The Second Great Lesson is the child’s first introduction to the interconnectedness of all life on Earth through the concept of evolution. This lesson is meant to give a big picture overview of how life and Earth has changed over time, and this lesson will be revised and referenced frequently during the child’s six elementary years.

Each of the Great Lessons serves as a springboard and an orientation for the domain it covers. Together, all five great lessons touch on every piece of what is learned in the six-year elementary classroom. This is a sample of the domains and lessons that would follow from The Coming of Life depending on the age and readiness of the child:

• Biology: Animal studies, classification, life cycles
• Botany: Photosynthesis, classification, experiments
• Mycology: Mushroom studies, nature hikes
• Ecology: Habitats, food webs
• Atmospheric Science: The carbon cycle, the water cycle
• Ancient Life: Dinosaurs, fossils
• Geology: Rock cycle, volcanoes

Keys to Success

1. The goal of the Timeline of Life is to show, in broad strokes, how life on Earth has changed throughout time.
2. This Great Story is enormously simplified, it is not a college lecture. Every single bullet point represents enough information to fill a book. Keep in mind that we are giving children the “keys to the universe,” not the entire universe!
3. Don’t bog down the story with too many specific details, like dates, names of eras or creatures. If the children get overwhelmed with detail, they’ll tune out and find the presentation boring.
4. Living things do not change because they want different characteristics, evolution is not goal oriented (teleological). For example saying, “Animals wanted to move onto land to get better food,” would be incorrect and misleading. Instead say, “Animals who were able to move onto land could get more food. This helped them survive.” Priscilla Spears has a must-read article explaining this in more detail here.
5. Go chronologically, but prioritize the narrative story over putting details in their exact temporal location. This can be always be clarified in follow-up works and key lessons.

Set Up and Materials

You will need the Timeline of Life, rolled up with some weights to hold down the corners as you unroll. Slowly unroll the timeline as you go along the story. Expect a lot of comments and excitement as the story progresses and children see creatures they recognize.

This script might work with other commercially available timelines, but it was specifically designed to work with the Rhyme and Reason Timeline of Life. From my research, most other commercial timelines have a much looser interpretation of the evolutionary connections and have outdated information which will make it very difficult to use this script.

You can read more about the updates in the Rhyme and Reason Timeline of Life here.

Presentation

2023 UPDATED: Priscilla Spears has worked with me to update, and provide clarity to this lesson and the Timeline.

First and foremost, the purpose of this lesson is to provide a framework for children to organize the entire history of of Earth, no small task! This story will become an integrated framework for zoology, botany, mycology and geology. The more easily the child can hold the “big picture” in their minds, the more they’ll be able to integrate the rest of the curriculum.

The children have the most fun when you are having fun- this is an epic, exciting story full of fascinating creatures and other-worldly environments! This is a story of destruction! A story of triumph! Find the parts that get you excited and share those with your students.

There are a lot of different ways you could go about teaching the Second Great Lesson, and I’ve chosen to go chronologically, picking out the details of the Timeline of Life that help carry along a narrative story. I try to keep specific details minimal during the story, so as not to overwhelm the children with too much information. Later lessons and follow up work will draw attention to specific lineages and interdependencies.

The lesson below is not written as a word-for-word script. When I give the lesson, I have my notes next to me and I improvise and interact with the students as I go. If they get really interested in a particular part of the story, I might expand a bit, and let them know that they can explore more in follow up work, and if one area is losing attention I can speed up a little and skip some parts. I never tell the exact same story twice.

The Timeline should be left out for a bit after the presentation so that the children can look at it. You should remain for a little while to answer any questions the children might have, but not to ask the children questions, or to give a lot more information. Answers to the questions should not be too detailed at this time. After a few days have lapsed the timeline can be rolled up and put away. It is not put up on the wall as a permanent fixture. The Timeline can always be brought out by children wishing to work with it, or look at it, and the teacher can bring it out periodically, as well, when it may attract new interest.

The Precambrian

Early Earth – The Hadean Eon

Recap the First Great Lesson.

• Planet Earth was formed 4.5 Billion years ago in a swirl of rocks and dust left over from the formation of our solar system.
• The early earth was a terribly unpleasant place to be.
  • Hadean is named after ‘Hades’ the Greek god of the underworld.
• The Earth was entirely covered in magma, no liquid water, constant lightning, meteorite impacts were a daily occurrence.
• You can point to Theia and talk about how this proto-planet collided with Earth to create the Moon.
• No life yet! Everything was quiet, no rustling leaves, no chirping birds, no splashing fish. Just the harsh wind across the water and rocks.
• No breathable air- Early atmosphere smelled like ammonia.
• Small asteroids continuously bombarded Earth. They brought water from other places in the solar system.
• Through the Cosmic Dance, Earth cooled enough to form warm oceans.

First Life – The Archaean Eon

Life begins on Earth. At this time, we honestly don’t know specific details how it began, but we do know that early life was very simple and very fragile. See the resource section below to learn more.

• And then something happened- something moved all on it’s own.
• We don’t know how many times this movement may have happened in flickers and flutters. It moved and lived for a short time and then died. Life from this time was delicate and weak. Maybe a million years later it happened again. And maybe it happened many many times, always dying before it could reproduce. But eventually it was able to reproduce, and make more of itself. There was now and forever life on Earth. 
• New living things continued to come from those first forms, and over time, they changed slightly. Always on accident, sometimes those changes would kill them, and sometimes those changes helped them survive.

Life Becomes More Complex – The Proterozoic Eon

• Land masses were small, everything still lived in the warm oceans.
• A new kind of creature, Cyanobacteria, took energy from the sun to make its own food – like plants, it released oxygen.
  • One particle at a time, these unassuming creatures changed our planet.
  • Oxygen gave the multicellular creatures more energy, which lead to many experiments.
• From these first experiments with life, major lineages appeared that are still in existence
  • Plants – make their own food
  • Fungi- decompose the dead and waste
  • Animals- eat other organisms

The Paleozoic Era

The Cambrian Explosion – The Cambrian Period

• Climate was hot, oxygen was abundant
• There were many strange beings quite unlike anything today, and some creatures we share characteristics with.
  • They left fossils in the Burgess Shale, a famous fossil site in Canada Rocky Mountains.
  • Kimberella – A bilateral. This creature is symmetrical on both sides of its body. (Students notice their own symmetry)
  • Pikaia- Early creature with a backbone. (Students feel their own backbone)
  • Anomalocaris – Hunters evolved- so did their victims. Among them, the abundant trilobites with their complex eyes.
• Bacteria, molds, and slimes came on land and started to breakdown the hard volcanic rocks. Soon plants would be able to venture onto land too.

Ordovician Extinction Event – The Ordovician Period

• On land, there were the Prototaxites and small land plants
• Prototaxites- Mushrooms the size of a two story house!
  • They were the first large creatures to populate land.
  • Allowing the first simple plants to venture on land.
  • They helped break down the volcanic rock – (as part of the rock cycle, this absorbs CO₂)
• Carbon dioxide levels dropped, cooling the Earth.
  • This was the largest Ice Age living things ever experienced.! 
  • Glaciers lower the sea levels. Killed almost everything, only about 15% of life survived.

The Silurian Period

This is a shortened version of the Story of Plants.

• Vascular plants – Move water and nutrients all around their bodies.
  • Taller than before, but still small. They sprawled from the shorelines.
• Roots Develop – plants can hold tight to the earth and reach water that is further away.
  • Plants get a little bigger as they move further away from water sources.
• True Leaves – Leaves opened up to collect more sunlight and more carbon dioxide.
  • Bigger plants! But if they got too big, they’d flop over (no woody stems).

Vertebrates Come Onto Land – The Devonian Period

Amphibians were not the first animals on land, arthropods were, but they are our first ancestors to come onto land, so they’re given a special emphasis.

• The coming of amphibians
  • Some fish were able to venture onto shore, where they had access to new foods and were safer from predators. Those with stronger limbs and larger lungs could get further away and stay on land longer.
  • These new creatures, called amphibians, were able to live their adult lives out of water, but they returned to water to lay their eggs.
• The bigger plants with their roots cause erosion
  • Nutrients are washed into oceans
    • Extra nutrients feed massive algae blooms.
  • Plants flourish and suck down even more carbon dioxide, plunging Earth into another ice age.
    • (Carbon dioxide levels stay dangerously low until fungus reset the carbon cycle in The Great Dying)

The Carboniferous Period

The Carboniferous and Permian have a lot of overlap narratively. For the Carboniferous, I like to talk about the creatures and for the Permian I like to tell the Story of Coal because it pairs well with the extinction event.

• High amount of oxygen from all of the plants! Arthropods grew to enormous sizes.
  • Arthropleura is as long as the timeline (2 meter centipede)
  • Meganeura 30 inches wide (giant dragonfly)
  • Can you imagine how big the spiders would have been?
• Amphibians stayed on land, and those who were able to stay longer, ate the arthropods
  • Some of these creatures were able to lay eggs out of water. These eggs had a hard shell and “water within” them. These creatures are the ancestors to reptiles.

The Great Dying – The Permian Period

Tree trunks and other “woody” parts of plants are made of complex hydrocarbon molecules. Lignin is one of these molecules and is responsible for very hard wood, like the kind we use to build houses. This chemical invention came to prominence in the Carboniferous Period, and it is indigestible by animals and most bacteria and fungi.

Without the ability to break this molecule down, carbon was trapped in wood even after trees died. The trunks simply piled up on each other and did not decompose. Over time, these “piles” could become coal. Fires and volcanic eruptions would periodically release CO₂ back into the atmosphere, but it was a precarious balance.

This is the lowest the CO₂ levels would be until the Pleistocene ice age.

Carbon Dioxide is plant food, without enough carbon dioxide in the air, plants will starve (see Needs of The Plant experiment!). Certain kinds of fungus evolved the ability to break down lignin and release the CO₂ back into the atmosphere, resetting the carbon cycle.

The Siberian Traps were a massive volcanic eruption happening at the same time as this biological activity. They played a large part in releasing CO₂ from rocks and coal deposits which decreased oxygen concentrations. This is explored more in follow-up work.

• Plants had pulled down nearly all CO₂ for the past 150 million years
  • Wood trapped CO₂, and it couldn’t be released!
    • As the trees died, they piled up on each other, but didn’t decompose. This created coal.
• All life had adapted to the high oxygen/low CO₂ levels.
  • Plants struggled to survive from lack of CO₂ food.
  • Fungus saved all life on Earth…
    • Fungus were able to break down trees (lignin)- Reset the carbon cycle!
  • … and also caused the extinction event.
    • By increasing the percentage of CO₂ in the atmosphere, there was proportionately less oxygen available for animals. This dramatic drop in oxygen suffocated animals.
• This event is called “The Great Dying”
  • Finally killed off trilobites
  • Killed more than half of insect species
  • Killed 90% of animals

The Mesozoic Era

Bookended by Mass Extinctions – The Triassic Period

• After The Great Dying, ecological niches were empty, and new kinds of creatures adapted and dominated.
• This was a time of great innovations for life. New monstrous reptiles and early mammals too.
• Earth was very warm, about 10 degrees warmer than today. No ice at the poles.
  • Life flourished (cold kills much more than heat).
• Extinction at end Triassic
  • Caused by a volcanic region the size of Canada that erupted for 600,000 years. Blocked out the sun, poisoned the air and oceans.

Dinosaurs Reign – The Jurassic Period

Dinosaur lineage begins in the Triassic, but dinosaurs truly dominate during the Jurassic and Cretaceous after the Triassic extinction.

• Dinosaurs!
  • Lizard-hipped dinosaurs walked on all fours- these are the HUGE dinosaurs.
  • Bird-hipped dinosaurs walked upright on two legs.
    • Some were large like Tyrannosaurus
    • Some were small like archaeopteryx – who were fast feathers
• Unassuming tiny mammals
  • Early mammals probably laid eggs.
  • They kept their eggs inside their bodies instead of laying them in nests.
  • They had fewer babies, but they were more protected from predators which increased their chance for survival.

Impact! – The Cretaceous Period

• SO DEADLY
• A meteor 6-10 miles (10-15km) wide struck Earth in the Gulf of Mexico.
• The impact made volcanoes erupt, and caused worldwide fires
  • Burned most of the world’s forests.
  • The soot caused a year of black skies, which killed many plant survivors.
  • The soot poisoned the water
• The hot blast cooked large animals alive in a matter of minutes.
• The impact caused earthquakes and tsunamis
• The only creatures that survived were those that were small enough to shelter underground or in water during the initial blast and those that ate very little when the plants had mostly died.

The Cenozoic Era

This Era is only covered briefly, and for the Second Great Lesson, we want to emphasize the modern flora and fauna that are present, especially mammals. In the Third Great Lesson we will talk extensively about the coming of human beings, so we want to give a cliffhanger for that story and not give it all away now!

Please note: The Cenozoic Era is not to scale. I needed to “stretch it out” to make space. To scale it would be smaller than the Cretaceous.

• Birds are the only living descendants from the dinosaurs.
• Extinction left many niches open, tiny mammals that survived the meteor quickly diversified to fill them. Variety tripled!
  • Some mammals returned to the water- like whales and dolphins
  • Mammals spread into every kind of environment.
  • Like every other part of the Timeline, only a small amount of variety is shown.
  • I’m sure you can think of many creatures that are missing!
• Humans have only arrived in the last tiny part of the Timeline. Our species has existed for a very, very short time.
  • But that’s a story for another day.

Resources for The Second Great Lesson

The Second Great Lesson is going to take the most amount of time for you to become familiar with the subject, and it is almost certainly going to be an absolute favorite in the class.

What made this lesson so intimidating to me, is how much content it covers. The history of all life on Earth is a staggeringly complex and vast body of knowledge. When I started my Montessori training, I knew almost none of it and had to teach myself. I’ve gathered up all the best resources I’ve used to help you teach yourself.

There is a lot of information here- I hope it doesn’t overwhelm you. My goal is to provide a wealth of information and for you to pick and choose what interests you most and dive in. This content is meant to be teacher-facing, but most of it will be appropriate for Upper Elementary students as well.

I have included Amazon affiliate links to products that I use and enjoy. These are for your convenience. I do receive a small percentage from every purchase without any increase to your price.

Resources from Rhyme and Reason Academy

• Blog Post: The Timeline of Life – Redesigned and Scientifically Accurate
• Product: The Timeline of Life
• Product: Mute Timeline of Life
• Product: Clock of Eons
• Product: Geologic Time Cards
• Blog Post: The First Great Lesson – Complete Script
• Product: The First Great Lesson Charts

Resources for Overall Understanding

• Priscilla Spears: Outline of Geologic Time and the History of Life (A must have)
• Priscilla Spears: It’s time to take out the teleology
• Wikipedia: Timeline of Evolutionary History of Life
• Wikipedia: History of Life
• Wikipedia: Evolution of Fungi
• Wikipedia: Evolutionary History of Plants
• Wikipedia: Geological History of Earth
• History of the Earth: Chronological Playlist (Videos)
• Carl Sagan: Evolution (Video)
• Book: Cosmos by Carl Sagan
• Book: Continental Drift

The Hadean Eon

• History of the Earth: How Did the Earth Form? (Video)
• History of the Earth: What Was Earth Like 4 Billion Years Ago? (Video)
• History of the Earth: The Mystery of the Late Heavy Bombardment (Video)
• History of the Earth: Where Did Earth’s Water Come From? (Video)

The Archaean Eon

• History of the Earth: How Did Life Begin? (Video)
• History of the Earth: What Was the Last Common Ancestor of All Life on Earth? (Video)

The Proterozoic Eon

• History of the Earth: How Bad Was the Great Oxidation Event? (Video)
• History of the Earth: How Did Life Recover The First Mass Extinction? (Video)
• History of Earth: What Was The First Complex Life? (Video)

The Cambrian Period

• Wikipedia: Cambrian Period
• Wikipedia: Cambrian Explosion
• Paleos: Cambrian

The Ordovician Period

• PBS Eons: How Plants Caused the First Mass Extinction (Video)
• PBS Eons: When Giant Fungi Ruled (Video)
• History of Earth: What Was The First Fungi? (Video)
• Wikipedia: Ordovician Period
• Wikipedia: Late Ordovician Mass Extinction
• Paleos: Ordovician

The Silurian Period

• Wikipedia: Silurian Period
• Paleos: Silurian

The Devonian Period

• Wikipedia: Devonian Period
• Wikipedia: Late Devonian Extinction
• Paleos: Devonian

The Carboniferous Period

• PBS Eons: The Age of Giant Insects (Video)
• Wikipedia: Carboniferous Period
• Paleos: Carboniferous
• Carboniferous Climate
• Atmospheric oxygen level and the evolution of insect body size

The Permian Period

• Hypoxia, Global Warming, and Terrestrial Late Permian Extinctions
• The terminal Paleozoic fungal event: evidence of terrestrial ecosystem destabilization and collapse.
• Wikipedia: Permian Period
• Wikipedia: Permian-Triassic Extinction
• Paleos: Permian

The Triassic Period

• PBS Eons: Why Triassic Animals Were Just the Weirdest (Video)
• PBS Eons: The Age of Reptiles in Three Acts (Video)
• Book: Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages
• Wikipedia: Triassic Period
• Wikipedia: Triassic-Jurassic Extinction
• Paleos: Triassic

The Jurassic Period

• Wikipedia: Jurassic Period
• Paleos: Jurassic

The Cretaceous Period

• Survival in the First Hours of the Cenozoic (Awesome paper)
• Wikipedia: Cretaceous Period
• Wikipedia: K-T Extinction
• Paleos: Cretaceous

The Cenozoic Era

• Paleos: Cenozoic
• Priscilla Spears: The Story of Mammals

Still Overwhelmed?

This video by Carl Sagan gives an excellent overview of the Story of Animals. If this is your first time presenting, and you’re intimidated, tell this much shorter story. Every year after, you can add more detail. I know you can do it!