Answer key · Unit 8

Evolution.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 5

Lesson 1 of 5 · About 25 minutes

Natural selection

Moths on tree bark: variation already in the population, birds that select, adaptation over generations, and why without selection almost nothing changes.

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The 6 steps and their answers

Step 1 of 6
Open this step
Variation
Practice · Choose
Question
The bark has been blackened by soot, and a pale moth stands out on it. Where will the dark moths that multiply over the next generations come from?

Correct answer
- Correct From the fairly dark moths already in the population
Setup, choices and feedback
The Scene at this step
Generation
0
Bark darkness
85 %
Bird predation
50 %
Sort by shade
Sorted by shade
Choices
Shown in a random order.
- Correct From the fairly dark moths already in the population
- Wrong The moths will darken to hide
  Misconception: Thinks living things change because they need to: moths darken in order to hide.
  Feedback from the catalogue
  A moth cannot decide to darken, nor darken because it would help. A moth’s colour is set by the genes it got from its parents. What happens is different: the population already holds moths of different shades, the birds eat more of those that stand out, and the survivors pass their genes on. Need does not make the change. It only decides who survives out of what is already there.
- Wrong The soot will stain the moths and their young
  Misconception: Thinks living things change because they need to: moths darken in order to hide.
  Feedback from the catalogue
  A moth cannot decide to darken, nor darken because it would help. A moth’s colour is set by the genes it got from its parents. What happens is different: the population already holds moths of different shades, the birds eat more of those that stand out, and the survivors pass their genes on. Need does not make the change. It only decides who survives out of what is already there.
Feedback for a right answer
The population has variation: moths of different shades, inherited. On dark bark the darker among them survive more, and pass their shade to their young. Small mutations add new variation every generation, at random and not to order.

Hint
Look with Sort by shade at Sorted by shade: are all the moths the same shade?

Worked solution
1. Wing shade is inherited, and differs from moth to moth: that is the variation.
2. The birds eat more of the ones that stand out on the bark.
3. The survivors breed, and their young inherit their shade.
4. So the dark moths of the coming generations descend from today’s fairly dark ones.
A moth cannot change its genes because it needs to, and soot on the wings is not inherited.
Step 2 of 6
Open this step
Fifty generations
Practice · Predict, then look
Question
The bark is sooty, and the birds are hunting. What will happen to the moths’ mean darkness after $50$ ⁠ generations?

Correct answer
- Correct It will rise, close to the bark’s shade
Setup, choices and feedback
The Scene at this step
Generation
0fixed
Bark darkness
85 %fixed
Bird predation
50 %fixed
Sort by shade
On the bark
Choices
- Correct It will rise, close to the bark’s shade
- Wrong It will stay as it is: pale moths have pale young
  Feedback
  Each moth does have young of a shade like its own, but not every moth breeds as much. The darker ones survive more, so there are more dark young.
- Wrong It will fall: the pale moths will adapt
  Misconception: Thinks a single animal evolves during its life.
  Feedback from the catalogue
  An individual does not evolve. A moth that hatched pale stays pale all its life. What changes is the population: from generation to generation the share of dark moths grows, because they survive and breed more. Evolution is a change in how common inherited traits are in a population, over generations.
Once the prediction is locked in, the Scene changes
Generation
50fixed
The Scene then shows: 76.74 · Mean wing darkness of the generation shown, 0 pale to 100 black/nodes/moths/outputs/meanDarkness
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The mean rose from about $19$ ⁠ to about $77$ ⁠. Each generation the birds ate more of the pale ones, and the survivors bred the next. That is what happened in 19th-century England: the dark form of the peppered moth went from rare to most, and after pollution fell, it declined again.

Hint
Which moths stand out on dark bark, and so are eaten more?

Worked solution
1. On dark bark pale moths stand out, and are eaten more.
2. The fairly dark moths survive and breed more.
3. Each generation inherits the shades of the survivors, so the mean moves a little towards dark.
4. After $50$ ⁠ generations the Scene shows a mean of about $77$ ⁠, close to the bark, whose darkness is $85$ ⁠.
Step 3 of 6
Open this step
The catch
Practice · Predict, then look
Question
In the first generation the birds eat $46$ ⁠ of the $120$ ⁠ moths. How many will they eat in generation $50$ ⁠, when the population is already dark?

Correct answer
10 ± 6
A number prediction counts as right within ± 6
Setup, choices and feedback
The Scene at this step
Generation
0fixed
Bark darkness
85 %fixed
Bird predation
50 %fixed
Sort by shade
On the bark
Once the prediction is locked in, the Scene changes
Generation
50fixed
The Scene then shows: 10 · Moths the birds eat in the generation shown/nodes/moths/outputs/eaten
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The birds now eat only about $10$ ⁠ moths. The population has adapted: most moths are camouflaged, so selection hardly changes them any more. That is also the end of the change: natural selection slows when there is no longer much difference between who survives and who does not.

Hint
How easy is it for a bird to find a dark moth on dark bark?

Worked solution
1. In the first generation most moths are pale and stand out, and the birds find many.
2. After $50$ ⁠ generations most moths are dark, close to the bark’s shade.
3. A camouflaged moth is unlikely to be eaten, so the number eaten falls: the Scene shows $10$ ⁠.
The same birds, the same level of predation: what changed is the moths.
Step 4 of 6
Open this step
No birds
Practice · Predict, then look
Question
Now there are no birds at all. The bark is still sooty. What will happen to the mean darkness after $50$ ⁠ generations?

Correct answer
- Correct It will hardly change
Setup, choices and feedback
The Scene at this step
Generation
0fixed
Bark darkness
85 %fixed
Bird predation
0 %fixed
Sort by shade
On the bark
Choices
- Correct It will hardly change
- Wrong It will rise anyway, because the bark is dark
  Misconception: Thinks a population keeps changing in one direction even when nothing selects among its members.
  Feedback from the catalogue
  Evolution by natural selection needs three things: variation, inheritance, and something that selects, such as birds eating more moths of one shade. With no birds, no shade gives an advantage, and the average only drifts a little by chance from generation to generation. There is no built-in direction of improvement.
- Wrong It will fall
  Misconception: Thinks a population keeps changing in one direction even when nothing selects among its members.
  Feedback from the catalogue
  Evolution by natural selection needs three things: variation, inheritance, and something that selects, such as birds eating more moths of one shade. With no birds, no shade gives an advantage, and the average only drifts a little by chance from generation to generation. There is no built-in direction of improvement.
Once the prediction is locked in, the Scene changes
Generation
50fixed
The Scene then shows: 24.14 · Mean wing darkness of the generation shown, 0 pale to 100 black/nodes/moths/outputs/meanDarkness
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The mean only wandered from about $19$ ⁠ to about $24$ ⁠, by chance. With no birds no shade has an advantage, so there is no selection. The bark alone does not change the moths: selection is what links them.

Hint
With no birds, does a dark moth have a better chance of surviving?

Worked solution
1. Natural selection needs variation, inheritance and a difference in survival.
2. With no birds every moth survives, and there is no difference in survival.
3. The mean moves only a little, at random, because each generation is a chance sample of its parents and small mutations add noise.
4. The Scene shows a mean of about $24$ ⁠ after $50$ ⁠ generations, against about $77$ ⁠ with birds.
Step 5 of 6
Open this step
Matching the bark
Practice · Reach the target
Question
Generation is fixed at $50$ ⁠, and Bird predation at $50\%$ ⁠. Change Bark darkness until, after the $50$ ⁠ generations, the moths’ mean darkness is about half, $50$ ⁠, then press Check.

Goal
- 50 ± 5
  At the start of the step: 12.78
Setup, choices and feedback
The Scene at this step
Generation
50fixed
Bark darkness
10 %
Bird predation
50 %fixed
Sort by shade
On the bark
Read from the Scene at
Mean wing darkness of the generation shown, 0 pale to 100 black/nodes/moths/outputs/meanDarkness
One setting that reaches it
Bark darkness
43 %
Feedback for a right answer
On mid-shaded bark the population reaches a middle shade. Selection follows the surroundings, which is why one species can look different in different places.

Feedback for a wrong answer
Not yet. Move the bark, and remember the moths do not always get all the way to its shade within $50$ ⁠ generations.

Hint
The birds push the moths towards the bark’s shade, but start from pale moths.

Worked solution
1. Selection brings the moths closer to the bark’s shade.
2. Within $50$ ⁠ generations they get close, but not always all the way, because they started pale.
3. Try bark of about $55\%$ ⁠: the Scene shows a mean of about $52$ ⁠.
Step 6 of 6
Open this step
Populations, not individuals
Practice · Choose
Question
One pale moth lives its whole life on sooty bark. What happens to it?

Correct answer
- Correct It stays pale. Only the population changes, from generation to generation
Setup, choices and feedback
The Scene at this step
Generation
50
Bark darkness
85 %
Bird predation
50 %
Sort by shade
Sorted by shade
Choices
Shown in a random order.
- Correct It stays pale. Only the population changes, from generation to generation
- Wrong It slowly darkens over its life
  Misconception: Thinks a single animal evolves during its life.
  Feedback from the catalogue
  An individual does not evolve. A moth that hatched pale stays pale all its life. What changes is the population: from generation to generation the share of dark moths grows, because they survive and breed more. Evolution is a change in how common inherited traits are in a population, over generations.
- Wrong Its young will be born darker, because it lived on dark bark
  Misconception: Thinks living things change because they need to: moths darken in order to hide.
  Feedback from the catalogue
  A moth cannot decide to darken, nor darken because it would help. A moth’s colour is set by the genes it got from its parents. What happens is different: the population already holds moths of different shades, the birds eat more of those that stand out, and the survivors pass their genes on. Need does not make the change. It only decides who survives out of what is already there.
Feedback for a right answer
Its shade is set by the genes it got, and the genes it passes on do not change because of the bark. It is simply at high risk of being eaten. Evolution happens in populations, not in individuals.

Hint
Do the surroundings change the genes a moth passes to its young?

Worked solution
1. Wing shade is set by genes, which do not change during life.
2. The bark does not change the genes a moth passes on.
3. What the bark changes is who survives. So the population changes, generation by generation, while each moth stays as it was born.

Lesson 2 of 5 · About 25 minutes

Antibiotic resistance

Natural selection within days: a few resistant mutants among a billion bacteria, a selection window that picks them out, why you finish the course, and why a low dose is dangerous.

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The 6 steps and their answers

Step 1 of 6
Open this step
A few mutants
Practice · Calculate
Question
The dish holds a billion bacteria, and about one in every ten million happens to carry a mutation that protects it from the drug. How many resistant bacteria are in the dish before the drug is added?

Correct answer
100 ± 1
Setup, choices and feedback
The Scene at this step
Drug level
4 × MICfixed
Course
7 daysfixed
Days after the drug
Drug addedfixed
Accepted within
± 1
Computed from the Scene at this step’s setting
Resistant cells on the day shown/nodes/dish/outputs/resistant
Wrong answers it recognises
- 0 ± 0.5
  Misconception: Thinks the drug creates resistance, or bacteria get used to it, rather than selecting mutants that were already there.
  Feedback from the catalogue
  The drug neither creates resistance nor “gets the bacteria used to it”. A colony of a billion cells already holds, by chance, about a hundred cells with a mutation that protects them. The drug kills the others and leaves them room, and they multiply. It is natural selection, fast: a bacterium divides every few hours, so what took the moths decades takes bacteria days.
Feedback for a right answer
$\frac{10^9}{10^7} = 100$ ⁠ resistant bacteria, before there is any drug. In the Scene they are the pink ones. The mutation happened by chance, like any mutation, and without the drug it gives no advantage.

Hint
A billion divided by ten million.

Worked solution
1. A billion: $10^9$ ⁠. Ten million: $10^7$ ⁠.
2. One in every $10^7$ ⁠: $\frac{10^9}{10^7} = 10^2 = 100$ ⁠.
3. A hundred resistant bacteria, hidden among a billion.
If you answered $0$ ⁠, you thought the drug creates the resistance. It only picks who survives.
Step 2 of 6
Open this step
The selection window
Practice · Predict, then look
Question
The drug level is $4$ ⁠ times the MIC: enough to kill the ordinary bacteria, but not the resistant ones, whose MIC is $16$ ⁠ times higher. The course lasts seven days. What will the dish look like after two weeks?

Correct answer
- Correct Full again, almost all of it resistant bacteria
Setup, choices and feedback
The Scene at this step
Drug level
4 × MICfixed
Course
7 daysfixed
Days after the drug
Drug addedfixed
Choices
- Correct Full again, almost all of it resistant bacteria
- Wrong Empty: the drug killed the colony
  Misconception: Thinks any dose of an antibiotic that kills ordinary bacteria clears the infection.
  Feedback from the catalogue
  A dose that kills the ordinary cells does not necessarily kill the resistant ones. Between the ordinary cells’ MIC and the resistant cells’ lies a selection window: there the drug wipes out the ordinary cells and leaves the resistant ones with no competitors. Only above the window does the drug kill both.
- Wrong As at the start: a few resistant among many ordinary
  Misconception: Thinks the drug creates resistance, or bacteria get used to it, rather than selecting mutants that were already there.
  Feedback from the catalogue
  The drug neither creates resistance nor “gets the bacteria used to it”. A colony of a billion cells already holds, by chance, about a hundred cells with a mutation that protects them. The drug kills the others and leaves them room, and they multiply. It is natural selection, fast: a bacterium divides every few hours, so what took the moths decades takes bacteria days.
Once the prediction is locked in, the Scene changes
Days after the drug
Two weeks onfixed
The Scene then shows: 1 · Share of the living cells that are resistant, 0..1 (0 when none are left)/nodes/dish/outputs/resistantShare
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Every bacterium in the dish is resistant. The drug cleared the space for them, and they filled it. That is the selection window: a level that kills the ordinary cells and not the resistant ones selects resistance.

Hint
Who survives the drug, and what happens to it when all the rest are dead?

Worked solution
1. A level of $4$ ⁠ times the MIC is above the ordinary cells’ MIC and below the resistant ones’, $16$ ⁠ times.
2. The ordinary cells die, and the resistant ones are hardly touched.
3. With no competition, the hundred resistant cells divide every few hours and fill the dish.
4. After the reveal the Scene shows that $100\%$ ⁠ of the living cells are resistant.
Step 3 of 6
Open this step
How fast
Practice · Predict, then look
Question
In the same course, after how many days will the resistant bacteria be more than half the living cells? Look at the chart beside the Scene.

Correct answer
1.563 d ± 0.5 d
A number prediction counts as right within ± 0.5 d
Setup, choices and feedback
The Scene at this step
Drug level
4 × MICfixed
Course
7 daysfixed
Days after the drug
Drug addedfixed
Once the prediction is locked in, the Scene changes
Days after the drug
Two weeks onfixed
The Scene then shows: 1.563 d · Time from the start until the resistant cells first outnumber the rest; NaN if never/nodes/dish/outputs/takeoverTime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Under two days: after about a day and a half the resistant cells are already the majority. In real life that is what happens when an infection does not respond to treatment: within a few days most of the bacteria in the body are resistant.

Hint
The ordinary bacteria die fast, within hours. The resistant ones only need to overtake them.

Worked solution
1. The drug kills the ordinary cells fast, and their number plunges a million-fold within hours.
2. Meanwhile the resistant cells divide, about every $4.6$ ⁠ hours.
3. The lines on the chart cross when the rising resistant cells pass the falling ordinary ones: the Scene shows about $1.6$ ⁠ days.
Step 4 of 6
Open this step
Finishing the course
Practice · Reach the target
Question
Now the drug level is $32$ ⁠ times the MIC, above the window: it kills the resistant cells too. But the course lasts only two days, and the bacteria grow back. Change Course until the dish is completely empty after two weeks, then press Check.

Goal
- 5 ± 0.1
  At the start of the step: 8
Setup, choices and feedback
The Scene at this step
Drug level
32 × MICfixed
Course
2 days
Days after the drug
Two weeks onfixed
Read from the Scene at
0 start, 1 no drug, 2 below the MIC, 3 selecting, 4 both dying, 5 cleared, 6 taken over, 7 grown back resistant, 8 grown back killable/nodes/dish/outputs/stage
One setting that reaches it
Course
3 days
Feedback for a right answer
Three days are enough: the last cell dies after about $2.9$ ⁠ days. Two days stopped the course a few hours before the end, and the few cells left filled the dish again. That is why you finish the course even when you feel well.

Feedback for a wrong answer
Not yet. Move Course and watch on the chart when the number of cells reaches zero.

Hint
The dish looks empty by day two, but a few cells are still alive. How long does it take to kill the last one?

Worked solution
1. At this level the drug kills every cell, but not at once.
2. The chart shows the last cell dies after about $2.9$ ⁠ days.
3. Any course that ends before that leaves cells alive, which grow back.
4. Set Course to $3$ ⁠ days or more.
Step 5 of 6
Open this step
Below the MIC
Practice · Predict, then look
Question
The drug level is half the MIC, for ten days: too little to kill the bacteria. What will happen to the resistant bacteria?

Correct answer
- Correct They will slowly multiply, and become a bigger part of the colony
Setup, choices and feedback
The Scene at this step
Drug level
0.5 × MICfixed
Course
10 daysfixed
Days after the drug
Drug addedfixed
Choices
- Correct They will slowly multiply, and become a bigger part of the colony
- Wrong Nothing: a level like that does nothing
  Misconception: Thinks a dose too low to kill the bacteria does nothing at all.
  Feedback from the catalogue
  Below the MIC the drug does not kill the colony, but it does slow the ordinary cells more than the resistant ones. So the resistant share grows slowly, unseen. That is how low doses, for instance antibiotics in animal feed, breed resistance.
- Wrong They will die, like all the rest
  Misconception: Thinks any dose of an antibiotic that kills ordinary bacteria clears the infection.
  Feedback from the catalogue
  A dose that kills the ordinary cells does not necessarily kill the resistant ones. Between the ordinary cells’ MIC and the resistant cells’ lies a selection window: there the drug wipes out the ordinary cells and leaves the resistant ones with no competitors. Only above the window does the drug kill both.
Once the prediction is locked in, the Scene changes
Days after the drug
Two weeks onfixed
The Scene then shows: 1902 · Resistant cells on the day shown/nodes/dish/outputs/resistant
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
From $100$ ⁠ to about $1900$ ⁠ resistant cells, with the colony hardly noticing. A low level slows the ordinary cells more than the resistant ones, and so gives them a small edge. In the world, levels like this in farming and sewage breed resistant bacteria.

Hint
Whom does the drug slow more: the ordinary cells or the resistant ones?

Worked solution
1. Below the MIC the drug does not kill, but slows the growth of the ordinary cells.
2. It hardly slows the resistant ones.
3. A small edge over many generations: the number of resistant cells grows about $19$ ⁠ times, to $1900$ ⁠.
Their share is still tiny, but it rose, and that is how resistance begins.
Step 6 of 6
Open this step
Bacteria, not viruses
Practice · Choose
Question
Why do doctors not give antibiotics for flu?

Correct answer
- Correct Flu is caused by a virus, and antibiotics harm only bacteria
Setup, choices and feedback
The Scene at this step
Drug level
4 × MIC
Course
7 days
Days after the drug
0.5 (Drug added → Two weeks on)
Choices
Shown in a random order.
- Correct Flu is caused by a virus, and antibiotics harm only bacteria
- Wrong Because flu is too mild to be worth the drug
  Misconception: Thinks antibiotics work against viruses such as flu or a cold.
  Feedback from the catalogue
  Antibiotics attack the machinery of bacteria, such as their cell wall or their ribosomes. A virus has no wall and no ribosomes: it uses the machinery of the cell it invades. So antibiotics do not help against flu or a cold, and every needless use only selects resistant bacteria in the body.
- Wrong Because the flu virus is already resistant to antibiotics
  Misconception: Thinks antibiotics work against viruses such as flu or a cold.
  Feedback from the catalogue
  Antibiotics attack the machinery of bacteria, such as their cell wall or their ribosomes. A virus has no wall and no ribosomes: it uses the machinery of the cell it invades. So antibiotics do not help against flu or a cold, and every needless use only selects resistant bacteria in the body.
Feedback for a right answer
Antibiotics attack the cell wall, ribosomes or proteins of bacteria. A virus has none of these. Giving antibiotics for flu does not help, and only selects resistant bacteria in the body.

Hint
What does an antibiotic attack? Does a virus have one?

Worked solution
1. A bacterium is a living cell, with a wall, ribosomes and machinery of its own, and that is what antibiotics attack.
2. A virus is genetic material in a protein coat, which multiplies only inside other living things’ cells.
3. So an antibiotic has nothing to attack in a virus. Against viruses there are other drugs, and vaccines.

Lesson 3 of 5 · About 20 minutes

The fossil record

Why most living things never become fossils, which conditions keep even the soft parts, and how rock layers and dated ash beds tell how old a fossil is.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
A rare event
Practice · Predict, then look
Question
A fish dies and sinks to a well-aired sea floor, and mud covers it only after weeks. What of it will be left as a fossil?

Correct answer
- Correct Scattered bones
Setup, choices and feedback
The Scene at this step
What dies
Bony fishfixed
Oxygen at the sea floor
100 %fixed
Covered by mud
After weeksfixed
Time
Deathfixed
Choices
- Wrong The whole fish, with the outline of its skin
  Misconception: Thinks most dead plants and animals end up as fossils.
  Feedback from the catalogue
  Most living things leave no fossil at all. A body left exposed is eaten by scavengers, dug through by burrowers, and rots. For anything to remain it needs quick burial in mud, ideally little oxygen, and even then usually only the hard parts are left: bones, teeth and shells. So the fossil record is full of gaps, and soft creatures, such as jellyfish, hardly appear in it.
- Wrong A whole skeleton
  Misconception: Thinks most dead plants and animals end up as fossils.
  Feedback from the catalogue
  Most living things leave no fossil at all. A body left exposed is eaten by scavengers, dug through by burrowers, and rots. For anything to remain it needs quick burial in mud, ideally little oxygen, and even then usually only the hard parts are left: bones, teeth and shells. So the fossil record is full of gaps, and soft creatures, such as jellyfish, hardly appear in it.
- Correct Scattered bones
- Wrong Nothing
  Feedback
  Bones are hard enough to last weeks outside, even if not whole. Nothing is left of soft creatures, or of a fish left exposed for decades.
Once the prediction is locked in, the Scene changes
Time
Exposedfixed
The Scene then shows: 1 · 0 nothing, 1 pieces, 2 whole hard parts, 3 soft parts too/nodes/cliff/outputs/fossil
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Scattered bones: in the weeks the fish lay exposed, scavengers ate the flesh and scattered the skeleton, and animals burrowing in the mud stirred the rest. That is the usual fate: most fossils are pieces, not whole bodies.

Hint
Who lives on a sea floor with plenty of oxygen, and what does it do to a body?

Worked solution
1. A well-aired sea is home to scavengers and to animals that burrow in the mud.
2. Weeks of exposure are enough for the flesh to be eaten and the skeleton to fall apart.
3. What is left are the hard bones, scattered: the Scene shows $1$ ⁠, pieces.
Change Oxygen at the sea floor and Covered by mud and compare.
Step 2 of 5
Open this step
Soft bodies
Practice · Predict, then look
Question
A jellyfish, with no bones or shell, dies on a sea floor with little oxygen, and is covered by mud after weeks. What will be left of it?

Correct answer
- Correct Nothing
Setup, choices and feedback
The Scene at this step
What dies
Jellyfishfixed
Oxygen at the sea floor
10 %fixed
Covered by mud
After weeksfixed
Time
Deathfixed
Choices
- Correct Nothing
- Wrong An imprint of the body in the mud
  Misconception: Thinks most dead plants and animals end up as fossils.
  Feedback from the catalogue
  Most living things leave no fossil at all. A body left exposed is eaten by scavengers, dug through by burrowers, and rots. For anything to remain it needs quick burial in mud, ideally little oxygen, and even then usually only the hard parts are left: bones, teeth and shells. So the fossil record is full of gaps, and soft creatures, such as jellyfish, hardly appear in it.
- Wrong The whole jellyfish, turned to stone
  Misconception: Thinks most dead plants and animals end up as fossils.
  Feedback from the catalogue
  Most living things leave no fossil at all. A body left exposed is eaten by scavengers, dug through by burrowers, and rots. For anything to remain it needs quick burial in mud, ideally little oxygen, and even then usually only the hard parts are left: bones, teeth and shells. So the fossil record is full of gaps, and soft creatures, such as jellyfish, hardly appear in it.
Once the prediction is locked in, the Scene changes
Time
Exposedfixed
The Scene then shows: 0 · 0 nothing, 1 pieces, 2 whole hard parts, 3 soft parts too/nodes/cliff/outputs/fossil
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Nothing: a soft body rots within days, and weeks of exposure wipe it out. A jellyfish imprint is left only if mud covers it within hours. Try Within hours (a storm).

Hint
A soft body rots within days. How long did it lie exposed?

Worked solution
1. A jellyfish has no hard parts.
2. Even with little oxygen, a soft body rots within days.
3. After weeks nothing of it is left that could turn to stone: the Scene shows $0$ ⁠.
So the fossil record is biased: creatures with bones and shells appear in it far more than soft ones.
Step 3 of 5
Open this step
Perfect preservation
Practice · Reach the target
Question
Palaeontologists hunt for rare fossils in which the soft parts are kept too: skin, eyes, even stomach contents. Change Oxygen at the sea floor and Covered by mud until the fish is kept with its soft parts, then press Check.

Goal
- 3 ± 0.1
  At the start of the step: 1
Setup, choices and feedback
The Scene at this step
What dies
Bony fishfixed
Oxygen at the sea floor
100 %
Covered by mud
After weeks
Time
Buried
Read from the Scene at
0 nothing, 1 pieces, 2 whole hard parts, 3 soft parts too/nodes/cliff/outputs/fossil
One setting that reaches it
Oxygen at the sea floor
0 %
Covered by mud
Within hours (a storm)
Feedback for a right answer
Little oxygen, and burial within hours: no scavengers, no burrowers, and rot hardly gets started. That is how famous sites such as the Burgess Shale in Canada formed, keeping soft creatures more than half a billion years old.

Feedback for a wrong answer
Not yet. Is there time for rot before the mud arrives? And who lives on a sea floor with plenty of oxygen?

Hint
Two things slow the destruction: little oxygen, and quick burial.

Worked solution
1. Soft parts rot within days, so burial must come within hours.
2. Plenty of oxygen means scavengers and burrowers that wreck the body, so you need little oxygen.
3. Set Covered by mud to Within hours (a storm), and Oxygen at the sea floor to $10\%$ ⁠ or less.
Step 4 of 5
Open this step
Reading the layers
Practice · Choose
Question
In the cliff, an ash bed below the fish’s layer is dated to $183.4$ ⁠ million years, and an ash bed above it to $182.6$ ⁠ million years. What can be said about the fish’s age?

Correct answer
- Correct It is between $182.6$ ⁠ and $183.4$ ⁠ million years
Setup, choices and feedback
The Scene at this step
What dies
Bony fish
Oxygen at the sea floor
10 %
Covered by mud
Within hours (a storm)
Time
Uplifted
Choices
Shown in a random order.
- Correct It is between $182.6$ ⁠ and $183.4$ ⁠ million years
- Wrong It is younger than $182.6$ ⁠ million years
  Misconception: Reads rock layers the wrong way round: takes a deeper layer to be younger.
  Feedback from the catalogue
  Rock layers form as sediment builds up on the sea floor, layer upon layer. Each new layer is laid on top of the older ones, so in an undisturbed sequence the deepest layer is the oldest. That is the law of superposition. A fossil lying between two dated ash beds is younger than the one below it, and older than the one above.
- Wrong It is older than $183.4$ ⁠ million years
  Misconception: Reads rock layers the wrong way round: takes a deeper layer to be younger.
  Feedback from the catalogue
  Rock layers form as sediment builds up on the sea floor, layer upon layer. Each new layer is laid on top of the older ones, so in an undisturbed sequence the deepest layer is the oldest. That is the law of superposition. A fossil lying between two dated ash beds is younger than the one below it, and older than the one above.
- Wrong You cannot tell without dating the fish itself
  Feedback
  A fish’s bone cannot be dated directly by radioactive methods, but the layers round it can. The order of the layers gives a range.
Feedback for a right answer
The fish sank onto a layer already there, and was covered by layers that came after it. So it is younger than the ash below it and older than the ash above: about $183$ ⁠ million years, in the Jurassic.

Hint
Which layer was laid down first: the one below the fish or the one above it?

Worked solution
1. The law of superposition: in undisturbed layers, what is lower was laid first.
2. The ash below the fish, $183.4$ ⁠ million years, was laid before the fish died.
3. The ash above it, $182.6$ ⁠ million years, was laid after.
4. So the fish lies between the two ages.
Ash beds are dated by the radioactive decay of uranium in tiny crystals inside them: uranium turns to lead at a steady rate, so the ratio between them shows how long ago the crystal formed.
Step 5 of 5
Open this step
Deep time
Practice · Calculate
Question
How much time passed between the two ash beds, $183.4$ ⁠ and $182.6$ ⁠ million years? Answer in millions of years.

Correct answer
0.8 ± 0.01
Setup, choices and feedback
The Scene at this step
What dies
Bony fish
Oxygen at the sea floor
10 %
Covered by mud
Within hours (a storm)
Time
Uplifted
Accepted within
± 0.01
A fixed value in the question
Wrong answers it recognises
- -0.8 ± 0.01
  Misconception: Reads rock layers the wrong way round: takes a deeper layer to be younger.
  Feedback from the catalogue
  Rock layers form as sediment builds up on the sea floor, layer upon layer. Each new layer is laid on top of the older ones, so in an undisturbed sequence the deepest layer is the oldest. That is the law of superposition. A fossil lying between two dated ash beds is younger than the one below it, and older than the one above.
Feedback for a right answer
$183.4 - 182.6 = 0.8$ ⁠ million years, that is $800{,}000$ ⁠ years, for a thin stretch of rock. Sediment on a deep sea floor builds up very slowly.

Hint
The difference between two ages: the older minus the younger.

Worked solution
1. The lower ash is older: $183.4$ ⁠ million years.
2. The upper ash: $182.6$ ⁠ million years.
3. The difference: $183.4 - 182.6 = 0.8$ ⁠ million years.
4. That is $800{,}000$ ⁠ years.
If you got a negative number, you subtracted the wrong way round: time elapsed is always positive.

Lesson 4 of 5 · About 25 minutes

Classifying life

The ranks from domain to species, the difference between sorting by looks and sorting by ancestry, shared ancestors on the tree of life, and why a whale is close to a hippo.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Ranks
Practice · Calculate
Question
The table beside the Scene lists the ranks of a human and an Egyptian fruit bat, from domain to species. In how many ranks, from the top, do they match?

Correct answer
4 ± 0.1
Setup, choices and feedback
The Scene at this step
Focus on
Humanfixed
Compare with
Fruit batfixed
Sort by ancestry
By looks
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Ranks the two share from the domain down, 0 to 8/nodes/tree/outputs/sharedRanks
Wrong answers it recognises
- 1 ± 0.1
  Misconception: Groups living things by how they look or live, taking that for how closely they are related.
  Feedback
  A bat does not look like a human, but it is a mammal: kingdom, phylum and class are shared too.
Feedback for a right answer
Four: eukaryotes, animals, chordates and mammals. At order they part: the human is a primate, and the bat in the bat order.

Hint
Read from the top: domain, kingdom, phylum, class… until the names part.

Worked solution
1. The ranks, broad to narrow: domain, kingdom, phylum, class, order, family, genus, species.
2. Human and bat: Eukaryota, Animalia, Chordata, Mammalia match.
3. At order: Primates against Chiroptera.
4. Shared ranks: $4$ ⁠.
Every living thing belongs to a group at every rank, and each group holds the groups below it, like an address: country, city, street, house.
Step 2 of 5
Open this step
Looks and ancestry
Practice · Predict, then look
Question
Sorted by looks, the whale sits with the shark: both swim with fins. To which is the whale more closely related by descent: the shark, or the hippo?

Correct answer
- Correct The hippo
Setup, choices and feedback
The Scene at this step
Focus on
Whale
Compare with
Sharkfixed
Sort by ancestry
By looksfixed
Choices
- Correct The hippo
- Wrong The shark
  Misconception: Groups living things by how they look or live, taking that for how closely they are related.
  Feedback from the catalogue
  Looking alike does not always mean being related. A whale and a shark both swim with fins, because both live in water, but the whale is a mammal: it breathes air, suckles its young, and its DNA is close to the hippo’s. A similar shape that evolved separately because the lives are similar is called convergent evolution. Real kinship is set by shared ancestry, and DNA shows it.
- Wrong Both equally
  Misconception: Groups living things by how they look or live, taking that for how closely they are related.
  Feedback from the catalogue
  Looking alike does not always mean being related. A whale and a shark both swim with fins, because both live in water, but the whale is a mammal: it breathes air, suckles its young, and its DNA is close to the hippo’s. A similar shape that evolved separately because the lives are similar is called convergent evolution. Real kinship is set by shared ancestry, and DNA shows it.
Once the prediction is locked in, the Scene changes
Compare with
Hippofixed
Sort by ancestry
By ancestryfixed
The Scene then shows: 55 · Age of the last ancestor the two share, millions of years (0 for the same organism)/nodes/tree/outputs/ancestorAge
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The hippo: their last shared ancestor lived about $55$ ⁠ million years ago, and the whale’s and the shark’s about $465$ ⁠. The whale’s forebears lived on land, and its flippers evolved again, separately from those of fish.

Hint
A whale breathes air and suckles its young. How is that like a shark?

Worked solution
1. Sort by ancestry rebuilds the tree by DNA.
2. The marker where the whale’s and the hippo’s lines meet: $55$ ⁠ million years.
3. Compare the shark: the whale’s and shark’s lines meet only $465$ ⁠ million years ago.
4. The younger the shared ancestor, the closer the kinship: the whale is closer to the hippo.
Step 3 of 5
Open this step
The tree of life
Practice · Reach the target
Question
Focus on is fixed at Human, and the tree is sorted by ancestry. Choose in Compare with the living thing whose last ancestor shared with the human lived about $320$ ⁠ million years ago, then press Check.

Goal
- 320 ± 1
  At the start of the step: 465
Setup, choices and feedback
The Scene at this step
Focus on
Humanfixed
Compare with
Shark
Sort by ancestry
By ancestry
Read from the Scene at
Age of the last ancestor the two share, millions of years (0 for the same organism)/nodes/tree/outputs/ancestorAge
One setting that reaches it
Compare with
Sparrow
Feedback for a right answer
The sparrow: birds and mammals share a land ancestor that laid shelled eggs, about $320$ ⁠ million years ago. The reptiles came from that line too, which is why birds are closer to crocodiles than to any mammal.

Feedback for a wrong answer
Not yet. Check the age at the marker where the two lit lines part.

Hint
$320$ ⁠ million years is before the first mammals, but after the vertebrates came onto land.

Worked solution
1. Go through the living things in Compare with, and look at the age at the marker.
2. Bat, hippo and whale: $90$ ⁠ million years, all placental mammals.
3. Sparrow: $320$ ⁠ million years.
4. Shark: $465$ ⁠. Bee: $600$ ⁠.
Step 4 of 5
Open this step
Wings twice
Practice · Predict, then look
Question
The bat and the sparrow both fly, and sorted by looks they are in the same group. To which is the bat more closely related: the sparrow, or the hippo?

Correct answer
- Correct The hippo
Setup, choices and feedback
The Scene at this step
Focus on
Fruit bat
Compare with
Sparrowfixed
Sort by ancestry
By looksfixed
Choices
- Correct The hippo
- Wrong The sparrow
  Misconception: Groups living things by how they look or live, taking that for how closely they are related.
  Feedback from the catalogue
  Looking alike does not always mean being related. A whale and a shark both swim with fins, because both live in water, but the whale is a mammal: it breathes air, suckles its young, and its DNA is close to the hippo’s. A similar shape that evolved separately because the lives are similar is called convergent evolution. Real kinship is set by shared ancestry, and DNA shows it.
Once the prediction is locked in, the Scene changes
Compare with
Hippofixed
Sort by ancestry
By ancestryfixed
The Scene then shows: 80 · Age of the last ancestor the two share, millions of years (0 for the same organism)/nodes/tree/outputs/ancestorAge
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The hippo: a shared ancestor about $80$ ⁠ million years ago, against $320$ ⁠ with the sparrow. A bat’s wing is a hand, with skin stretched between long fingers, and a bird’s wing is made of feathers. The two evolved separately.

Hint
A bat is a mammal. What does that say about its body?

Worked solution
1. A bat is a mammal: it has fur, and suckles its young.
2. By DNA, bat and hippo parted about $80$ ⁠ million years ago.
3. Bat and sparrow parted about $320$ ⁠ million years ago.
4. So the bat is closer to the hippo. Flight evolved twice, by convergent evolution.
Step 5 of 5
Open this step
Fungi
Practice · Choose
Question
The fly agaric is a fungus. To which is it more closely related: the oak, or the human?

Correct answer
- Correct The human
Setup, choices and feedback
The Scene at this step
Focus on
Fly agaric
Compare with
Oak
Sort by ancestry
By ancestry
Choices
Shown in a random order.
- Correct The human
- Wrong The oak: both grow from the ground and stay put
  Misconception: Takes fungi such as mushrooms for plants.
  Feedback from the catalogue
  Fungi do not move about, so they were once sorted with the plants. But they have no chlorophyll, do not photosynthesise, and feed like animals, on organic matter they break down. DNA shows they are closer to the animals: the last ancestor a mushroom shares with a human lived about $1.1$ ⁠ billion years ago, and with an oak about $1.6$ ⁠ billion.
- Wrong Neither: fungi are a separate kingdom, with no relatives
  Feedback
  Fungi are a separate kingdom, but every living thing has relatives. The only question is when they parted.
Feedback for a right answer
The human: fungi and animals parted about $1.1$ ⁠ billion years ago, and plants split from both about $1.6$ ⁠ billion years ago. Like animals, fungi feed on matter others built, and their cell walls are made of chitin, the stuff of insect skeletons.

Hint
Check the tree sorted by ancestry: where do the fungus’s lines meet each of them?

Worked solution
1. Set Focus on to Fly agaric, and Compare with to Oak: an ancestor about $1.6$ ⁠ billion years ago.
2. Switch Compare with to Human: about $1.1$ ⁠ billion years.
3. The ancestor shared with the human is younger, so the fungus is closer to the human.

Lesson 5 of 5 · About 25 minutes

The clock of life

All 4.5 billion years of the Earth squeezed into one day and one year: microbes alone for most of the day, animals arriving in the evening, dinosaurs in December, and our species in the last seconds.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
A day of history
Practice · Predict, then look
Question
The first sure signs of life are about $3500$ ⁠ million years old. The Earth formed $4540$ ⁠ million years ago, at midnight. Predict how many hours after midnight life appears on the one-day clock.

Correct answer
5.498 h ± 0.5 h
A number prediction counts as right within ± 0.5 h
Setup, choices and feedback
The Scene at this step
Squeeze it into
One dayfixed
Dial shows
All 4.5 billion yearsfixed
Through time
Earth forms
Once the prediction is locked in, the Scene changes
Through time
First life
The Scene then shows: 5.498 h · Time since the start of the day or year at the hand, s/nodes/clock/outputs/clockTime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About five and a half hours, at 05:30 in the morning. The Earth was too hot and too battered in its first few hundred million years, but after that life appeared fairly quickly. The surprise is what came next: for most of the day it stayed single cells.

Worked solution
1. Gone by: $4540 - 3500 = 1040$ ⁠ million years since the Earth formed.
2. The share of the day: $\dfrac{1040}{4540} \approx 0.229$ ⁠.
3. $0.229 \times 24 \approx 5.5$ ⁠ hours after midnight.
If you got about $18.5$ ⁠, you placed “ $3500$ ⁠ million years ago” from the start of the day instead of from its end.
Step 2 of 6
Open this step
Animals come late
Practice · Calculate
Question
The first animals appeared about $575$ ⁠ million years ago. On the one-day clock, how long before midnight do they arrive?

Correct answer
3.04 h ± 3%
Setup, choices and feedback
The Scene at this step
Squeeze it into
One dayfixed
Dial shows
All 4.5 billion yearsfixed
Through time
First animalsfixed
Accepted within
± 3%
Computed from the Scene at this step’s setting
Time left until the end of the day or year at the hand, s/nodes/clock/outputs/toGo
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 20.96 h ± 3%
  Misconception: Places how long ago something happened on the clock by counting forwards from the start of the day, instead of back from the end.
  Feedback from the catalogue
  “How long ago” is measured back from the present, and the present is the end of the day, the next midnight. An event $575$ ⁠ million years ago sits $\dfrac{575}{4540}$ ⁠ of the day before the end, about three hours before midnight. Its time on the clock is what is left after taking that from $24$ ⁠ hours.
Feedback for a right answer
About three hours, $\dfrac{575}{4540} \times 24 \approx 3.04$ ⁠, a little before nine in the evening. Nearly seven-eighths of the day went by without a single animal you could see. Every animal that ever lived, from Dickinsonia to you, fits in the last eighth.

Hint
“Ago” is measured from the end of the day. What share of $4540$ ⁠ million years is $575$ ⁠ million?

Worked solution
1. An event $575$ ⁠ million years ago sits $\dfrac{575}{4540}$ ⁠ of the day before the end.
2. $\dfrac{575}{4540} \approx 0.127$ ⁠.
3. $0.127 \times 24 \approx 3.04$ ⁠ hours before midnight, at about 20:57.
If you answered about $21$ ⁠ hours, you gave the time on the clock, the time since the previous midnight, not the time left until midnight.
Step 3 of 6
Open this step
Microbes alone
Practice · Choose
Question
Life appeared about $3500$ ⁠ million years ago, and the first many-celled life about $1000$ ⁠ million years ago. For what share of all the time there has been life on Earth were all living things single cells?

Correct answer
- Correct About 70%
Setup, choices and feedback
The Scene at this step
Squeeze it into
One dayfixed
Dial shows
All 4.5 billion years
Through time
Many cells
Choices
Shown in a random order.
- Wrong About a tenth
  Feedback
  That is roughly the share with animals, not the share with only single cells.
- Wrong About a third
  No feedback of its own; the learner sees the standard message
  Not yet. Look again and try once more.
- Correct About 70%
- Wrong Almost all of it, 99%
  Feedback
  A billion years is a sizeable share of $3500$ ⁠ million, not one percent.
Feedback for a right answer
About 70%: $\dfrac{3500 - 1000}{3500} \approx 0.71$ ⁠. For $2500$ ⁠ million years, from half past five in the morning to twenty to seven in the evening on the clock, every living thing was microscopic. In that time they changed the Earth: they filled the air with oxygen.

Worked solution
1. All the time with life: $3500$ ⁠ million years.
2. Only single cells: from $3500$ ⁠ to $1000$ ⁠ million years ago, $2500$ ⁠ million years.
3. $\dfrac{2500}{3500} \approx 0.71$ ⁠, about 70%.
Step 4 of 6
Open this step
A year of history
Practice · Predict, then look
Question
The Earth’s whole history is squeezed into one year. The first dinosaurs appeared about $230$ ⁠ million years ago. Predict when in the year they arrive.

Correct answer
- Correct In mid-December
Setup, choices and feedback
The Scene at this step
Squeeze it into
One yearfixed
Dial shows
All 4.5 billion yearsfixed
Through time
Plants on land
Choices
- Wrong In July
  No feedback of its own; the learner sees the standard message
  Not yet. Look again and try once more.
- Wrong In October
  No feedback of its own; the learner sees the standard message
  Not yet. Look again and try once more.
- Correct In mid-December
- Wrong On 31 December
  Feedback
  That is our species’ date. The asteroid that ended the dinosaurs falls on 26 December; the dinosaurs themselves came earlier.
Once the prediction is locked in, the Scene changes
Through time
Dinosaurs
The Scene then shows: 346.5 d · Time since the start of the day or year at the hand, s/nodes/clock/outputs/clockTime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
On $13$ ⁠ December. $\dfrac{230}{4540}$ ⁠ of the year is about $18.5$ ⁠ days before the end. The whole age of dinosaurs, some $165$ ⁠ million years, fits in the last two weeks of the year, and plants reached land only at the end of November.

Worked solution
1. The share of the year left: $\dfrac{230}{4540} \approx 0.051$ ⁠.
2. $0.051 \times 365 \approx 18.5$ ⁠ days before the end of the year.
3. $31 - 18.5 \approx 12.5$ ⁠: $13$ ⁠ December.
Step 5 of 6
Open this step
Dinosaurs, mammals and us
Practice · Choose
Question
Which of these lived at the same time as the dinosaurs other than birds, between $230$ ⁠ and $66$ ⁠ million years ago? Choose all of them.

Correct answer
- Correct Small mammals
- Correct Birds
Setup, choices and feedback
The Scene at this step
Squeeze it into
One year
Dial shows
Last 70 million
Through time
Asteroid
Choices
Shown in a random order. More than one choice is correct: every one must be selected.
- Correct Small mammals
- Correct Birds
- Wrong The first people
  Misconception: Thinks people lived at the same time as the dinosaurs.
  Feedback from the catalogue
  More than $60$ ⁠ million years separate the last dinosaurs (other than birds) from the first people. The asteroid struck $66$ ⁠ million years ago; our lineage split from the chimpanzees’ $6$ ⁠ to $7$ ⁠ million years ago, and our species appeared about $300\,000$ ⁠ years ago. On the one-day clock, that is the gap between 23:39 and six seconds to midnight.
- Wrong Trilobites
  Feedback
  Trilobites vanished in the Great Dying, $252$ ⁠ million years ago, before the dinosaurs appeared.
- Wrong None of these: mammals and birds appeared only after the dinosaurs died out
  Misconception: Thinks mammals appeared only after the dinosaurs died out.
  Feedback from the catalogue
  The first mammals appeared soon after the first dinosaurs, more than $200$ ⁠ million years ago, and lived beside them for over $150$ ⁠ million years: small, and active mostly at night. The extinction did not create mammals; it made room for them, and only after it did they grow large and spread into every way of life.
Feedback for a right answer
Mammals and birds. The first mammals appeared soon after the dinosaurs, and birds evolved from small dinosaurs during the age of dinosaurs. People arrived more than $60$ ⁠ million years after the asteroid; on the year of history, on the evening of $31$ ⁠ December.

Worked solution
1. Dinosaurs (not birds): from $230$ ⁠ to $66$ ⁠ million years ago.
2. Mammals: from more than $200$ ⁠ million years ago, so within that time.
3. Birds: descendants of dinosaurs, from about $150$ ⁠ million years ago, also within it.
4. Trilobites: until $252$ ⁠ million years ago, before it. People: from a few million years ago, long after it.
Step 6 of 6
Open this step
The last seconds
Practice · Calculate
Question
Our species appeared about $300\,000$ ⁠ years ago. On the one-day clock, how many seconds before midnight?

Correct answer
5.709 s ± 0.5 s
Setup, choices and feedback
The Scene at this step
Squeeze it into
One dayfixed
Dial shows
All 4.5 billion yearsfixed
Through time
Our speciesfixed
Accepted within
± 0.5 s
Computed from the Scene at this step’s setting
Time left until the end of the day or year at the hand, s/nodes/clock/outputs/toGo
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 86390 s ± 1%
  Misconception: Places how long ago something happened on the clock by counting forwards from the start of the day, instead of back from the end.
  Feedback from the catalogue
  “How long ago” is measured back from the present, and the present is the end of the day, the next midnight. An event $575$ ⁠ million years ago sits $\dfrac{575}{4540}$ ⁠ of the day before the end, about three hours before midnight. Its time on the clock is what is left after taking that from $24$ ⁠ hours.
Feedback for a right answer
About six seconds: $\dfrac{300\,000}{52\,500} \approx 5.7$ ⁠. Farming began a quarter of a second before midnight, and all written history fits in the last tenth of a second. Even when Dial shows only the last $7$ ⁠ million years, our species takes up only about a twenty-third of the dial.

Hint
A day has $86\,400$ ⁠ seconds, and each second is about $52\,500$ ⁠ years.

Worked solution
1. One second on the clock: $\dfrac{4\,540\,000\,000}{86\,400} \approx 52\,500$ ⁠ years.
2. $\dfrac{300\,000}{52\,500} \approx 5.7$ ⁠ seconds.
3. About six seconds before midnight: 23:59:54.

Across the unit · 15 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

Natural selection

Variation

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

Fifty generations

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Hint

Worked solution

The catch

Question

Correct answer

The Scene at this step

Once the prediction is locked in, the Scene changes

Hint

Worked solution

No birds

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Hint

Worked solution

Matching the bark

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Populations, not individuals

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

Antibiotic resistance

A few mutants

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

The selection window

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Hint

Worked solution

How fast

Question

Correct answer

The Scene at this step

Once the prediction is locked in, the Scene changes

Hint

Worked solution

Finishing the course

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Below the MIC

Question

Correct answer

The Scene at this step

Choices