

d. 5

question_image

In what era did the Jurassic period exist?

a. Cenozoic

-->  b. Mesozoic

c. Paleozoic

d. Neoproterozoic

question_image

How many eras are there?

a. 9

-->  b. 10

c. 12

d. 11

question_image

Identify which period came before the Jurassic period.

-->  a. Triassic

b. Palaeogene

c. Neogene

d. Cretaceous

question_image

How many millions of years did the Jurassic Period last?

a. 54

b. 50

c. 48

-->  d. 44

question_image

What period was 145 million years ago?

a. Neogene

b. Triassic

-->  c. Jurassic

d. Permian

question_image

How many millions of years ago was the Triassic Period?

a. 23 million years ago

-->  b. 200 million years ago

c. 400 million years ago

d. 500 million years ago

question_image

What is the present period we live in?

a. Palaeogene

b. Jurassic

-->  c. Quaternary

d. Neogene

question_image

How many time periods end in gene?

a. 6

-->  b. 2

c. 4

d. 8

question_image

How many million years ago was the Devonian period?

a. 488

b. 2.6

-->  c. 359

d. 65

question_image

Which of these occurred before the Cambrian period?

a. Devonian period

b. Cretaceous period

-->  c. Precambrian period

d. Jurassic period

question_image

What is the most recent period shown in the diagram?

a. Neogene

b. Jurassic

c. Cambrian

-->  d. Quaternary

question_image

What is the period between Triassic and Cretaceous?

-->  a. Jurassic

b. Permian

c. Paleogene

d. Neogene

question_image

If you traveled back in time 310 million years, in which era would you find yourself?

-->  a. The Paleozoic era

b. The Mesozoic era

c. The Cenozoic era

d. The Precambrian era

question_image

What era is the Neogene period part of?

a. paleozioc

-->  b. cenozoic

c. mesozoic

d. precambrian

question_image

From the diagram, how many periods are there in the Mesozoic era?

a. 2

-->  b. 3

c. 4

d. 5

question_image

What is the Mesozoic Era made up of?

a. Neogene

-->  b. Cretaceous, Jurassic, Triassic

c. Quaternary

d. Paleogene

question_image

Which of these periods is oldest?

a. Permian

-->  b. Cambrian

c. Jurassic

d. Neogene

question_image

Identify which is part of the 89% of geological time.

a. Jurassic

b. Silurian

-->  c. Proterozoic Eon

d. Quaternary

question_image

Approximately how many million years ago was the Devonian period?

a. 200 million years ago

b. 65 million years ago

c. 300 million years ago

-->  d. 440 million years ago

question_image

What do you call the mass extinctions years ago?

a. The Extinctions FIve

-->  b. The Big Five

c. The Mass Five

d. The Five

question_image

When was the Origin of Earth?

-->  a. 4,600 million years ago

b. 200 million years ago

c. 300 million years ago

d. 65 million years ago

question_image

Jurassic period was in which era?

-->  a. Mesozoic

b. Cenozoic

c. Paleozoic

d. Triassic

question_image

What period occurred approximately 508 million years ago?

-->  a. Ordovician

b. Jurassic

c. Devonian

d. Cretaceous

question_image

How many eras does the earth have?

-->  a. 3

b. 10

c. 4

d. 5

question_image

How many periods are there in the Mesozoic era?

a. 2

b. 5

-->  c. 3

d. 4

question_image

Which period occurred between 66.4 million and 1.6 million years ago?

-->  a. Tertiary period

b. Triassic period

c. Jurassic period

d. Permian period

question_image

In which era does the Jurassic period take place?

a. Cenozoic

b. Paleozoic

-->  c. Mesozoic Era

d. Phanerozoic

question_image

What is the period before Triassic called?

-->  a. Jurassic

b. Cambrian

c. Mesozoic

d. Permian

question_image

How many periods does the Mesozoic era have?

a. 7

-->  b. 3

c. 12

d. 5

question_image

How many sections are there in the era column?

a. 3

b. 2

c. 4

-->  d. 6

question_image

Which period lies between Jurassic and Permian?

a. Palaeogene

-->  b. Triassic

c. Cambrian

d. Devonian

question_image

Which period occurred between the Triassic and Cretaceous periods?

a. Silurian period

b. Devonian period

-->  c. Jurassic period

d. Cambrian period

question_image

How many eras are there in this diagram?

a. 4

b. 6

c. 1

-->  d. 2

question_image

From the diagram, identify the era which is known as the age of dinosaurs.

-->  a. mesozoic

b. phanerozoic

c. quaternary

d. cenozoic

question_image

Which period comes after Triassic?

-->  a. jurassic

b. quaternary

c. cretaceous

d. tertiary

question_image

How many periods are there in the Mesozoic era?

a. 5

b. 4

-->  c. 3

d. 6

question_image

How many periods were during Mesozoic era?

a. 2

b. 5

-->  c. How many periods were during Mesozoic era?

d. 3

question_image

Which era was the Triassic in?

-->  a. Mesozoic

b. Phanerozoic

c. Cenozoic

d. Tertiary

question_image

What happened in the Carboniferous period?

a. Soft bodied animals

-->  b. First reptiles appeared

c. First fishes appeared

d. Earliest hominids

question_image

During what epoch did Homo sapiens appear on Earth?

a. Paleocene

-->  b. Pleistocene

c. Eocene

d. Miocene

question_image

In which period did the mammals start its dominance?

a. Quaternary.

-->  b. Tertiary.

c. Triassic.

d. Ordovician.

question_image

What dominated earth after the widespread extinctions of the dinosaurs?

-->  a. mammals

b. flowering plants

c. Fishes

d. Homo sapiens

question_image

How many Epochs are in the Tertiary Period?

a. 3

-->  b. 5

c. 2

d. 4

question_image

How many periods are there in the Paleozoic Era?

a. 9

-->  b. 6

c. 3

d. 2

question_image

How many periods are in the Cenozoic era?

-->  a. 3

b. 2

c. 8

d. 1