Unit 4 — Dating Planet Earth

1.1 How do we know that dinosaurs lived millions of years ago?

• Nobody saw dinosaurs, yet we know a great deal about them.
• The evidence comes from fossils.
• Imagine you find a line of footprints in a sandy area.
• What do they tell you?
• You can tell what kind of animal walked there (from the shape and size), roughly how big it was, how many animals, the direction they were going, and even whether the animal was walking or running (from spacing).
• In the same way, scientists "read" fossils as clues to events of long ago.

1.2 What is a fossil and how does it form?

Fossils are the remains or traces of organisms (plants, animals, micro-organisms) that lived in ancient geologic times and were preserved because of special environmental conditions after they died.

In rare cases, a whole organism is preserved.

• Fossils usually form when the organism is buried quickly after death in a medium that protects it from decay (decomposition by bacteria and fungi, and scavengers).
• Suitable media include:

1. Sediments: fragments of clay, sand or limestone that bury the remains.
2. Permafrost (ice): ground frozen for very long periods without melting. (Mammoths have been found preserved in ice with skin and hair.)
3. Natural tar: a form of crude oil that traps and preserves animals.
4. Amber: hardened resin, the sticky substance secreted by some trees, which can trap insects perfectly.

• Why are fossils of soft-bodied organisms rare? Soft tissues (such as jellyfish, worms, skin, muscles) decay quickly after death, before burial can protect them.
• Hard parts (bones, teeth, shells) last longer, so most fossils are of hard parts.

1.3 Types of fossils

(a) Body fossils

These include parts of the organism's body: bones, teeth, shells, tree trunks, leaves, or a whole organism, such as an insect trapped in amber or a mammoth preserved in ice.

(b) Trace fossils

• These do not include the body itself, but signs of its activity: footprints, tracks, burrows, or fossil droppings (coprolites).
• They show the organism's behaviour and feeding habits (for example, how it moved or what it ate).

(c) Mold and cast fossils

• Mold: an impression of the outside surface of an organism preserved in rock. When an organism (for example a shell) is buried and later dissolves or decays away, it leaves a hollow in the hardened sediment with the shape of its outer surface.
• Cast: the filling of that hollow (or of the inner cavity of a shell) with mineral solutions such as silica or calcite, or with sand and clay that harden over time. The cast has the shape of the inside (or the original form) of the organism. Example: mold and cast of ammonites.

• You can make a model of this: press a shell into soft clay and remove it; the impression is a mold (model A).
• Pour plaster into the impression, let it harden, and lift it out: it is a cast (model B).

(d) Petrified fossils

• If an organism stays buried for millions of years, minerals such as silica or calcium carbonate gradually replace the original organic matter in its tissues, and the remains turn into stone.
• Example: the petrified tree trunks of the Petrified Forest east of Maadi, in Cairo Governorate.

Geological Museum in Cairo. It contains igneous, metamorphic and sedimentary rock samples and fossils of animals and plants from different periods, including a complete ancient whale skeleton, a Moon rock and a Martian meteorite.

1.4 The fossil record

The fossil record is the collection of all fossils that have been naturally preserved in rock layers or in other media (ice, tar, amber) and have been discovered.

1.5 The importance of the fossil record

1. Tracing the appearance and development of life

• Sedimentary rock layers are deposited one on top of another, so in an undisturbed sequence the deeper layers are older than the ones above.
• Organisms whose fossils are found in deeper layers appeared on Earth earlier than those found in higher layers.

Example: fossils of invertebrates are found in deeper layers than fossils of vertebrates, so most invertebrates appeared before vertebrates.

2. Identifying mass extinctions

• A mass extinction is an event in which a huge number of species die out in a relatively short time.
• The fossil record shows that certain layers contain many kinds of fossils, but the layers above them lack those fossils.
• For example, about 66 million years ago a mass extinction wiped out about 75% of all species on Earth, including most dinosaurs.

Hypotheses for mass extinctions:

1. cosmic catastrophes such as the impact of a large meteorite (asteroid);
2. severe climate change and changes in the composition of the atmosphere;
3. massive volcanic activity;
4. changes in sea level and the changing positions and shapes of continents.

Cause and effect. Cosmic catastrophes, severe climate change or massive volcanic activity in ancient times (causes) led to the extinction of many organisms (effect).

3. Identifying evolutionary relationships between ancient and modern organisms

• Palaeontology (the study of fossils) works with anatomy and genetics to link ancient organisms to those alive today and shows how species branched from common ancestors.
• Example: the modern Asian elephant is the closest living relative of the extinct woolly mammoth, since both come from a common ancestor.

4. Inferring ancient environments and climate

• Skeletons of primitive whales in Wadi Al-Hitan (Fayoum) show that the area was once covered by an ancient sea.
• The Petrified Forest in Cairo shows that the region was once a humid forest, and the climate there has changed from humid to dry.

• Worked reasoning example 1. Petrified tree trunks are found in a dry desert.
• Trees need water, so the region was once moist with plenty of rain and rivers (a forest).
• Since then the climate changed to dry.

• Worked reasoning example 2. Marine shells are found in rocks high on a mountain.
• Marine organisms live only in seas; so the rock formed on an ancient sea floor, and was later lifted up when plates collided.

1.6 Causes of the diversity of life forms through time

Scientists give several explanations for the great variety of living things and the appearance of new species through geologic time.

1. Adaptation to different environments (the deep sea, forests, deserts), producing species adapted to each environment.
2. Evolution by natural selection, gradually over long periods, or relatively quickly in some cases.
3. Accumulated genetic mutations within a group of organisms.
4. Geographical isolation: a group of organisms is separated from the rest of its species by a natural barrier such as a mountain range, a wide river or a deep valley. Over time, this can lead to new species. The steps are:
   • (i) the barrier interrupts gene flow: the two groups no longer mate, so genes are no longer exchanged;
   • (ii) each group accumulates different mutations independently;
   • (iii) natural selection in each different environment preserves the favourable traits in each group;
   • (iv) differences grow until mating between the groups becomes impossible, and a new species has appeared.

• Change and stability. Earth and living things have changed a great deal, but some rock formations stayed relatively stable.
• The rock layers record these ancient events.

Check.

• Geographical isolation is a cause of the appearance of new species (not of mass extinction).
• The fossil record supports (does not deny) the evolutionary link between the Asian elephant and the mammoth.

1.7 Comparative anatomy

Comparative anatomy is the study of similarities and differences in the body structures of different species to deduce evolutionary relationships.

• Compare the forelimb of a human, a bat and a whale.
• Each has the same basic bones (one upper-arm bone, two forearm bones, wrist bones, finger bones) in the same arrangement, though they are used differently: a human arm for grasping, a bat's wing for flying, a whale's flipper for swimming.

• Homologous structures: organs with the same basic structure (and origin) but which may perform different functions, because organisms adapted to different environments and lifestyles. They are evidence of a common ancestor. The similar bones of the human arm, bat wing and whale flipper are evidence that these animals are all mammals.
• Analogous structures: organs with similar function but different structure. Example: the wings of a pterosaur (extinct flying reptile), a bat (a mammal) and a butterfly (an insect). All enable flight, but:
  • a pterosaur's wing is a skin membrane supported by one very long finger;
  • a bat's wing is supported by four long, spread fingers like umbrella ribs;
  • an insect's wing is an extension of the exoskeleton and has no bones or muscles in it; it is moved by chest (thoracic) muscles. Analogous structures show that having wings does not mean sharing a recent common ancestor; it shows adaptation to a similar way of life (flying).
  1Structure

  Homologous

  • Similar

  Analogous

  • Different

  2Function

  Homologous

  • May be different

  Analogous

  • Similar

  3Meaning

  Homologous

  • Common ancestor

  Analogous

  • Adaptation to a similar environment

  4Example

  Homologous

  • Human arm, bat wing, whale flipper

  Analogous

  • Bat wing and butterfly wing

  Importance of comparative anatomy

  1. Finding common ancestors. The more similar the internal structures of two species, the closer their evolutionary relationship. Fossils show that modern birds share many features with a group of meat-eating dinosaurs called theropods, such as hollow bones, posture and feathers.
  2. Following the evolution of organs. For instance, bones of the reptile jaw evolved into the bones of the middle ear in mammals.
  3. Reconstructing extinct animals. By comparing the skeleton of an extinct animal with the skeletons of living animals, scientists can picture what it looked like, as with Archaeopteryx, which became extinct about 150 million years ago.

  Transitional fossils

  • A transitional fossil shows features of two different groups and supports the idea of a gradual evolutionary change.
  • Archaeopteryx had teeth and a long bony tail like reptiles, and feathered wings like birds.
  • It is an example of a transitional fossil between reptiles and birds.

  1.8 Summary of Lesson 1

  • Fossils are remains or traces of ancient organisms, formed by rapid burial in sediments, ice, tar or amber.
  • Types: body, trace, mold, cast and petrified.
  • The fossil record shows the order of appearance of life, mass extinctions, evolutionary relationships and ancient climates.
  • Diversity arises through adaptation, natural selection, mutation and geographical isolation.
  • Comparative anatomy: homologous (same structure, common ancestry) and analogous (same function, different structure); transitional fossils like Archaeopteryx.

2.1 How old is Earth?

Scientists were not alive in ancient times, yet they have worked out much of Earth's history by studying two records:

1. The fossil record: all fossils preserved in rocks, ice or amber and discovered.
2. The rock record: all the rock layers and formations of Earth's crust.

Together they show past environmental and climate changes and help to arrange the major events in order.

2.2 The geologic time scale

• We divide ordinary time into units (years, months, days, hours).
• In the same way scientists divide Earth's 4.6 billion year history into units, based on rock and fossil evidence.
• This division is the geologic time scale.
• It helps us understand the sequence of:

1. great geologic changes over millions of years;
2. the appearance of different living organisms;
3. major mass extinctions.

Units, from the largest to the smallest:

Eons → Eras → Periods → Epochs

• An eon is the largest unit. Each eon is divided into eras.
• Each era is divided into periods, each with distinct climate conditions.
• Each period is divided into epochs.

Reference outline (approximate, millions of years ago):

The Phanerozoic Eon has three eras: Paleozoic (about 541–252 million years ago), Mesozoic (about 252–66 million years ago; the age of dinosaurs) and Cenozoic (about 66 million years ago to the present; the age of mammals).

Making a timeline. To model Earth's history, choose a scale such as 1 cm = 500 million years. A line for 4.6 billion years then needs 4600/500 = 9.2 cm. Mark events: formation of Earth (4.6 billion years ago, left end), first life (about 3.5 – 4 billion years ago), extinction of dinosaurs (66 million years ago, only 0.13 cm from the right end), and the appearance of modern humans (a few hundred thousand years ago, just a tiny fraction of a millimetre). Such a scale shows how recent the human era is.

2.3 Reading Earth's history from rocks and fossils

Studying geologic structures, the sequence of rock layers and the fossils helps us infer events that happened in the past. Examples:

1. The Red Sea mountains and Uweinat mountains are made of igneous (granite) and metamorphic rocks. Igneous rocks like granite form deep within Earth by the cooling of magma; metamorphic rocks form from other rocks under great heat and pressure. So these mountains are among the oldest parts of Earth's crust, formed deep inside the Earth.
2. Basalt covering limestone at Abu Rawash. Basalt is an igneous (volcanic) rock; limestone is a sedimentary rock that forms on the floor of a sea. So the area was first under the sea (limestone formed), the sea retreated, and later volcanic eruptions covered the limestone with basalt.
3. Marine fossils on the peaks of the Himalayas. Marine fossils form only in the seas. Their presence on mountain peaks shows the area was an ancient sea floor, which was uplifted when tectonic plates collided to form the mountain range.
4. A fossil type found in one layer worldwide and then absent from higher layers, such as the dinosaurs, indicates the extinction of those organisms (a sudden global mass extinction, probably due to a meteorite impact or severe climate change).

2.4 Determining the age of rocks

There are two methods: relative dating and absolute dating.

2.5 Relative age

The relative age tells whether a rock is older or younger than another rock without giving its age in years. It depends on:

1. The law of superposition

• In an undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top.
• (Each new layer is laid down on top of the previous layers.)

2. Index fossils

Index fossils

• are fossils of organisms that lived for a short period of geologic time but were widely distributed geographically, then became extinct.
• Examples are trilobites and ammonites.

How to use them:

• If the same index fossil is found in two rock layers far apart, the two layers have the same relative age, even if the rocks above and below them are different.
• The layer containing the index fossil is used as a reference: the layer below it is older and the layer above is younger.

Why are index fossils good for dating? Because their short existence limits the layer's age to a narrow range of time, and their wide distribution allows matching of layers in different places.

Worked example 1

• In rock sequence 1 (from bottom to top) the layers are A, B, C, D.
• Fossil X is found in layer B and fossil Y in layer D.
• Which organism appeared first?
• B is below D, so B is older, so organism X lived earlier than organism Y.

Worked example 2

• In an area, layer P contains trilobite fossils; in a distant area, a layer Q contains the same trilobite species.
• We conclude that P and Q formed at the same time, even if P is limestone and Q is sandstone.
• A layer above P that holds fossils of a later species is younger than P.

• Relative dating tells us the order of events, not the number of years.
• It is used when rocks cannot be dated in years, such as when there are no suitable radioactive minerals in a sedimentary rock.

2.6 Absolute age

The absolute age is the real age of a rock or fossil in years. It is found using radioactivity.

• Some minerals contain unstable (radioactive) elements such as uranium-238 and carbon-14.
• Their nuclei give off radiation and change into more stable elements.
• A radioactive isotope decays at a constant rate which is characteristic of that element and is not changed by heat, pressure or chemical reactions.
• That makes it a reliable clock.

Half-life: the time required for half the quantity (half the nuclei) of a radioactive element to decay.

• After 1 half-life, 1/2 remains; after 2 half-lives, 1/4 remains; after 3, 1/8; after 4, 1/16; and so on.
• After n half-lives the fraction left is (1/2)ⁿ.

By measuring how much of the radioactive element remains in the rock and how much of its decay product has formed, scientists can calculate how much time has passed since the rock formed.

Examples of isotope pairs:

• Carbon-14 (half-life about 5730 years) is used for dating organic remains (bones, wood, shells) up to about 50 000 years old. Living things take in carbon-14 from the air while alive; when they die the intake stops and the carbon-14 decays.
• Uranium-238 (half-life about 4.5 billion years) decays eventually into lead-206; it is used for very old rocks and minerals.

Worked example 3: carbon-14

A wooden object has only 25% of its original carbon-14. How old is it? (Half-life = 5730 years.) 25% = 1/4 = (1/2)², so 2 half-lives have passed. Age = 2 × 5730 = 11 460 years.

Worked example 4: fraction remaining

How much of a 80 g sample of a radioactive isotope remains after 3 half-lives? Fraction = (1/2)³ = 1/8; mass = 80/8 = 10 g. (After the first half-life 40 g; after the second 20 g; after the third 10 g.)

Worked example 5: age of a rock

A rock contains 12.5% of the original amount of a radioactive isotope whose half-life is 100 million years. How old is the rock? 12.5% = 1/8 = (1/2)³, so 3 half-lives have passed. Age = 3 × 100 = 300 million years. Reality check: any age you calculate for a rock must be less than the age of Earth (about 4.6 billion years). For uranium-238, whose half-life is 4.5 billion years, a rock that formed with Earth would have lost only about half of its uranium.

Worked example 6: the reverse problem

A fossil bone is 17 190 years old. How much of its original carbon-14 remains? Number of half-lives = 17 190 ÷ 5730 = 3. Fraction left = (1/2)³ = 1/8 = 12.5%.

• Zircon crystals. Tiny zircon crystals from Jack Hills in Australia are very hard, and they contain uranium that decays into lead; this makes them an accurate "geologic clock".
• Some are about 4.4 billion years old.
• Studies show Earth is about 4.6 billion years old.

2.7 Comparing relative and absolute dating

• Critical thinking. Relative dating is needed when no radioactive material suitable for measurement is present (for example in many sedimentary rocks), or when only the order of layers is needed.
• Often both methods are used together.

• Index fossil vs transitional fossil. An index fossil is a fossil of an organism with a short time range and a wide distribution, used to date and match rock layers.
• A transitional fossil shows features of two groups and supports evolutionary change (Archaeopteryx).

2.8 Exploiting resources and extinction (issue)

• Human exploitation of natural resources, such as hunting and destroying habitats, can cause species to die out much faster than natural processes.
• Studying past mass extinctions helps us understand the risks today.

2.9 Summary of Lesson 2

• Geologic time scale: eons, eras, periods, epochs (largest to smallest).
• Rock and fossil records reveal events such as mountain building and volcanic activity.
• Relative age: law of superposition, index fossils.
• Absolute age: radioactive isotopes and half-life; fraction left = (1/2)ⁿ.
• Earth is about 4.6 billion years old.

How a fossil forms, step by step

1. An organism dies in a place where sediment is being deposited, such as a river delta, a lake bottom or the sea floor.
2. It is buried quickly by sand, clay or mud, which keeps out oxygen and scavengers, so decay is slow or stops.
3. Over a very long time, more layers pile on top. Pressure and the minerals in the water turn the sediment into sedimentary rock.
4. The hard parts may be dissolved (leaving a mold), filled with minerals (cast) or slowly replaced by minerals (petrification).
5. Later, movements of the crust and erosion bring the rock to the surface, where a scientist finds the fossil.

• This sequence explains why most fossils are found in sedimentary rocks (rather than igneous rocks, where the heat of lava would destroy a body) and why fossils of hard parts are far more common than fossils of soft bodies.
• It also shows how rare fossilisation is: only a tiny proportion of the organisms that ever lived became fossils, so the fossil record has gaps.

How to read a rock column (relative dating)

Suppose a cliff has five undisturbed layers from top to bottom: sandstone (contains fossil Y), shale, limestone (contains fossil X), shale, and conglomerate.

1. Number the layers from the bottom: 1 = conglomerate, 2 = shale, 3 = limestone with X, 4 = shale, 5 = sandstone with Y.
2. By the law of superposition, layer 1 is the oldest and layer 5 the youngest.
3. Fossil X is in a lower layer than fossil Y, so the organism X lived before organism Y.
4. If a second cliff far away has a layer with fossil X, that layer is the same age as layer 3 and the layers above it are younger.
5. If layer 5 held fossils of more complex organisms than layer 3, the layers also show the order in which these groups appeared.

• Be careful: the law of superposition applies to undisturbed sequences.
• If the crust has been folded or tilted, the order has to be worked out with more care; the question will tell you if the sequence is undisturbed.

Choosing a dating method

• If you are asked "which rock is older?" or "arrange the layers", use relative dating.
• If you are asked "how many years ago?", use absolute (radiometric) dating: count the half-lives.
• Use carbon-14 for remains of once-living things younger than about 50 000 years; use isotopes with very long half-lives, such as uranium-238, for very old rocks.

Half-life calculation recipe

1. Write the fraction left (or find it from the percentage: 50% = 1/2, 25% = 1/4, 12.5% = 1/8, 6.25% = 1/16).
2. Count the number of half-lives n, using fraction = (1/2)ⁿ.
3. Multiply n by the length of one half-life to get the age.
4. Check that the answer is reasonable (not older than Earth).

Common mistakes

1. Writing the units of the geologic time scale in the wrong order. Remember: Eons (largest), Eras, Periods, Epochs (smallest). A helpful phrase: "Every Elephant Pushes Eggs".
2. Mixing up mold and cast. The mold is the hollow impression; the cast is the solid copy that fills it.
3. Thinking that analogous structures prove a common ancestor. They do not; they show adaptation to similar functions. Homologous structures are the ones that point to a common ancestor.
4. Saying that half of the sample disappears in one half-life and all of it in two. After two half-lives one quarter is left, not zero; the amount never reaches exactly zero but becomes too small to measure after many half-lives.
5. Forgetting that a mass extinction is an event, not a cause of the appearance of new species. Mass extinctions remove many species; geographic isolation, mutation and natural selection explain the appearance of new species.
6. Confusing the fossil record with the rock record. The fossil record is all the fossils found; the rock record is the sequence of rock layers and formations. Scientists use both together.