Geological time !
Student reading worksheet
Introduction:
Nearly 80 percent of Earth’s history passed before multi-cellular life evolved. Up until then, all organisms
existed as single cells. Why did multi-cellular organisms evolve? What led up to this major step in the evolution
of life? To put the evolution of multi-cellularity in context, let’s return to what was happening on planet Earth
during this part of its history.
Before Reading: Assessing Prior Knowledge
How much do you know about the history of life on Earth? For each of the following statements, circle
whether it is true or false.
1. True/False: Dinosaurs and early man lived together for a brief period of time.
2. True/False: The extinction of the dinosaurs is the only known mass extinction.
3. True/False: Scientists think that at one point the Earth was completely covered with snow and
ice.
4. True/False: During one period of the Earth’s history, all of the continents used to be joined
together.
The Late Precambrian:
The late Precambrian was 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.
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One ice age was so cold that snow and ice completely covered the planet
Reading Check:
1. What is a “greenhouse effect”? ___________________________________________________________
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2. What caused the rise and fall of carbon dioxide levels during the late Precambrian?
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3. Explain how continental drift could be responsible for major climate change.
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4. Right now our Earth is experiencing an increase in carbon dioxide levels. What do you know about the
effect this is having right now and what factors may be contributing to the increase in CO2?
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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 multi-cellularity.
• 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.
• By 1 billion years ago, the first multi-cellular 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.
Reading Check:
1. What are three new adaptations that developed during the late Precambrian?
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2. Explain why sexual reproduction would lead to more variety in offspring than asexual reproduction.
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3. Why would this cause a faster rate of evolution?
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4. Recall what you learned about evolution and adaptive radiation in the previous chapter. Why would mass
extinctions lead to even more diversity of life?
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5. Consider the probable causes of the Precambrian mass extinction. Why do you think some scientists think
we are currently going through a sixth mass extinction?
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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 3.11. 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.
The Paleozoic Era includes the six periods described here.
Cambrian Period
(544–505 million years ago)
* Following the Precambrian mass
extinction, there was an explosion of new
kinds of organisms in the Cambrian.
* Many types of primitive animals called
sponges evolved.
* Small ocean invertebrates (animals
without a backbone), called Trilobites,
were very abundant.
Ordovician Period
(505–440 million years ago)
* The oceans were filled with invertebrates
of many types.
* The first fish evolved.
* Plants colonized the land for the first
time, but animals still remained in the
water.
Silurian Period
(440–410 million years ago)
* In the oceans, corals appeared and fish
continued to evolve.
* On the land, vascular plants appeared.
With special tissues to circulate water and
other materials, they could grow larger
than earlier, nonvascular plants.
Devonian Period
(410–360 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.
* Fish with lobe fins evolved. They could
breathe air when they raised their heads
above water. Breathing would be
necessary for animals to eventually
colonize the land.
Carboniferous Period
(360–290 million years ago)
* Widespread forests of huge plants left
massive deposits of carbon that eventually
turned to coal.
* The first amphibians evolved to 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 land.
Permian Period
(290–245 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 ended with a mass
extinction
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.
Reading Check:
1. What are three key adaptations that allowed plants to move from water to being on land and
able to withstand drier conditions?
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2. What evolutionary steps led to the colonization of land by animals?
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