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Part 2-Lesson 1. The Environment of the Phanerozoic eon. Conditions for life on Earth.

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part 2 lesson 1

Part 2-Lesson 1

The Environment of the Phanerozoic eon

conditions for life on earth
Conditions for life on Earth

The Earth’s atmosphere began to change around the beginning of the Proterozioc. (Mark this on your timelines) Cyanobacteria were becoming more abundant and adding more and more oxygen to the atmosphere through photosnythesis.

conditions for life on earth1
Conditions for life on Earth

The oxygen cyanobacteria produced reacted with iron dissolved in the oceans and was taken out of the atmosphere and ended up on the bottom of the ocean. (precipitation) This is how BIF’s formed.

conditions for life on earth2
Conditions for life on Earth

Eventually the dissolved iron was used up and oxygen began to accumulate in the atmosphere. As the oxygen accumulated it began to form a very important layer. The ozone layer.

conditions for life on earth3
Conditions for life on Earth

The ozone layer is in the lower part of the stratosphere (10-50km above the surface). This layer blocks out 93-99% ultra violet radiation from the sun.

conditions for life on earth4
Conditions for life on Earth

The energy from the sun causes oxygen molecules to split. The single atoms of oxygen react with other oxygen molecules in a process called photolysis to produce Ozone O3.

conditions for life on earth5
Conditions for life on Earth

This reaction absorbs some of the UV radiation which would otherwise reach the Earth’s surface. How would this affect early life on Earth?

conditions for life on earth6
Conditions for life on Earth

The first organisms evolved in very hostile environments on Earth. Until the ozone layer, ultraviolet radiation from the sun would have made living on land impossible.

conditions for life on earth7
Conditions for life on Earth

As the oxygen concentrations increased, the ozone concentrations increased blocking more and more UV radiation. Organisms were now faced with new living conditions. They could now evolve to live on land and use this new gas (oxygen) to produce energy instead of chemosynthesis.

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Conditions for life on Earth

The ozone layer allowed organisms to evolve and live in terrestrial environments around 500 million years ago, this marks the start of the Phanerozoic Eon. (find this on your timeline)

  • Read page 71HSC Spotlight Text
  • Update electronic vocab list
  • Complete “To Think About” SET 2 pg 73 HSC Spotlight Text
  • Complete DOT Points 2.1-2.4 pg 33-34
part 2 lesson 2

Part 2-Lesson 2

The Cambrian Explosion

dating the cambrian explosion
Dating the Cambrian Explosion

The Cambrian period marks the start of the Phanerozoic Eon. This is when environmental conditions allowed new life to form and evolve and become more complex than it had in the Archaean and Proterozoic.

dating the cambrian explosion1
Dating the Cambrian Explosion

Fossils around the world provide us with evidence about what these times were like and the organisms that lived during them.

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Dating the Cambrian Explosion

Finding fossils can be easy but determining how old they are is not. Because fossils are found in sedimentary rocks we can get a relative age based on the fact that the layers below are older than those on top. This is called the law of superposition and is the basis of relative dating.

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Dating the Cambrian Explosion

By using the law of superposition and stratigraphic sequences, geologists can construct a geological time scale. This then allows scientists to break the large eons into smaller periods.

  • Read pages 11-12 ‘A snapshot’ handout from the TAFE Environments Through Time Module
    • Underline key words and phrases
  • Complete the Dating Game Activity on page 13
dating the cambrian explosion4
Dating the Cambrian Explosion

To determine the exact age of a rock or fossil geologists use radiometric dating. This method measures the radioactive decay of isotopes. These isotopes come from the minerals that make up rocks. What does this mean for sedimentary rocks?

dating the cambrian explosion5
Dating the Cambrian Explosion

We know that minerals form in igneous rocks, so the minerals and sediments that make up sedimentary rocks are likely to have formed millions of years before. Radiometric dating therefore cannot be used to date fossils or sedimentary rocks.

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Dating the Cambrian Explosion

Relative dating can give us accurate information for determining the order in which life forms appeared on Earth. Once this order is established, then absolute dating of available igneous rocks provide an exact age for the fossils.

  • Read pages 75 HSC Spotlight Text
  • Update Electronic Vocabulary
  • Complete DOT Point 3.1
part 2 lesson 3

Part 2-Lesson 3

Radiometric Dating

radiometric dating
Radiometric Dating
  • Relative dating is based on position in a strategraphic column as well as a comparison of the lithology of the rocks and their fossils. To determine an absolute date of a rock or fossil, scientists use radiometric dating.
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Radiometric Dating
  • RD is based on the radioactive decay of isotopes to measure age. Radioactive decay happens when the unstable parent isotope breaks down into a more stable daughter isotope.
radiation and decay
Radiation and Decay

Below is a list of other examples of atoms decaying to become more stable :

  • Uranium-235 to lead-207
  • Thorium-232 to lead 208
  • Rubidium-87 to strontium-87
  • Potassium-40 to argon-40
  • Carbon-14 to nitrogen-14

In these examples, the unstable atoms on the left are called the parent material and the more stable atoms on the right are called the daughter product or remnant isotope.

determining ages from half lives
Determining ages from half-lives

The time taken for each of the parent atoms to decay to their daughter products varies from millions of years to minutes.


The time taken for half of the parent material to decay into its daughter product is know as the Half Life of that parent material.

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Determining ages from half-lives

The half-life for each radioactive element remains the constant. For example it takes 5370 years for half a sample of carbon-14 to decay to nitrogen-14

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Determining ages from half-lives

It will take another 5370 years for half the remaining carbon-14 to decay into nitrogen-14. This is known as the second half-life.

This means it has taken 10740 years for ¾ of the original carbon-14 to decay. (two half-lives)

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Determining ages from half-lives
  • This diagram shows the relationship between half-life and amount of radioactive parent material
dating rocks
Dating Rocks


Using the half-lives from radioactive elements to calculate age is known as radiometric dating.

Dating rocks using radioactive elements can indicate a particular time of formation.

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Dating Rocks

When minerals in igneous rocks are first formed after cooling, the amount of each radioactive mineral present at this time is 100%

Immediately after formation these unstable radioactive minerals begin to break down

dating rocks2
Dating Rocks

Think about this:

If you were given two samples of rock, one sample containing 95% of the original radioactive parent element and the other sample had 30% of the same original radioactive parent element, which of these rocks do you think would be oldest? Why?

dating the oldest rocks in australia
Dating the oldest rocks in Australia

Radiometric dating methods have enabled scientists to determine the time of mineral formation of some of the oldest rocks on Earth.

The oldest minerals ever dated in Australia are zircon crystals found in quartzite rock at Mt. Narryer in the Murchison region of Western Australia. These minerals are dated at 4.15byo

methods used to date precambrian rocks
Methods Used to Date Precambrian Rocks

The correct types of radiometric isotopes have to be used to date Precambrian rocks. Precambrian time starts about 4.7 billion years ago when the Earths crust began to form to the start of the Cambrian about 542 million years ago.

methods used to date precambrian rocks1
Methods Used to Date Precambrian Rocks

Because the Precambrian dates back to far, uranium/lead isotopes are used to date rocks within this period of time. Naturally occurring uranium contains two radioactive isotopes both with very long half-lives.

Uranium usually occurs as

trace elements in minerals

such as zircon.

  • Read pages 76-77 HSC Spotlight Text
  • Update electronic vocabulary
  • Complete Activity 2.7 pg 77 HSC Spotlight Text
  • Complete DOT Point 3.3
part 2 lesson 4

Part 2-Lesson 4

The Cambrian Event

the cambrian event
The Cambrian Event

The appearance of numerous and varied fossils separates the Precambrian from Cambrian. We know that life existed well before the Cambrian but was not preserved as fossils. During the Cambrian, existing life evolved very rapidly in the oxygen rich environment which allowed more complex evolution to occur.

the cambrian event1
The Cambrian Event

For example, more complex organisms began reproducing sexually versus asexually which had happened up until this period in time. Sexual reproduction allows individuals to pass on genetic material to their offspring. This mixing of genetic material means that the offspring were not identical to their parents.

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The Cambrian Event

Genetic variations happen because of mutations. If such mutation is beneficial to the organism, it can then be passed through the population by sexual reproduction.

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The Cambrian Event
  • If a ‘new’ population is separated from the ‘original’ population by geographical isolation or other isolating mechanism, over many generations they can evolve into a new species.
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The Cambrian Event
  • Organisms with hard parts and shells did not exist before the Cambrian period (Precambrian). A well known fossil site for these soft bodied organisms is in Ediacara, South Australia. This site is very important geologically because fossils of these organisms are very rare.



the cambrian event5
The Cambrian Event
  • The first ‘hard-bodied’ organisms had an advantage over their soft bodied competitors. They could use this to hide from predators and for protection against environmental elements.
  • Do you think it allowed them to colonise new environments?
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The Cambrian Event
  • These organisms with hard parts have a much greater chance of being preserved as a fossil than those that were soft. The most famous fossil site for Cambrian organisms is the Burgess Shale in British Colombia.
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The Cambrian Event
  • Man of the late Cambrian organisms such as brachiopods and trilobites had very specialised armour to protect them from predators. These organisms also evolved long spines which allowed them to burry themselves in the sand to prevent being washed around by currents.
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The Cambrian Event
  • These hard bodied organisms became more abundant and still exist today. Examples include molluscs and crustaceans.
  • Complete activity 2.8 and 2.9 pg 79 HSC Spotlight Text in class.

Read pages 78HSC Spotlight Text

Update electronic vocabulary

Complete DOT Points 3.4, 3.5, 3.7

part 2 lesson 5

Part 2-Lesson 5

Science Skills

science skills activity
Science Skills Activity
  • Read and discuss the bottom of page 79
  • Complete ‘To Think About’ pg 80-81
  • Complete any outstanding homework
the theory of evolution
The theory of evolution

Evolution is organisms changing over time. Fossils provide evidence of these changes. Charles Darwin (1809-1882) proposed a theory to explain why evolution occurs.

the theory of evolution1
The theory of evolution
  • Darwin believed that evolution occurred because of natural selection. Certain traits passed on from generation to generation make certain organisms more likely to survive and reproduce. Those better suited to survive will do just that, and those who aren’t, die and do not pass on their genetics to others.
the theory of evolution2
The theory of evolution

An observable example of such evolution occurred in the United Kingdom during the industrial revolution. The peppered moth can exist in light and dark colours. During the industrial revolution, trees and forests were covered in soot which gave the darker moth an advantage because it was better able to hide from predators. In just a few generations the majority of the moths were dark.

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The theory of evolution

The four points of Darwin’s theory of evolution are:

  • Genetic variation can occur in any population and individuals within a species are not identical.
  • Offspring do not always survive to adults. As they die, the characteristics they posses are not passed on.
  • The offspring that do survive and reproduce pass on favourable genetic variations as they are well adapted to the environment.
  • These favourable genetic variations are passed on to offspring and become more common in the population.
the theory of evolution4
The theory of evolution

Scientists studying organisms today can see similarities between certain species and are able to trace these characteristics to distant relatives.

the theory of evolution5
The theory of evolution

Today scientists have a number of lines of evidence to support the theory of evolution. This includes:

  • Anatomy
  • Physiology
  • Biogeography
  • Embryonic development
  • Biochemistry
  • Read and discuss pages 83-88 HSC Spotlight Text

Update electronic vocabulary

Complete ‘To Think About’ page 89-91 HSC Spotlight Text