Ausgeology. Senior secondary lessons

SACE Stage 1 Earth and Environmental Science · Topic 6: Biosphere

Life through deep time.

Life has been on Earth for more than three and a half billion years, and animals for only the last part of that. Follow how scientists worked out when life began, how old the first animals are and what wiped most of them out, case by case and in the order the evidence came in.

How the lesson works

1 · Where life began

Screens 2 to 4. Place the history of life on the time-scale, test how life might have begun with the Miller–Urey experiment, and weigh the evidence for the oldest life.

2 · Reading the record

Screens 5 to 7. Learn how fossils form and how rocks show which way is up, then follow Reg Sprigg’s fossils and the Cambrian Explosion.

3 · Extinction

Screens 8 to 10. Weigh the evidence on what killed the dinosaurs and Australia’s megafauna, look at biodiversity loss today, then show what you know.

What we are learning

How life began and evolved from prokaryotes to eukaryotes, how fossils and sedimentary rocks record past environments and the history of life, what the fossil record shows about mass extinctions, and how scientists come to accept or reject an explanation.

I can

  • 1place the main events in the history of life on the geological time-scale.
  • 2describe the early Earth and explain how the Miller–Urey experiment tested ideas about how life began, and its limits.
  • 3explain why prokaryotes matter as the first life and how eukaryotes evolved from them.
  • 4explain ways fossils form and use sedimentary structures to tell which way up rocks are.
  • 5infer a past environment by comparing fossils with living things.
  • 6explain why the Ediacaran fossils matter and how scientists came to accept their age.
  • 7describe the Cambrian Explosion and explain why some fossils make good index fossils.
  • 8weigh the evidence for the causes of a mass extinction.
  • 9explain why current biodiversity loss is almost entirely caused by humans.

SACE Stage 1 Earth and Environmental Science, Topic 6: Biosphere. This lesson covers the topic’s statements on early life, the origin of life, past environments, the fossil record and extinction. The teacher notes at the end quote them in full.

My notebook

Printable activity

Case notebook

Three pages, one box for each screen or pair of screens: a time-scale table, the Miller–Urey apparatus, way-up sketches, a specimen table, the dinosaur evidence and the megafauna. For each case, students write their own conclusion before the scientists’. Print one per student.

Case notebook (PDF) Editable (Word)

1Where life began

Deep time.

How long has there been life on Earth, and for how much of that time were there animals?

Geologists divide Earth’s 4,567 million years into eons, eras and periods. The fossil record fills only the last part of that line with animals. Use the time-scale to see how the events in this lesson fit together, before looking at how scientists worked each of them out.

This screen · about 20 minutes

  1. Read how the time-scale is divided.
  2. Open each event on the time-scale, in both views.
  3. Put eight events in order, then answer the check.

Eons, eras and periods

The largest units of the geological time-scale are eons. Earth’s history starts with the Hadean, from Earth’s formation 4,567 million years ago, followed by the Archaean from 4,031 million years ago, the Proterozoic from 2,500 million, and the Phanerozoic from 538.8 million years ago to today. The Phanerozoic is split into three eras: the Palaeozoic, Mesozoic and Cenozoic. These dates come from the International Commission on Stratigraphy, which keeps the official chart and updates it as new dates come in (ICS, 2026).

The fossil record and the eras are linked. Two of the five great mass extinctions mark the ends of eras: the end-Permian extinction, 252 million years ago, closes the Palaeozoic, and the end-Cretaceous extinction, 66 million years ago, closes the Mesozoic. The Precambrian, everything before the Cambrian, makes up more than 80% of the geological record (Britannica), but it has few fossils, so the SACE outline notes that time spans older than the Ediacaran “are defined by absolute age”: by dates measured from radioactive decay, not by fossils.

Put them in order

Tap the events from oldest to youngest. Use the time-scale above if you need it.

Imagine Earth’s history as one day, starting at midnight. Roughly when does the Cambrian Period begin?

My notebook

1Where life began

How life began.

Could the chemicals of life have formed by themselves on the early Earth, and how could anyone test that?

No one can go back four billion years, but scientists can build the conditions they think the early Earth had and see what forms. This screen follows the most famous attempt, step by step, as the evidence came in.

This screen · about 30 minutes

  1. Read what the Archaean Earth was probably like.
  2. Step through the Miller–Urey apparatus.
  3. Work through the case file. Choose your own conclusion at each question before you read what happened next.
  4. Answer the check and write your conclusion.

The Archaean Earth

The air of the Archaean Eon had almost no free oxygen: less than a millionth of today’s level at the surface. Nitrogen was a main gas, as it is now, and there was far more carbon dioxide and methane than today. The Sun was 25 to 30% fainter than today, yet average surface temperatures were probably between 0 and 40 °C. Some studies of oxygen isotopes argued for oceans of 50 to 85 °C, but most researchers doubt them. The deep ocean held dissolved iron and had no oxygen (Catling and Zahnle, 2020).

Iron and oxygen meet in the banded iron formations of the Pilbara and elsewhere: layers of iron oxide and silica laid down on the sea floor as oxygen made by microbes reacted with the iron dissolved in sea water (Australian Museum).

What is the main limitation of the 1953 Miller–Urey experiment?

My notebook

1Where life began

The first life.

What were the first living things, how old is the evidence for them, and how did more complex cells come from them?

For nearly two billion years, every living thing was a prokaryote, made of cells without a nucleus. Some of those cells built structures that still survive in Australian rocks, and some of them changed the air.

This screen · about 35 minutes

  1. Read about prokaryotes and stromatolites.
  2. Work through the case file on the oldest evidence for life.
  3. Read how oxygen built up in the air.
  4. Put the steps of endosymbiosis in order, then answer the checks.

Prokaryotes and stromatolites

A prokaryote is an organism that “lacks a distinct nucleus and other organelles” (Britannica). Bacteria are prokaryotes. The oldest evidence for life, in rocks of the Archaean Eon, comes from prokaryotes: the oldest widely accepted fossils of cells with a nucleus, eukaryotes, are about 1,650 million years old (Nanjing Institute of Geology and Palaeontology, 2024), nearly two billion years later.

Much of that evidence is in stromatolites, layered structures built as mats of microbes trap and bind the sediment that washes over them. “Like the rings of a tree, the stromatolite layers represent the life history of the colony of microbes that built it” (National Museum of Australia).

Drawing · not to scale

Stromatolites cut through: each thin layer was once the surface of a mat of microbes.

Then and now · Shark Bay, Western Australia

Living stromatolites

Hamelin Pool in Shark Bay has the most diverse and abundant living stromatolites, the first modern examples to be recognised. Structures like them “dominated marine ecosystems on Earth for more than 3,000 million years” (UNESCO). The living ones are young: the oldest dated heads are about 1,900 radiocarbon years old, they grow by less than 0.1 to 0.5 mm a year, and the water of Hamelin Pool has a salinity of 56 to 70, well above that of the open ocean (Collins and Jahnert, 2014). Studying how they grow now helps geologists read the ancient ones.

UNESCO World Heritage Centre; Collins and Jahnert (2014)

The air changes

Cyanobacteria used sunlight, water and carbon dioxide to make carbohydrates, “releasing oxygen as a by-product” (National Museum of Australia). Oxygen first built up in the atmosphere in the Great Oxidation Event, about 2.43 billion years ago, and levels rose and fell for about 200 million years before oxygen stayed for good, about 2.22 billion years ago (Poulton and others, 2021). Not every early stromatolite was built by cyanobacteria: the Western Australian Museum notes that early stromatolites “may not have been formed largely by photosynthesising cyanobacteria”. The Climate drivers lesson follows the evolution of the atmosphere further.

From prokaryotes to eukaryotes

In 1967 Lynn Margulis, publishing as Lynn Sagan, argued that the mitochondria and the plastids that photosynthesise inside eukaryote cells were once free-living prokaryotes, taken in by another cell: endosymbiosis. Her paper was sent to at least fifteen journals before it was accepted (Embryo Project Encyclopedia, 2014). Her ideas “were frequently greeted with skepticism and even hostility”, but the theory “has since been widely accepted” (Britannica).

The oldest widely accepted eukaryotes are single-celled forms about 1.65 billion years old, and multicellular eukaryotes appear in North China about 1.63 billion years ago. Older claims exist, but “most of them are controversial due to their simple morphology and lack of cellular structure” (Nanjing Institute of Geology and Palaeontology, 2024).

The Northern Territory has a famous example of such a debate. In 1965 Barghoorn and Schopf described microfossils in chert from the Bitter Springs limestone of central Australia, “interpreted as both green and blue-green algae” (Barghoorn and Schopf, 1965). The rocks are about 830 million years old (Hill and others, 2000). Blue-green algae are cyanobacteria, which are prokaryotes; green algae are eukaryotes. Ten years later Barghoorn himself, with Knoll, compared the fossils with partly decayed modern cyanobacteria and concluded that there was “no compelling evidence for the presence of eukaryotic microfossils” in the Bitter Springs rocks (Knoll and Barghoorn, 1975). The fossils are still important. One kind of filament, made of “chains of box-like cells”, was examined again in 2019 and found to “fulfil all established biogenicity criteria”, the tests for whether a structure was once alive (Wacey and others, 2019).

Endosymbiosis, in order

Why are prokaryotes so important in the history of life?

Why did scientists come to doubt that the Bitter Springs microfossils include eukaryotes?

My notebook

2Reading the record

Reading fossils and rocks.

How does a living thing become a fossil, and what can the rock around it tell us?

Before using fossils as evidence, it helps to know how they form, why most living things never become one, and how to tell which way up a layer of rock was laid down.

This screen · about 35 minutes

  1. Step through how a shell becomes a fossil, and sort body and trace fossils.
  2. Decide which way up each block of rock is.
  3. Match fossils to the environments they lived in.
  4. Read why the record is incomplete, then answer the check.

Other ways fossils form

Replacement

The shell or bone is replaced completely by other minerals, keeping its shape (Britannica).

Mould and cast

A shell dissolves away and leaves a hollow in the rock, a mould (Australian Museum). If new minerals or sediment fill the hollow, they make a cast.

Carbonisation

Leaves, stems and other plant matter can be left as a thin film of carbon (Britannica).

Amber

Organisms trapped in tree resin can be preserved in amber (Australian Museum).

Body or trace?

Body fossils are parts of the organism. Trace fossils record what it did (Australian Museum).

Which way up?

Beds of sedimentary rock are laid down one on top of another, so the youngest is normally at the top. Folding can turn a whole sequence over. Some structures formed in the sediment show which side was the top, the bed’s facing. In cross-bedding the sloping layers are cut off sharply at the top and curve gently into the base. A graded bed is coarse at the bottom and fine at the top. Ripple crests are sharper than their troughs, and mud cracks narrow downwards (Newfoundland and Labrador Geological Survey; Ricketts; Hauptvogel and Sisson, 2021; University of Alberta).

Is each block the right way up?

The present is the key to the past

The principle of uniformitarianism, from James Hutton, holds that the processes at work today also worked in the past: “the present is the key to the past” (Britannica). Fossils show past environments because “different kinds of plants and animals were confined to specific environments, just as they are today” (Indiana Geological and Water Survey). The same reasoning reads the rock: sand settling into graded beds, ripples forming in shallow water and mud drying into cracks can all be watched today.

Where did it live?

A record with gaps

For an organism to become a fossil it usually needs hard parts and quick burial, so “Only a small fraction of ancient organisms are preserved as fossils” (Britannica). Fossils do not occur in igneous or metamorphic rock (NCSE), and erosion wears away rock that holds them. The gaps are widest before about 600 million years ago. The start of the Cambrian is marked by the evolution of hard body parts such as shells (Britannica, “Cambrian explosion”), so most earlier life was soft and rarely preserved. And in only a few places in the world can palaeontologists even look for the ancestors of Cambrian animals (NCSE).

In a cliff, a layer of mudstone has V-shaped cracks filled with sand. The points of the Vs face up. What does this tell you?

My notebook

2Reading the record

Sprigg’s fossils.

How did imprints in South Australian sandstone change the story of when animals appeared?

In 1946 a geologist looking at old mine sites in the Flinders Ranges found impressions of soft-bodied animals. It took more than half a century for the time-scale to catch up with what he had found.

This screen · about 25 minutes

  1. Work through the case file. Choose your own answer at each question before you read on.
  2. Read about the Ediacaran sea floor.
  3. Answer the check and note what changed scientists’ minds.
Nilpena Ediacara National Park · about 555 million years ago

A sea floor covered in mats

Layers of sandstone at Nilpena give “a snapshot of life in a shallow sea”. The sea floor was covered with a mat of microbes, and the organisms living on it about 20 m down were periodically buried (South Australian Museum, 2025). Dickinsonia grew up to 1.4 m long (ANU, 2018), and specimens at Nilpena reach 90 cm (Flinders Ranges Ediacara Foundation).

South Australian Museum (2025); ANU (2018); Flinders Ranges Ediacara Foundation

Brachina Gorge · Ikara-Flinders Ranges National Park

The golden spike

The base of the Ediacaran Period is marked at Enorama Creek, at the base of a layer of carbonate rock called the Nuccaleena Formation (ICS). This point in the rock is the period’s GSSP. Visitors can find its marker “just next to the Trezona campground along the Brachina Gorge Road” (National Parks and Wildlife Service SA).

International Commission on Stratigraphy; National Parks and Wildlife Service SA

Why are the Ediacara fossils so important?

My notebook

2Reading the record

The Cambrian Explosion.

What happened at the start of the Cambrian, and how do fossils let geologists date rocks?

At the start of the Cambrian Period, animals with shells and many new body plans appeared in a geologically short time. South Australia has two important early Cambrian sites.

This screen · about 30 minutes

  1. Read about the Cambrian Explosion and its Australian sites.
  2. Look at each of the seven specimens: when it lived and where.
  3. Sort good and poor index fossils, then answer the check.

What exploded

The Cambrian Explosion brought “the appearance of many of the major phyla (between 20 and 35) that make up modern animal life” (Britannica). A phylum is a group with one basic body plan, such as the arthropods. The start of the Cambrian “is marked by the evolution of hard body parts such as calcium carbonate shells”, and large predators such as Anomalocaris lived in Early Cambrian seas (Britannica).

How long it lasted depends on the source. Britannica gives about 541 to 530 million years ago. A study of trilobite evolution found that “modern-style marine communities emerged in just 20 million years”, ending by 521 million years ago (University of New England, 2019). Both use 541 million years for the start of the Cambrian; the current ICS chart gives 538.8.

Kangaroo Island · about 515 million years ago

The Emu Bay Shale

The Emu Bay Shale preserves details of anatomy that are rare even in Burgess Shale-type fossils (Gaines and others, 2024). Its fossils include the eyes of Anomalocaris, each with at least 16,000 lenses (Paterson and others, 2011).

Gaines and others (2024); Paterson and others (2011)

Flinders Ranges · about 530 to 509 million years ago

The first reefs

Archaeocyaths, extinct sponges with mineral skeletons, played “a dominant role in constructing the Earth’s first reefs” (South Australian Heritage Council, 2014). Their reefs at Wilkawillina Gorge and the Ajax Mine near Leigh Creek formed in shallow seas (South Australian Heritage Council, 2014, 2024).

South Australian Heritage Council (2014, 2024)

For comparison · Canada, about 505 million years ago

The Burgess Shale

The Burgess Shale in Yoho National Park, British Columbia, preserves the soft tissues of many organisms “in exquisite detail” (Royal Ontario Museum, 2025).

Royal Ontario Museum (2025)

Fossils that tell the time

An index fossil lets geologists match rock layers of the same age in different places. To be useful it “must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time” (Britannica). It helps if the group was “evolving rapidly” and could live over a wide range of places (Indiana Geological and Water Survey).

Good or poor index fossil?

Which statement best describes the significance of the Cambrian Explosion?

My notebook

3Extinction

Mass extinctions.

What wipes out most of life in a short time, and how do scientists work out the cause?

The fossil record shows long stretches of slow change broken by a few catastrophes. The cause of the most famous one, the end of the dinosaurs, was argued over for decades.

This screen · about 35 minutes

  1. Read the chart of the five mass extinctions and their causes.
  2. Work through the case file on the end of the dinosaurs.
  3. Sort the evidence, then answer the check.

The big five

In a mass extinction, Earth “loses more than three-quarters of its species in a geologically short interval” (Barnosky and others, 2011). More than 40 years ago, Raup and Sepkoski picked out five such episodes in the fossil record of sea life (Marshall, 2023). Between them, species die out at a slower background rate.

The causes the SACE outline names, asteroid strikes, volcanic eruptions and natural climate shifts, all appear in the chart. One review argues that “large scale volcanism is the main driver of mass extinction”, that most extinctions are linked to global warming, and that the only convincing link between an impact and an extinction is Chicxulub at the end of the Cretaceous (Bond and Grasby, 2017).

Show the numbers as a table

Sources give different figures. The end-Permian loss is “more than 96 percent of marine species” in one report (MIT News, 2015) and “may have eliminated more than three-quarters of species of marine animals” in another (Sam Noble Museum). The teacher notes list the differences.

Sort the evidence

Why was the iridium in the boundary clay evidence for an asteroid impact?

My notebook

3Extinction

The sixth wave?.

Who or what killed Australia’s giant animals, and are humans causing a sixth mass extinction now?

The SACE outline says that “Scientists recognise six waves of extinction in the past half billion years”. Some scientists argue that a sixth is happening now; others say it is not yet a mass extinction. This screen starts with an Australian case where the evidence is thin, then looks at the present.

This screen · about 35 minutes

  1. Work through the case file on the megafauna, and note the limits of the evidence.
  2. Read what is driving biodiversity loss today, and sort the examples.
  3. Sort the arguments about the Anthropocene, then answer the check.

Biodiversity loss today

In 2019 the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), drawing on 145 expert authors from 50 countries, reported that “around 1 million animal and plant species are now threatened with extinction”. It found the current rate of extinction is tens to hundreds of times higher than the average over the last 10 million years, and that three-quarters of the land and about 66% of the ocean have been “significantly altered by human actions”. It ranked the direct causes “in descending order: (1) changes in land and sea use; (2) direct exploitation of organisms;” then “(3) climate change; (4) pollution and (5) invasive alien species.” All five are human activities.

Kakadu’s mammals

The same drivers are at work in the Top End. Monitoring plots in Kakadu National Park, sampled from 1996 to 2009, showed that “Plot-level species richness and total abundance decreased significantly, by 54% and 71%, respectively”. The authors named “too frequent fire, predation by feral cats and invasion by cane toads” as the most plausible causes (Woinarski and others, 2010). Because the decline happened inside a national park, Woinarski and colleagues concluded that conservation reserves on their own are not enough to keep biodiversity (Woinarski and others, 2011). Later monitoring found that “the mammal community has not recovered from declines that occurred between 1995 and 2005” (NESP Northern Hub, 2021).

Which driver?

Is it a mass extinction yet? The sources disagree

Ceballos and others, 2015

Vertebrates have been lost over the last century at up to 114 times the background rate, so “a sixth mass extinction is already under way”.

Barnosky and others, 2011

Current extinction rates are higher than the fossil record would lead us to expect, and “a sixth mass extinction may be under way”.

Marshall, 2023

“Not yet, but it could be”: the number of species lost is “still relatively small”, so “there is still time to prevent this becoming a genuine mass extinction.”

They agree on the cause and on the rate being high. They differ on whether the share of species lost so far is enough to call it a mass extinction.

The Anthropocene

In 2000 Paul Crutzen and Eugene Stoermer proposed the name Anthropocene for the current geological epoch (Crutzen and Stoermer, 2000). A working group set up in 2009 (Ellis, 2024) proposed starting it in the mid-twentieth century, with a sediment core from Crawford Lake in Canada as the reference site (UCL, 2023). The Subcommission on Quaternary Stratigraphy voted on the proposal, and voting closed on 4 March 2024: 4 in favour, 12 against and 3 abstaining. The ICS Executive recognised the vote as valid, and a statement from the International Union of Geological Sciences said the term will still be used informally and “will remain an invaluable descriptor in human-environment interactions” (IUGS, 2024).

For or against an Anthropocene epoch?

Why is current biodiversity loss said to be almost entirely caused by humans?

My notebook

4Show what you know

Show what you know.

Nine questions, one for each of the I can statements on screen 1. You get one try at each, so think before you tap.

Which of these happened first?

A student says: “Miller and Urey proved how life began.” Which reply is most accurate?

How did eukaryote cells come to have mitochondria?

In a sandstone, the sloping layers of the cross-bedding curve gently along the top of each bed and are cut off sharply at the bottom. What does this show?

A limestone contains fossil corals. Using uniformitarianism, what environment did it probably form in?

What changed scientists’ minds about the age of the Ediacara fossils?

Which would make the best index fossil?

Which conclusion about the end-Cretaceous extinction fits the evidence best?

According to IPBES (2019), which is the biggest direct driver of biodiversity loss?

Printable activity

Case notebook

Three pages, one box for each screen or pair of screens: a time-scale table, the Miller–Urey apparatus, way-up sketches, a specimen table, the dinosaur evidence and the megafauna. For each case, students write their own conclusion before the scientists’. Print one per student.

Case notebook (PDF) Editable (Word)
For teachers

Curriculum

SACE Board of South Australia, Stage 1 Earth and Environmental Science Subject Outline (reissued for 2025), Topic 6: Biosphere. NT students take the subject as part of the NTCET. The topic outline begins:

The fossil record provides evidence of the diverse life forms that have existed over time. Students interpret and evaluate different types of fossil evidence and explore the connections of fossils to past environments.

The topic’s Science Understanding statements and dot points that this lesson covers:

Evidence indicates that life first appeared on Earth approximately 3.7 billion years ago.

  • Discuss the significance of prokaryotes as the first life on Earth in the Archaean Eon.
  • Explain how life then evolved into eukaryotes.

Laboratory experimentation has informed theories about how life emerged.

  • Explain how scientists use systematic investigation to replicate primeval conditions on Earth that could possibly produce chemicals that are essential for life to begin.
  • Describe the atmospheric and oceanic conditions that are thought to have existed in the Archaean Eon.

The characteristics of past environments and communities can be inferred from the sequence and internal textures of sedimentary rocks, and from enclosed fossils and trace fossils.

  • Explain ways in which fossils form.
  • Relate the present environment of living organisms to the past environment of fossils of similar organisms.
  • Describe the formation of sedimentary structures that are used to determine the facing of beds.

The diversification and proliferation of living organisms over time and the catastrophic collapse of ecosystems can be inferred from the fossil record.

  • Explain the relationship between the fossil record and the eras in the geological time-scale.
  • Explain why some fossils are useful as index fossils.
  • Explain why the fossil record is inevitably incomplete, especially for organisms that lived more than 600 million years ago.
  • Explain the significance of fossils within the geological time-scale and as indicators of past environments.
  • Describe the Cambrian Explosion and its significance.
  • Discuss possible theories about the extinction of dinosaurs at the end of the Cretaceous Period.

Scientists recognise six waves of extinction in the past half billion years.

  • Explain how past mass extinctions were caused by events like asteroid strikes, volcanic eruptions, and natural climate shifts.
  • Explain why current biodiversity loss is almost entirely caused by humans.

In the outline’s table the statement on diversification and collapse runs on into a second row, which holds the dot points on the Cambrian Explosion, the dinosaurs and the six waves of extinction; the lesson treats them as part of that statement, and quotes the sentence on six waves separately because it reads as a statement in its own right. The topic’s other statements, on ecosystems, photosynthesis, carrying capacity and the carbon, nitrogen and phosphorus cycles, are left for a second Topic 6 lesson. The outline’s list of topics and the topic heading both call it “Topic 6: Biosphere”.

Where the lesson covers them:

  • Life first appeared about 3.7 billion years ago; prokaryotes in the Archaean: screen 4 (prokaryotes and stromatolites, the case file on Isua and the Pilbara, living stromatolites at Shark Bay, the Great Oxidation Event) and screen 2 (the time-scale), x1 and x3.
  • How life evolved into eukaryotes: screen 4 (endosymbiosis, the oldest eukaryote fossils, the Bitter Springs debate, the ordering task), x3.
  • Laboratory experimentation; replicating primeval conditions; Archaean atmosphere and oceans: screen 3 (the Archaean Earth, the apparatus, the Miller–Urey case file), x2.
  • Past environments from sedimentary rocks, fossils and trace fossils; ways fossils form; relating present to past environments; facing of beds: screen 5 (the fossilisation model, body and trace fossils, the way-up blocks, the environment sort), and screen 4 (Shark Bay), x4 and x5.
  • The fossil record and the eras; index fossils; why the record is incomplete: screen 2 (eras and extinctions), screen 5 (gaps in the record), screen 7 (index fossils), x7.
  • Significance of fossils; the Cambrian Explosion: screens 6 and 7 (the Ediacara case file, the Cambrian sites, seven specimens), x6 and x7.
  • Theories about the extinction of the dinosaurs; past mass extinctions and their causes: screen 8 (the big five, the Alvarez case file, the evidence sort), x8.
  • Six waves of extinction; current biodiversity loss caused by humans: screen 9 (the megafauna case file, IPBES drivers, Kakadu’s mammals, the sixth extinction debate, the Anthropocene), x9.

The lesson uses these of the outline’s possible contexts:

Link the appearance of photosynthetic organisms in stromatolites to the changes in oxygen production discussed in Topic 4: The Earth’s atmosphere.

Identify and explain the importance of Ediacaran fauna.

Explain how the Miller–Urey experiment is used to replicate primeval conditions on Earth that could possibly produce chemicals that are essential for life to begin.

Describe possible environmental settings and conditions in which this could occur.

Describe the limitations of the Miller–Urey experiment.

Investigate sedimentary structures that are used to determine the facing of beds, including cross-bedding and way up, graded bedding, and ripple marks.

Identify specimens of each of the following and describe the time and environment in which they lived:

Investigate the limitations of the available fossil evidence in determining the cause of the death of the megafauna.

Debate current geological time being named the Anthropocene.

The specimen list in the outline is Ediacaran fauna, trilobites, dinosaurs, ammonites, archaeocyatha, graptolites and megafauna (diprotodonts); screen 7 has a drawing and a card for each. The drawings are not a substitute for hand specimens: if your school has a fossil kit, use it alongside the screen. The outline also suggests a museum visit to see Ediacaran fossils and fieldwork at Maslin Bay or Port Willunga; the case notebook works for both.

Science as a human endeavour runs through the lesson, since the method is built on it. Key concepts from the outline, and where they are used:

Development of complex scientific models and/or theories often requires a wide range of evidence from many sources and across disciplines.

Screen 8: geochemistry (iridium), mineralogy (shocked quartz), geophysics (the buried crater) and dating, together; screen 4: shapes, organic matter and dating for the oldest life.

New technologies improve the efficiency of scientific procedures and data collection and analysis. This can reveal new evidence that may modify or replace models, theories, and processes.

Screen 3: Miller’s stored vials reanalysed in 2008; screen 6: cholesterol found in Dickinsonia in 2018; screen 9: modelling the uncertainty in megafauna dates.

The acceptance and use of scientific knowledge can be influenced by social, economic, cultural, and ethical considerations.

Screen 6: Sprigg’s find was not accepted for more than a decade; screen 4: Margulis’s paper sent to at least fifteen journals.

Science is a global enterprise that relies on clear communication, international conventions, and review and verification of results.

Screen 4: the Isua claim tested by a second team, and the Bitter Springs eukaryotes reassessed; screen 6: the Ediacaran Period ratified by the International Commission on Stratigraphy; screen 9: the vote on the Anthropocene.

Science informs public debate and is in turn influenced by public debate; at times, there may be complex, unanticipated variables or insufficient data that may limit possible conclusions.

Screen 9: the megafauna, where sparse fossils and uncertain dates limit the conclusions, and whether today’s losses count as a mass extinction.

Stage 1 is assessed through an investigations folio and skills and applications tasks. The case notebook and the notebook prompts suit a skills and applications task; the evidence sorts and the megafauna case give practice in evaluating evidence and the limits of conclusions.

How the lesson is taught

The lesson is a set of historical case studies. One well-known model is James Conant’s Harvard Case Histories in Experimental Science, which brought “the reader into the frame of mind of the earlier experimenters” through quotations from the original papers (Ratcliffe, 1958). Irwin (2000) used a historical approach to teach atomic theory to 14-year-olds and described positive effects. The research has a warning in it too: Abd-El-Khalick and Lederman (2000) found “very few and limited changes” in college students’ views of the nature of science after history of science courses, and that “explicitly addressing certain NOS aspects rendered the HOS courses relatively more effective”. Allchin’s historical inquiry cases answer that by stopping the story at points where students have to reason from the evidence as it stood, and then asking them to reflect on what the case shows about how science works (Allchin, 2012). A synthesis of studies in physics teaching found mostly positive effects of history and philosophy of science on learning concepts, though without consensus, and more divided results on attitudes (Teixeira, Greca and Freire, 2012).

Each case file follows that pattern. Evidence is revealed in the order it was found, and at each “You decide” card students must choose a conclusion before they can read what happened next. No choice is marked wrong: the feedback says what the choice assumes, and the next card shows what the scientists concluded. The notebook prompts and the case notebook ask students to state what each case shows about how science works: making the nature of science explicit is what Abd-El-Khalick and Lederman found made the courses more effective. The four cases (Miller and Urey, the oldest life, Sprigg, Alvarez) and the megafauna were chosen because each turned on a dispute about evidence, and in three of them the dispute is still going.

  • Timing. About four hours: five 50-minute lessons. Lesson 1: screens 1 and 2, then screen 3. Lesson 2: screen 4. Lesson 3: screens 5 and 6. Lesson 4: screens 7 and 8. Lesson 5: screens 9 and 10.
  • Running the case files as a class. The cases work well projected, with the class voting at each “You decide” card before anyone reads on. Ask students to justify their vote from the cards so far. Keep a record of the votes: comparing them with what the scientists concluded is a good discussion at the end of each case.
  • Pairs or alone. The sorts and the way-up blocks work well in pairs. The notebook prompts are better done alone.
  • Discussion points. After screen 4, ask why a single team’s claim of very old life is not enough. After screen 6, ask why Sprigg’s age was hard to accept in 1947. After screen 9, ask what new evidence would settle the megafauna question.
  • The case notebook has six boxes, one for each screen or pair of screens, and asks students to record their own conclusion before the scientists’ at each case. Collect it as evidence of learning.

Handle with care

  • The megafauna case involves the arrival of the first people in Australia. Questions about whether people caused the extinctions can be sensitive for Aboriginal and Torres Strait Islander students and communities. The lesson gives the published evidence on both sides with its limits, and draws no conclusion that the sources do not. Keep the discussion on the evidence and its limits.
  • The Ediacara Hills and Nilpena are in the northern Flinders Ranges, and “The Adnyamathanha people (meaning rock people) are the Traditional Owners of the Flinders Ranges” (National Parks and Wildlife Service SA). The lesson tells no Adnyamathanha stories. If your class visits, follow your school’s protocols and the guidance of the parks.
  • Some students or families may hold religious views about the age of the Earth or the origin of life. The lesson presents the scientific evidence and the debates within science; treat other views with respect and keep the focus on how scientists reason from evidence.
  • The current extinction can worry students. The screen ends with what drives it, which are human activities that people can change; Marshall (2023) says “there is still time to prevent this becoming a genuine mass extinction.”

Misconceptions the screens target

  • Animals have been around for most of Earth’s history (screen 2, q-time, x1). On the 24-hour clock the Cambrian starts at about 21:10.
  • The Miller–Urey experiment made life, or proved how life began (screen 3, q-origin, x2). It made amino acids, from a mixture of gases now in doubt.
  • The first life was simple animals, or plants (screen 4, q-life). For nearly two billion years there were only prokaryotes.
  • Most living things become fossils (screen 5). “Only a small fraction of ancient organisms are preserved as fossils” (Britannica).
  • The lowest layer of rock is always the oldest (screen 5, q-fossils, x4). Folding can overturn beds, and way-up structures show it.
  • Science accepts a discovery as soon as it is made (screens 4 and 6, x6). The Ediacara age took 13 years to be shown and 58 to reach the time-scale; the Isua claim is still argued over.
  • The dinosaur question has one simple answer (screen 8, x8). The impact is strongly supported; the part played by the Deccan eruptions is still argued over.
  • All mass extinctions were caused by asteroids (screen 8). One review links large volcanic eruptions to at least half of the major extinctions and argues volcanism is the main driver of mass extinction (Bond and Grasby, 2017).
  • Cane toads caused the decline of Kakadu’s mammals (screen 9). In 2010 the toads were “affecting particularly one native mammal species” (Woinarski and others, 2010), and later monitoring found “no evidence of them contributing to other mammal declines” beyond the northern quoll’s (NESP Northern Hub, 2021).
  • Hamelin Pool’s stromatolites are billions of years old (screen 4). The living stromatolites are about 1,000 to 2,000 years old (Collins and Jahnert, 2014); some public pages confuse them with the ancient ones.

About the data and the models

  • The time-scale (screen 2) uses the International Commission on Stratigraphy’s chart, version 2026/06: Hadean from 4,567 Ma, Archaean from 4,031 ± 3 Ma, Proterozoic from 2,500 Ma, Phanerozoic from 538.8 ± 0.6 Ma, Ediacaran from about 635 Ma, Palaeozoic to 251.902 Ma and Mesozoic to 66.0 Ma. Event dates are from the source named with each. The 24-hour clock is our arithmetic: (1 − age ÷ 4,567) × 24 hours.
  • The apparatus and fossilisation diagrams (screens 3 and 5) are schematic and not to scale. The way-up blocks are drawn from the descriptions in the sources named in the feedback. The fossil drawings (screen 7) are drawn for this lesson from general descriptions, not traced from specimens.
  • The chart of the five mass extinctions (screen 8) uses the figures Our World in Data compiled from Barnosky and others (2011), so its dates (444, 360, 250, 200 and 65 Ma) are that table’s, not the ICS chart’s.
  • Quotations in the screens are short excerpts checked against the source pages; where a sentence continues, the lesson quotes only the part shown.

Where sources disagree

  • The oldest life: Isua stromatolites, biological (Nutman and others, 2016, 2019) or deformation structures (Allwood and others, 2018). Britannica gives “about 3.5–3.7 billion years”. The SACE outline’s 3.7 billion years rests on the disputed Isua evidence; screen 4 says so.
  • Who built early stromatolites: cyanobacteria in Britannica’s account; the Western Australian Museum says early stromatolites “may not have been formed largely by photosynthesising cyanobacteria”.
  • The age of the Hamelin Pool stromatolites: a Western Australian parks page says they have been at Hamelin Pool for 3.5 billion years; dating puts the oldest heads at about 1,900 radiocarbon years (Collins and Jahnert, 2014). The lesson uses the dating.
  • The Great Oxidation Event: an onset between about 2,460 and 2,426 Ma (Gumsley and others, 2017) or about 2.43 Ga (Poulton and others, 2021), with permanent oxygen from about 2.22 Ga (Poulton). The lesson uses Poulton.
  • The oldest eukaryotes: about 1.65 Ga for the oldest unambiguous ones (Nanjing Institute, 2024); Changcheng fossils of 1.78 to 1.68 Ga only “possibly” eukaryotic, and Grypania “very likely” eukaryotic (Knoll and others, 2006).
  • Miller’s amino acids: secondary sources list different amino acids from the 1953 apparatus, so the lesson names only glycine and alanine, which all the sources we checked include.
  • Sprigg’s reception: a popular story says Nature rejected his paper. We found no reliable source for that; the Flinders Ranges Ediacara Foundation says a 1948 paper “was rejected for publication” without naming the journal, and IUGS Geoheritage says one of his papers appeared in Nature. The lesson uses the Foundation’s wording. The “first new period” is “in 120 years” in the Australian Dictionary of Biography and the South Australian Museum and “in more than a century” in Knoll and others (2004).
  • World Heritage: the Flinders Ranges was nominated in early 2026 and is not yet listed; some pages written before then describe it as listed or nearly so.
  • The start and length of the Cambrian Explosion: 541 to about 530 Ma (Britannica) or 541 to 521 Ma (University of New England, 2019). Both use the older 541 Ma for the base of the Cambrian; the ICS chart gives 538.8 Ma.
  • Diprotodon’s extinction: about 46,000 years ago (Britannica), about 25,000 years ago (Australian Museum), or around 45,000 or later (CSIRO). The megafauna as a whole: about 46,400 years ago (Roberts), a peak about 42,100 years ago (Saltré and others, 2016) or roughly 60,000 years ago (Naracoorte Caves page).
  • The megafauna’s cause: people (Saltré and others, 2016, reject climate as the main driver) or climate, with a human role possible but unproven (Wroe and others, 2013; Field and Dodson, 1999).
  • The big five: dates differ by a few million years between sources (end-Permian 250 or 252 Ma; end-Cretaceous 65, 65.5 or 66 Ma), and so do losses (end-Permian “more than 96 percent of marine species” or “may have eliminated more than three-quarters of species of marine animals”).
  • The end-Cretaceous cause: the impact triggered it (Schulte and others, 2010); the impact came about 300,000 years earlier and did not cause it (Keller and others, 2004); both events mattered (Schoene and others, 2019); most Deccan lava came after the boundary (Sprain and others, 2019).
  • The Gubbio iridium: about 30 times background (Alvarez and others, 1980) or about 600 times richer than the limestone either side (Lawrence Berkeley National Laboratory, 2010). They compare against different things; the lesson uses the 1980 paper.
  • Bitter Springs: about 830 million years old (Hill and others, 2000), 850 million (Wacey and others, 2019) or about 800 million (Britannica). Green algae (Barghoorn and Schopf, 1965) and cell division (Britannica, which still reports it) against “no compelling evidence” for eukaryotes (Knoll and Barghoorn, 1975).
  • The sixth extinction: “already under way” (Ceballos and others, 2015), “may be under way” (Barnosky and others, 2011), “Not yet, but it could be” (Marshall, 2023). Screen 9 shows all three.
  • The Anthropocene: proposed in 2000 (Crutzen and Stoermer) or 2001 (the IUGS statement); the proposed start “~1950” (IUGS), 1952 (the plutonium marker) or “the mid-twentieth century” (UCL).

Answers at a glance

  • Screen checks: q-time, about 21:10; q-origin, the gas mixture may not match the early atmosphere; q-life, the only life for nearly two billion years, oxygen and the origin of eukaryotes; q-bitter, decayed modern cyanobacteria look like the supposed eukaryote cells; q-fossils, upside down; q-ediacara, large animals before the Cambrian and a new period; q-cambrian, today’s major animal groups with shells and predators, and far more fossils; q-extinction, iridium scarce in the crust and the layer found far apart; q-now, all five IPBES drivers are human.
  • Ordering (screen 2): Pilbara stromatolites, oxygen, eukaryotes, Ediacaran animals, Cambrian Explosion, first dinosaurs, non-bird dinosaurs die out, megafauna die out. Endosymbiosis (screen 4): a cell takes in a bacterium, it is not digested, each comes to depend on the other, it becomes a mitochondrion, later chloroplasts.
  • Body or trace (screen 5): shell, tooth, carbon film of a leaf and insect in amber are body fossils; footprints, burrows and a dropping are trace fossils. Way up: blocks 1, 3 and 6 the right way up; 2, 4 and 5 upside down. Environments: corals, stromatolites, archaeocyaths and ammonites, shallow sea; graptolites, floating in the open sea; Diprotodon, land.
  • Index fossils (screen 7): graptolites and ammonites good; stromatolites and Diprotodon poor.
  • Evidence sort (screen 8): iridium, shocked quartz, the crater and the spherules support an impact; the Deccan pulse before the impact supports a role for the eruptions; 75% of the lava after the boundary counts against the eruptions alone.
  • Drivers (screen 9): bush clearing, land use; overfishing, exploitation; coral bleaching, climate change; fertiliser, pollution; foxes and cats, invasive species. Anthropocene: plutonium and Crawford Lake for; farming thousands of years ago and a start too recent and shallow against.
  • The case files have no wrong answers; each “You decide” card gives feedback on the reasoning behind every choice.
  • Exit check: x1 oxygen; x2 amino acids, but the gas mixture is doubted and no life; x3 a cell took in a bacterium; x4 upside down; x5 a warm, shallow sea; x6 Glaessner’s evidence, 1959; x7 a short-lived, widespread graptolite; x8 the impact triggered it, the eruptions’ part argued over; x9 changes in land and sea use.

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Sources by screen

Word list