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Session 8: DNA, Evolution and Extinction

Start thinking: Could you make these cookies without �the recipe?

Could this recipe exist if no one had ever baked the cookies before?

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Every living thing has a recipe inside it too.

They are made of a chemical called DNA.

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DNA needs proteins to copy itself. Proteins need DNA as their recipe. Neither can work without the other! Scientists think DNA was not the first instruction manual that life ever used. So what came before it?

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Today’s Big Question:

How has life survived for 3.7 billion years?

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PATHWAY 1

Replicating Life Molecules

What is RNA
Where DNA actually lives inside a cell
What is LUCA

10 mins

PATHWAY 2

Evolution

How DNA copying mistakes (mutations) drive evolution
The Big Five mass extinctions

10 mins

How has life survived for 3.7 billion years?

Finished? Complete the Story of Life Minigame

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Survive the Asteroid

A massive asteroid has just hit Earth. The sky is dark, temperatures have dropped, most plants are dying, and food is scarce. Decide whether these species will survive or go extinct based on their trait.

Example:

Trait	Survive or Extinct	Why?
Feeds only on one specific plant	Extinct	That plant dies in the dark. If a species ca›n only eat one host plant and it disappears, they starve. No flexibility.

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Trait	Survive or Extinct	Why?
Can eat almost anything organic	Survives

Dietary generalists thrive. If the fresh plants die, they eat rotting wood or dead matter.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Very large body, needs huge amounts of food	Extinct	Think T-Rex. Big bodies need lots of energy. When the food web crashes, the giants starve first.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Reproduces once every 10 years	Extinct	Too slow to adapt or recover the population. If 90% of the population dies, they can't rebuild their numbers fast enough to adapt or recover.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Reproduces every few weeks	Survives	More generations mean shorter generation times and more opportunities for DNA copying mistakes (mutations), which helps them adapt fast.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Found on every continent	Survives	Geographic spread is insurance. Even if three continents are destroyed, the population on the fourth survives.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Can survive in a wide range of climates	Survives	Flexibility is the key to survival.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Lives in only one small habitat	Extinct	If the asteroid impact or dust hits your specific valley or island, your entire species is wiped out in one second.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Found on every continent	Survives	Geographic spread is insurance. Even if three continents are destroyed, the population on the fourth survives.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Needs a very narrow temperature range	Extinct	The climate is completely broken now. If you can't handle hot and cold, you're out.

Survive the Asteroid

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Trait	Survive or Extinct	Why?
Small body, needs very little food	Survives	Think tiny mouse-like mammals. They can hide underground, eat scraps and survive on nearly nothing.

Survive the Asteroid

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Survive or Extinct	Why?
Extinct

This plant species is the absolute champion of photosynthesis - the most efficient green plant on Earth.

There is zero sunlight for two years because of the dust cloud. Being the absolute best at something is useless if the environment changes and removes the one thing you rely on. Specialisation is a trap during an extinction

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Survive or Extinct	Why?
Extinct

This species is a living fossil. It has existed, completely unchanged, for 300 million years.

Just because you survived past disasters doesn't mean you're immune to this one. If your DNA never changes and lacks genetic variation, you have no new genetic options to handle a completely new environment.

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There are only 50 individuals of this species left, but they are incredibly adaptable and eat anything.

Survive or Extinct	Why?
Extinct

Even if you are adaptable, 50 individuals isn't enough spread or genetic diversity. One bad disease or localised fire wipes out your entire genetic library (a genetic bottleneck). You need numbers to maintain a safe breeding population.

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Looking back at every species that survived, what did they all have in common?

Flexibility - dietary, climate, behavioural

Numbers and Spread (a buffer

against local disasters)

Genetic variation within the species - if every individual is identical, one change can wipe them all out!

"Looking back at every species that survived, what did they all have in common?" Guide towards three ideas: flexibility (dietary, climatic, behavioural); numbers and spread (a buffer against local disaster); and genetic variation within the species, the crucial one, because if every individual is identical, one change can wipe them all out, but variation means some individuals will always suit the new conditions.

If students want to talk through each example on the board:

Humans

Flexibility: Live in deserts, the Arctic, mountains, cities — eat almost anything
Numbers and spread: Over 8 billion people, on every continent
Genetic variation: Huge diversity across populations — different immune responses, tolerances (e.g. lactose tolerance, high-altitude adaptation in Andean and Tibetan populations) — meaning no single disease or environmental shift threatens everyone equally

Rats

Flexibility: Eat almost any food, thrive in sewers, farms, ships, cities, forests
Numbers and spread: Populations in the billions, found on every continent except Antarctica
Genetic variation: High reproductive rate constantly generates new variation, which is part of why they evolve resistance to poisons so quickly — some individuals are always naturally resistant

Beetles

Flexibility: Found in nearly every habitat on Earth, from deserts to rainforests to freshwater
Numbers and spread: Around 400,000 known species, roughly 1 in 4 of all known animal species is a beetle
Genetic variation: That sheer number of species represents an enormous pool of genetic variation, meaning as a group they're almost impossible to wipe out all at once — some lineage always fits the new conditions

Ants

Flexibility: Found in deserts, rainforests, grasslands, even inside buildings — different species eat fungi, seeds, other insects, or scavenge
Numbers and spread: An estimated 20 quadrillion ants alive at any time, on every continent except Antarctica — by some estimates their combined weight roughly equals that of all humans on Earth
Genetic variation: Around 12,000+ known species, each adapted to different conditions, so as a group they cover an enormous range of genetic and behavioural variation — no single threat could plausibly wipe out ants everywhere

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WE HAVE COVERED:

How DNA became the permanent instruction manual inside cells
Where DNA is stored in a cell
What LUCA is and why scientists believe all life descends from it
Define mutation and explain how copying mistakes in DNA drive evolution
Predict which species would survive a catastrophic event

Learner Journal

Part 1: Three Things I Learnt Today

Open your Learner Journal. Write down three things you learnt. These can be facts, ideas, or things that surprised you.

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99.9% of all animals that have

ever lived are now extinct. How does that make you feel differently about the animals alive today, including humans?

Write your response in your Learner Journal, then ﬁnish sections 3-5.

Learner Journal

Part 2: Reflective Question

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“Natural selection is daily … scrutinising … the slightest variations; rejecting that which is bad, preserving and adding up all that is good.” Charles Darwin, biologist (1859)

DNA copies itself with extraordinary accuracy, but not perfectly. Those rare copying mistakes are called mutations and they are what create the variety of life we see today. That variety is not decoration. It is insurance.

Lock-In Quote

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Extension Activities

Extension 1

Extension 2

The Theory of Evolution

Evolution's Strange Inventions