Chapter 2 of 8

All 8 chapters
  1. 1Natural selection, readunderstood
  2. 2Evidence for evolution, readunderstood
  3. 3New species and extinction, readunderstood
  4. 4Classification, readunderstood
  5. 5The carbon and nitrogen cycles, readunderstood
  6. 6Measuring populations, readunderstood
  7. 7Interdependence, readunderstood
  8. 8Biodiversity and people, readunderstood

S4.3.2 Chapter 2

Evidence for evolution

How do we know that living things have changed over millions of years, when nobody was there to see it?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how fossils and DNA show that species are related
  • Explain why the fossil record has gaps

Nobody saw the first birds or the first fish. But the past leaves records. Two of the most important are fossils and DNA. This chapter shows how they are used.

A fossil is the remains or the trace of a living thing from long ago, found in rock, ice or amber. There are three common kinds. The first kind is the hard parts of a body, such as shells, bones and teeth. Over a long time, minerals slowly turn them to stone.

The second kind is a trace, such as a footprint, a burrow or a root hole. The third kind is the living thing itself, kept from rotting, such as an insect in amber or a plant in peat. Amber is hard tree resin.

Most dead bodies rot or are eaten. Most fossils form when a body is buried quickly in mud or sand. A fossil can also form when something stops the rotting, such as ice, amber or peat. This is rare. So fossils are only a small part of what once lived.

Many fossils lie in sedimentary rock. This rock forms in layers. Mud and sand settle one layer on another, then slowly harden. So in layers that have not been moved, the lowest layer is the oldest. The youngest is at the top.

Draw it first

Layers build up from the bottom

  1. A cut through layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. The first picture shows layer 2 over layer 1. The second adds layer 3 on top. The third adds layer 4 on top. Layer 1 holds a shell, layer 2 holds a shell and a fish, layer 3 holds a fish and layer 4 holds a fern.layer 2layer 1youngestoldest

    1 Layer 1 is laid down first, with a shell in it. Layer 2 settles on top.

  2. A cut through layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. The first picture shows layer 2 over layer 1. The second adds layer 3 on top. The third adds layer 4 on top. Layer 1 holds a shell, layer 2 holds a shell and a fish, layer 3 holds a fish and layer 4 holds a fern.layer 3layer 2layer 1youngestoldest

    2 Layer 3 settles on top of layer 2.

  3. A cut through layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. The first picture shows layer 2 over layer 1. The second adds layer 3 on top. The third adds layer 4 on top. Layer 1 holds a shell, layer 2 holds a shell and a fish, layer 3 holds a fish and layer 4 holds a fern.layer 4layer 3layer 2layer 1youngestoldest

    3 Layer 4 settles on top. The first layer, layer 1, is the oldest and lies at the bottom.

Now line up the fossils from the oldest layer to the youngest. We see that the kinds of living thing change. Old layers hold kinds that are no longer alive. Younger layers hold kinds that look more like living species. This shows that species have changed over time.

Some fossils have features of two groups. One example is Archaeopteryx, found in rock about 150 million years old. It had feathers and wings like a bird. It also had teeth, claws on its wings and a long bony tail, like a small dinosaur.

A cut through two layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. The top layer, sandstone, holds a fern and is 66 million years old. The bottom layer, limestone, holds a fish and is 145 million years old. The column is headed million years old.sandstone66limestone145millionyears oldyoungestoldest
Example ages. The older layer is at the bottom. Its age is the larger number.

Worked example

A fish fossil is in rock that is 145 million years old. A fern fossil is in rock that is 66 million years old. How many million years older is the fish fossil?

  1. The ages are 145 million years and 66 million years.
  2. The fish is older. To find how much older, take away the smaller age.
  3. 145 − 66 = 79 million years.

Answer: 79 million years older

The second record is DNA. Species that share a recent common ancestor have had little time to collect different mutations. So their DNA is very alike. Species that shared an ancestor long ago have had longer to change. Their DNA differs more.

A table with the columns Species and DNA. Species 1 is ATGGCCTAAGCT. Species 2 is ATGGCCTAAGCA. Species 3 is ATGACCTTAGCA. Species 4 is CTGACGTTAGGA.SpeciesDNAspecies 1ATGGCCTAAGCTspecies 2ATGGCCTAAGCAspecies 3ATGACCTTAGCAspecies 4CTGACGTTAGGA
A short stretch of 12 bases from four species. These are example data. Real stretches are far longer.

Worked example

Species 1 and species 3 are compared over 12 bases. How many bases differ, and what percentage is that?

  1. Compare the two stretches one base at a time.
  2. The 4th, 8th and 12th bases differ. That is 3 places.
  3. 3 out of 12 differ. 3 ÷ 12 = 0.25, which is 25%.

Answer: 3 bases differ, which is 25%.

Species 1 and species 2 differ at only one place. Species 1 and species 4 differ at six. So species 1 and species 2 shared an ancestor more recently. DNA can be used to draw a family tree.

A tree of descent with time running to the right. The common ancestor, A, splits into the orangutan and branch point B. B splits into the gorilla and branch point C. C splits into the human and the chimpanzee. The arrow is labelled time.AorangutanBgorillaChumanchimpanzeetimecommon ancestor
The family tree of four apes, drawn from many DNA studies. Branch points show common ancestors, and time runs to the right.

In the tree, the human and the chimpanzee share branch point C. Their latest common ancestor is more recent than any ancestor they share with the gorilla. Fossils and DNA tell the same story. Two kinds of evidence that agree are strong.

Yet the fossil record has gaps. First, soft bodies such as worms and jellyfish usually rot before they can become fossils. Second, fossils form only in rare conditions, and most dead things leave none. Third, rocks wear away, are folded, or are carried deep underground. Fourth, many fossils have not been found yet.

A cut through three layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. Layer A holds a fern. Layer B holds no fossils. Layer C holds a fish.layer Alayer Blayer Cyoungestoldest
No fossils in layer B does not mean that nothing lived then. It means that none was kept, or none has been found.

Try it

Make layers like rock

You need: a tall clear plastic cup, sand and soil, a leaf, an empty snail shell, a spoon, a little water.

Be safe: Use a plastic cup, not glass. Use only an empty snail shell, and never take a shell with a live snail in it. Wash your hands after you touch soil.

  1. Put 2 cm of sand in the cup. Lay a leaf on it. This is layer 1.
  2. Spoon on 2 cm of soil. Press the shell into it. This is layer 2.
  3. Add 2 cm of sand as layer 3, with nothing in it. Add a little water and let it settle.
  4. Look at the cup from the side. Which layer did you make first, and where is it?

What you will see: The layer you made first is at the bottom. The last layer is at the top. The layers in the cup stand for layers of rock. Rock layers form in the same way, which is why the bottom ones are the oldest.

If you cannot try it, this is what happens.
A cut through the three layers in the cup, drawn like layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. Layer 3 at the top holds no fossil. Layer 2 holds a shell. Layer 1 at the bottom holds a leaf, drawn as a fern.layer 3layer 2layer 1youngestoldest

A wrong idea many people have

Gaps in the fossil record show that evolution is only a guess.

Why it feels right: A full record would be more convincing, and the fossil record has many holes.

The test that breaks it: A diary with pages torn out still shows the order of events. The fossils we have fit in order of age. The DNA of living species gives the same family tree as the fossils.

What is true: Gaps are expected, because fossils form so rarely. Many separate records agree, and that is why scientists trust the evidence.

Key words

fossil
the remains or trace of a living thing from long ago, found in rock, ice or amber.
fossil record
all the fossils we know, and what they tell us about life long ago.
common ancestor
a living thing from which two or more kinds of living thing came.
sedimentary rock
rock made of layers of mud, sand and other pieces that settled and hardened.

Back to your guess

The answer is C: The past leaves records. Fossils in rock show life of long ago, and DNA shows how closely living things are related.

Fossils and DNA are records. Fossils show what lived long ago, and DNA shows how close the relationships are. The two kinds of evidence agree with each other.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which of these is a fossil?

    Tap your answer.

    1. Yes, that is right. A fossil is a trace or remains of a living thing from long ago. A footprint is a trace fossil.

    2. Not this one. A fossil is a trace of something that lived long ago, kept in rock, ice or amber. A new shell is not a fossil. Try another.

    3. Not this one. A fossil is much older, and has been kept in rock, ice or amber. A bone from last year has not been kept in this way. Try another.

    See the answer

    A footprint made in mud long ago and turned to stone

    A fossil is a trace or remains of a living thing from long ago. A footprint is a trace fossil.

    • If you chose B: A fossil is a trace of something that lived long ago, kept in rock, ice or amber. A new shell is not a fossil.
    • If you chose C: A fossil is much older, and has been kept in rock, ice or amber. A bone from last year has not been kept in this way.

  2. Look at the layers. They have not been moved. Which layer holds the oldest fossils?

    A cut through four layers of rock. Time runs up the page, with the words oldest and youngest at the ends of the arrow. Layer A at the top holds a fern. Layer B holds a fish. Layer C holds a shell. Layer D at the bottom holds a shell and a fish.layer Alayer Blayer Clayer Dyoungestoldest
    Tap your answer.

    1. Not this one. Layer A is at the top. A layer on top settled after the ones under it, so it is the youngest. Try another.

    2. Yes, that is right. Layer D is at the bottom, so it was laid down first. In layers that have not been moved, the oldest is at the bottom.

    3. Not this one. Each layer settled at a different time, one on another. Their ages are different. Try another.

    See the answer

    Layer D

    Layer D is at the bottom, so it was laid down first. In layers that have not been moved, the oldest is at the bottom.

    • If you chose A: Layer A is at the top. A layer on top settled after the ones under it, so it is the youngest.
    • If you chose C: Each layer settled at a different time, one on another. Their ages are different.

  3. Look at the DNA of three species. Which species is most closely related to species X?

    A table with the columns Species and DNA. Species X is CCATGGTACTGA. Species Y is CCATGGTACAGA. Species Z is CGATCGTACAGT.SpeciesDNAspecies XCCATGGTACTGAspecies YCCATGGTACAGAspecies ZCGATCGTACAGT
    Tap your answer.

    1. Not this one. Species Z differs from species X in more places than species Y does. More differences mean a more distant relative. Try another.

    2. Yes, that is right. Species Y differs from species X at 1 place. Species Z differs at 4 places. Fewer differences mean a more recent common ancestor.

    3. Not this one. All DNA uses the same four bases, but the order of the bases differs. The number of differences tells how long ago the species shared an ancestor. Try another.

    See the answer

    Species Y

    Species Y differs from species X at 1 place. Species Z differs at 4 places. Fewer differences mean a more recent common ancestor.

    • If you chose A: Species Z differs from species X in more places than species Y does. More differences mean a more distant relative.
    • If you chose C: All DNA uses the same four bases, but the order of the bases differs. The number of differences tells how long ago the species shared an ancestor.

  4. Which sentence best explains why the fossil record has gaps?

    Tap your answer.

    1. Not this one. Life existed in those times. Its remains were just not kept, or have not been found. Try another.

    2. Not this one. It is the other way round. Most dead things rot or are eaten, and few become fossils. Try another.

    3. Yes, that is right. Soft bodies rot, fossils form only in rare conditions, rocks wear away or move, and many fossils are not found yet.

    See the answer

    Few dead things become fossils, and many fossils are destroyed or not yet found.

    Soft bodies rot, fossils form only in rare conditions, rocks wear away or move, and many fossils are not found yet.

    • If you chose A: Life existed in those times. Its remains were just not kept, or have not been found.
    • If you chose B: It is the other way round. Most dead things rot or are eaten, and few become fossils.

  5. Look at the family tree. Which two living things share the most recent common ancestor?

    A tree of descent with time running to the right. The common ancestor, A, splits into the orangutan and branch point B. B splits into the gorilla and branch point C. C splits into the human and the chimpanzee. The arrow is labelled time.AorangutanBgorillaChumanchimpanzeetimecommon ancestor
    Tap your answer.

    1. Not this one. Their paths meet only at A, the oldest branch point on the tree, far back to the left. Try another.

    2. Not this one. Their paths meet at B. The human and the chimpanzee meet later, at C, so they share a more recent ancestor. Try another.

    3. Yes, that is right. The human and the chimpanzee branch from point C, which is the latest branch point on the tree.

    See the answer

    The human and the chimpanzee

    The human and the chimpanzee branch from point C, which is the latest branch point on the tree.

    • If you chose A: Their paths meet only at A, the oldest branch point on the tree, far back to the left.
    • If you chose B: Their paths meet at B. The human and the chimpanzee meet later, at C, so they share a more recent ancestor.

  6. From an earlier chapter

    In a wood there are dark moths and light moths on dark bark. Birds eat the light moths more often. What do you expect after many generations?

    Tap your answer.

    1. Not this one. A moth does not change colour because it needs to. Selection changes the share of each kind. Go to S4.3.1. Try another.

    2. Yes, that is right. Dark moths are eaten less, so more of them breed and pass on their alleles.

    3. Not this one. Being there longest does not decide who survives. The ones less likely to be eaten breed more. Go to S4.3.1. Try another.

    See the answer

    A larger share of the moths will be dark.

    Dark moths are eaten less, so more of them breed and pass on their alleles.

    • If you chose A: A moth does not change colour because it needs to. Selection changes the share of each kind. Go to S4.3.1.
    • If you chose C: Being there longest does not decide who survives. The ones less likely to be eaten breed more. Go to S4.3.1.

  7. A shell fossil is in rock 120 million years old. A fern fossil is in rock 45 million years old. How many million years older is the shell fossil?

    See the answer

    75 million years older

    The shell is older. 120 − 45 = 75 million years.

  8. The DNA of two species is compared over 200 bases. They differ at 4 bases. What percentage of the bases are the same?

    See the answer

    98%

    4 out of 200 differ, which is 2%. So 100 − 2 = 98% are the same.

Keep it

Copy this into your exercise book.

Fossils lie in layers, the oldest at the bottom. More alike DNA means a more recent common ancestor. The record has gaps, because few things become fossils.

Draw: Three layers of rock with a fossil in each, an arrow for time pointing up, and the words oldest and youngest.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (1.0 MB). It opens on any phone.