History of Life on Earth, Grade 10: Timeline and Model Fossils

Three plaster of Paris moulds in plastic tubs showing the imprints of a seashell, a snail shell and a small bone, with the real shell, snail shell and bone in front of them

Two practicals for the Grade 10 history of life on Earth: a scaled timeline of Earth's history and a model fossil made in plaster of Paris. Below them is the topic in brief, with a class mapping activity of South Africa's key fossil sites.

Subject Life Sciences
Grade and term Grade 10, Term 4
Topic History of life on Earth
Practicals A timeline of Earth's history, and a plaster of Paris model fossil
Time Timeline about 45 minutes. Model fossil about 40 minutes, plus a day for the plaster to dry

Practical 1: a timeline of Earth's history

Aim

To build a timeline to scale, showing key events from the earliest life to today.

What you need

  • A strip of paper at least 4,7 m long, such as a till roll, or lengths of A4 paper taped end to end
  • A 5 m tape measure and a 30 cm ruler
  • Coloured markers
  • Prestik or tape to fix the strip along a wall or the corridor floor
  • The table of events below

Method

  1. Use the scale 1 mm = 1 million years, so 1 m = 1 000 million years.
  2. Draw a line along the strip. Mark one end "Today".
  3. Measure 4,6 m from "Today" and mark "Earth forms".
  4. Measure and mark each event in the table below, always measuring from "Today".
  5. Divide the last 540 mm into the three eras with different colours: Palaeozoic from 540 mm to 252 mm, Mesozoic from 252 mm to 66 mm, and Cenozoic from 66 mm to "Today".
  6. Draw a small picture beside each event.
  7. Walk the timeline from "Earth forms" to "Today", and note how much of it passes before the first animals appear.

What you should see

A timeline of Earth's history from 4 600 million years ago to today, showing the first life, the rise of oxygen, the Cambrian explosion and the three eras crowded into the last 540 million years

Event About how long ago Distance from "Today"
Earth forms 4 600 million years 4,6 m
First life: single-celled bacteria 3 500 million years 3,5 m
Oxygen starts to build up in the air, made by photosynthetic bacteria 2 400 million years 2,4 m
First soft-bodied many-celled animals 570 million years 57 cm
Cambrian explosion 540 million years 54 cm
First land plants 470 million years 47 cm
First four-legged animals (early tetrapods) About 365 million years, late in the Devonian 36,5 cm
The largest mass extinction 252 million years 25,2 cm
First mammals About 225 to 205 million years, late in the Triassic 22,5 to 20,5 cm
Archaeopteryx 150 million years 15 cm
Dinosaurs die out 66 million years 6,6 cm
Earliest human ancestors walking upright About 6 to 7 million years 6 to 7 mm
Modern humans, Homo sapiens 300 000 years 0,3 mm

About 88% of the strip is used up before the Cambrian explosion, and all of human history fits into the last third of a millimetre.

Why it happens

For most of Earth's history, life was made of single cells living in water. Oxygen made by photosynthetic bacteria slowly built up in the air and the sea, and complex animals became possible only much later. Once hard parts evolved in the Cambrian, fossils became far more common, which is why the record is so much richer in the last 540 million years. A scale timeline shows this better than a list of numbers.

Safety

  • If the timeline is laid on a corridor floor, tape it flat so nobody trips, and keep the walkway clear.
  • Keep the tape measure's steel blade under control when it rewinds. Its edge can cut.

Practical 2: a model fossil in plaster of Paris

Aim

To make a model of a mould fossil and a cast fossil, and see how each forms.

What you need

  • Plaster of Paris, from a hardware or craft shop
  • Objects to fossilise: a seashell, a snail shell, a chicken bone that has been boiled and cleaned, or a firm leaf
  • Petroleum jelly
  • A shallow plastic tub, such as a margarine tub, for each learner
  • An old ice-cream tub or a 500 ml plastic beaker, and a plastic spatula, for mixing
  • A 250 ml measuring cylinder and a 250 ml plastic beaker
  • Cold water
  • Safety goggles and nitrile gloves
  • Newspaper to cover the bench, and a bin bag for waste

Method

The mould

  1. Cover the bench with newspaper and put on goggles and gloves.
  2. Smear the object thinly with petroleum jelly so it does not stick.
  3. Pour 120 ml of cold water into the mixing tub.
  4. Add about 240 ml of plaster powder, measured in the beaker, slowly to the water. Stir gently until it is smooth, like thick cream.
  5. Pour the plaster into the tub to a depth of about 2 cm and tap the tub to remove bubbles.
  6. Wait 3 to 5 minutes, until the plaster starts to thicken.
  7. Press the object halfway into the plaster, flat side down.
  8. Leave it for about 20 minutes, until the plaster is firm, then lift the object out carefully.
  9. Leave the mould to harden overnight.

The cast

  1. Next day, smear the inside of the hardened mould with petroleum jelly.
  2. Mix a fresh batch of plaster, half the size, and pour it into the hollow of the mould.
  3. Leave it overnight, then tip the tub over and ease the cast out of the mould.

What you should see

Model What it looks like The real fossil it models
Mould A hollow imprint showing the outside shape of the object A mould fossil, left when a buried shell or bone dissolves away
Cast A solid copy of the object's shape A cast fossil, formed when minerals fill a natural mould

Fine details such as the ribs of a shell show clearly. Soft objects, such as a leaf, leave a much fainter mark than hard ones.

Why it happens

Plaster of Paris reacts with water and sets hard, much as soft sediment turns into rock over a long time. Pressing an object into it leaves a mould of the outside shape. In nature, a buried shell or bone can dissolve and leave a hollow in the rock, and minerals carried by water can later fill that hollow to make a cast. Hard parts keep their shape long enough to leave a mark, which is why most fossils are of shells, bones and teeth.

Safety

  • Plaster gets hot as it sets. Never put fingers, hands or any part of the body into setting plaster, as it can cause serious burns.
  • Wear goggles. Plaster dust irritates the eyes, and if any gets in, rinse with plenty of water.
  • Add the powder to the water gently so you raise as little dust as possible.
  • Never pour plaster or plaster water down the sink, because it sets in the pipes. Let left-over plaster harden in the mixing tub, then flex it out into the bin.
  • Boil and clean any bone first, and wash your hands after handling it.

The history of life in brief

Fossils and how they form

A fossil is the preserved remains of an organism, or a trace of it such as a footprint, that lived long ago. Palaeontology is the study of fossils.

Four-step diagram of fossil formation: an animal dies in water, it is buried by sediment, its bones are replaced by minerals as the sediment turns to rock, and the fossil is exposed by erosion

  1. An organism dies, usually in or near water.
  2. It is buried quickly by mud or sand, before it is eaten or broken up. The soft parts usually rot away.
  3. Over millions of years, layers of sediment press down and turn into sedimentary rock. Minerals in the water slowly replace the hard parts.
  4. Erosion or digging exposes the fossil.
Type of fossil Example
Body fossil, the remains themselves, often turned to stone Bones, teeth, shells, petrified wood
Mould and cast The imprint of a shell, and the copy formed when the imprint fills with minerals
Trace fossil Footprints, burrows and fossilised dung
Whole organisms preserved Insects in amber

Dating fossils

Diagram of five rock layers with a fossil in each, showing the youngest rock and fossils at the top and the oldest at the bottom

Method How it works What it gives
Relative dating In undisturbed rock, lower layers are older than the layers above them, so fossils in lower layers are older Which fossil is older, but no age in years
Radiometric dating Radioactive atoms in a sample decay at a known, steady rate. The half-life is the time for half of them to decay. Measuring how much is left gives the age An age in years

Carbon-14 is used for material that was once alive and is less than about 50 000 years old. Older rock, such as volcanic ash, is dated with other radioactive isotopes that decay much more slowly.

Scientists use inference to understand fossils: they reason from the evidence to the most likely explanation. A new find can change that explanation, and the fossil record still has many gaps.

The geological timescale

Earth's history is divided into eras, and the eras into periods. The names of the periods do not have to be learnt.

Era About how long ago Life
Palaeozoic, "old life" 540 to 252 million years The Cambrian explosion, the first land plants and the first four-legged animals
Mesozoic, "middle life" 252 to 66 million years Dinosaurs, and the first mammals and birds
Cenozoic, "new life" 66 million years to today Mammals spread, and humans appear

The Cambrian explosion, about 540 million years ago, is the short period in which almost all the major groups of animals first appear in the fossil record, many with hard shells and skeletons.

A changing planet

  • The atmosphere: early Earth had almost no oxygen. Photosynthetic bacteria slowly raised it, which allowed larger, more active organisms to evolve.
  • Climate: Earth has had ice ages and warm periods, and each changed where organisms could live.
  • Moving continents: the continents drift slowly. Southern Africa was once joined to South America, Antarctica, India and Australia, which is why fossils of Glossopteris are found on all of them, and fossils of Lystrosaurus in both South Africa and Antarctica.
  • Sea level: fossils of sea animals on dry land show where the sea once was, for example ammonites and bivalves in northern KwaZulu-Natal, whale fossils in the Sahara Desert, and trilobites in the rocks of the Western and Eastern Cape.

Mass extinctions

A mass extinction is the loss of a large share of all species in a short time. There have been five.

Mass extinction About how long ago
End of the Ordovician 445 million years
Late Devonian 372 million years
End of the Permian, the largest. About 90% of species died out 252 million years
End of the Triassic 201 million years
End of the Cretaceous, which ended the dinosaurs 66 million years

Two hypotheses explain the extinction 66 million years ago:

  • Meteorite impact: a layer of clay rich in iridium, an element rare on Earth but common in meteorites, is found in rocks of this age around the world. A huge crater of the same age lies at Chicxulub in Mexico.
  • Volcanoes: enormous lava flows in India, the Deccan Traps, erupted at the same time and released gases that changed the climate.

Species are now dying out faster than at any time in the past, mainly because of human activity such as habitat loss, pollution and alien species. This is called the sixth extinction.

Two famous fossils

  • Archaeopteryx, about 150 million years old, shows features of both reptiles and birds. Like a reptile it had teeth, claws on its wings and a long bony tail. Like a bird it had feathers, wings and a wishbone.
  • The coelacanth is a "living fossil": it was known only from fossils until a living one was caught off East London in 1938. Marjorie Courtenay-Latimer of the East London Museum recognised it, and J.L.B. Smith identified it. A living population was found off Sodwana Bay in northern KwaZulu-Natal in 2000.

Key events in South Africa's fossil record

Event Where
Some of the earliest life: fossilised bacteria and stromatolites, layered mounds built by bacteria Barberton area in Mpumalanga, and many other parts of South Africa
Soft-bodied animals Southern Namibia and the Northern Cape
Early land plants Around Makhanda (Grahamstown)
Forests of Glossopteris Near Mooi River and Estcourt
Mammal-like reptiles, such as Lystrosaurus and Thrinaxodon The Karoo
Dinosaurs and cone-bearing plants Foothills of the Drakensberg and Maluti mountains
Early mammals Eastern Cape and Lesotho
Humans and pre-humans The Cradle of Humankind in Gauteng, Taung in North West, Florisbad in the Free State, Border Cave in KwaZulu-Natal, Makapansgat in Limpopo

Fossil tourism

Fossil sites bring visitors, and visitors bring jobs and income for guides, guesthouses, shops and transport. Examples are the Cradle of Humankind and Maropeng in Gauteng, and the Taung and Makapan Valley parts of the same World Heritage Site.

Class activity: map the key fossil sites

  1. Give each learner an outline map of South Africa with the provinces marked.
  2. Mark each site from the table above with a dot and a number.
  3. Write a key beside the map: the number, the place, and what was found there.
  4. Compare the map with a fossils wall chart or photographs of the fossils, and discuss why so many sites are in the Karoo and the Cradle of Humankind.

Research tasks

  • Research Archaeopteryx as a link between dinosaurs and birds, or the coelacanth as a living fossil, and present a short verbal or written report.
  • Choose one hypothesis for the extinction 66 million years ago, the meteorite impact or the volcanoes in India, and describe the evidence that supports it.

Free worksheet and marking memo

Related practicals

References