Teachers' notes

Geological time

For many, the concept of time is difficult to grasp, but even in Key Stages 1 and 2, children are expected to understand long periods of time: the activities of the Egyptians 4000 years ago, the Romans 2000 years ago or the Vikings 1000 years ago. If 1000 years is a long time, how can a child comprehend one thousand million years? But not only is it possible, it is intellectually stimulating. Geological time means large numbers and it is necessary to break down these vast periods of time into more manageable pieces. Scaling is a useful tool.

There are a number of models that have been used to scale the passing of the 4600 million years of the Earth's geological history. Compressing geological time into, for example, a 460 page book, the twenty four hours of the day or even to a single hour have been used. These are misleading, however, as they give the impression that, with the appearance of humans 'a few seconds before midnight', geological time came to an end and that our species is the ultimate life form at the end of the long evolutionary process. This is far from the truth. If survival of the fittest is the key to evolution, we have a lot to learn from blue-green cyanobacteria!

The geological timeline

It has been estimated that the solar system is about half way through its life, so the model presented here scales geological time to that of a middle aged person. We consider the Earth not as a planet 4600 million years old, but as a person 46 years old today. Of course to an eight or 12 year old child, it is difficult to imagine that anybody could be as old as 46, but discussion about the age of relatives (parents and grandparents — and teachers?) makes this more understandable. For a child, the same mental agility is necessary to come to terms with 46 years, as for 460 years (the Tudor period), 4600 years (Egyptians were about to construct the pyramids at Giza) and 4600 million years (the age of the Earth).

The geological timeline introduces the question, what of the future? Our middle aged Earth is part way through its life. It is interesting to debate what the world's environment might be in another million or 4000 million years. Will humans still exist? What will be the effect of humans on biodiversity? What organisms might evolve? What might the atmosphere be composed of? How will it all end?

Classroom activities

A timeline is the ideal way to comprehend the passage of geological time and to demonstrate how life, environment and geography have changed throughout Earth's history. There are several ways that this can be achieved in school:

  • a piece of wallpaper 4.6 m long could be stretched around a room, divided up into forty-six rectangles
  • a 4.6 m length of rope or washing line can be stretched across a playground with a peg every 10 cm
  • an interesting project is to draw the timeline by computer

Whatever method is appropriate, subdivision into 46 units is useful as these give the idea of the 46 'birthdays' (each birthday represents 100 million years of geological time). It is helpful if the last division (representing the last year) is an elongated rectangle divided into 12 months — a lot happened during that last year. The children's work can be attached to the timeline: written work, photographs or art work.

The key geological events in the history of life are shown in the geological timeline and further details are shown in the table below. It should be noted that some of these events did not fall exactly on the Earth's birthdays, but they have been placed next to the nearest one. So the appearance of the dinosaurs 225 million years ago, for example, has been placed at the 44th birthday (so within 25 million years). Of course absolute ages are approximate anyway and the degree of error and uncertainty increases with age. Taking the Carboniferous as an example, its lower boundary been variously dated between 367 and 353 million years ago and the top between 280 and 301, and is, therefore, between 52 and 87 million years in duration.

Cross curricular activities

These will depend on the age and ability of the children:

  • mathematics: measuring (along a timeline), scales, calculations involving time
  • English: written work on a period of time, organisms, rocks, etc.
  • science: what is life; relationship between organisms and the environment they live in; changes in the environment; Earth studies
  • art: depiction of life in the past
  • IT: construct a time line on a PC. Student's artwork can be scanned and attached to their timeline (or digitally based artwork packages can be used). Pictures of fossils, rocks, minerals, maps, etc., perhaps taken from the Web or from clip-art packages can be used. Hyperlinks can be used to link the timeline to additional information (e.g. students' written work or art work).
  • theology: this is a difficult subject, but here we present the scientific view of geological time and the evolution of life. How does this compare with religious beliefs?

Further reading

A number of scientific palaeontology books are available, providing data that can be used on a timeline. The majority are for advanced students, but a brief outline of the fossil record, including a short reference list, is published by BGS: Rigby, S.  1997.  Fossils, the story of life.  British Geological Survey, Keyworth, 64pp.

The Fossil Focus series is also published by BGS. These laminated A3 cards colourfully explain the anatomy, distribution and environmental requirements of a number of fossil groups.

The timeline in 'birthdays' Approximate age of the Earth (millions of years) Approximate time before present (millions of years) (Ma) Notes on key events along the timeline
0 0 4600 Earth formed from a dust cloud with the Sun in centre. When Earth was about 80 per cent of its present size, it crashed with another planetoid. Debris around the Earth fused together to form the Moon.
1 100 4500 Earth's core formed when dense metals sank to the centre. Eventually the stony crust cooled and solidified. Little is known about Earth (no crustal rocks survive).
2 200 4400 Oldest known minerals to form on Earth are zircon crystals found in Australia. Inclusions in the crystals said to indicate oceans had formed by this time, although this is controversial.
5 500 4100 End of the Hadean Period (the name of the essentially unknown phase of Earth's history, not represented by crustal rocks).
6 600 4000 The Ancaster Gneiss (Greenland) is Earth's oldest known crustal rock (approx. 4000 Ma).
8 800 3800 The Akilia Gneiss (3 850 Ma) is said to have carbon traces of life, but this is controversial.
9 900 3700 Banded ironstone formations (BIFs) are considered to have been created by bacteria. Oxygen created by bacteria caused ferrous iron in the ocean water to oxidise and precipitate as a red layer of iron on the sea floor. At times when oxygen was not being created, grey cherts were precipitated instead. These layers built up alternately to form BIFs. BIFs provide the earliest signs of photosynthesis. They began to form about 3 700 Ma, but no fossils are known. However, photosynthesising bacteria must have evolved from non-photosynthesising ancestors, which in turn evolved via non-biological evolution.
11 1100 3500 The Apex Chert (western Australia), 3465 Ma, was once believed to contain the earliest fossils, but this is now considered unlikely. However, in the Pilbara region, north west Australia, silica rich rocks dating to about 3500 Ma contain tubes about 40 microns long and thinner than a human hair. Although some may have formed inorganically, there are some geologists who believe that they were formed by rock eating bacteria. This is still controversial.
16 1600 3000 Stromatolites formed by blue-green cyanobacteria: microscopic, single celled, photosynthesising organisms. The organic mats precipitated calcite and trapped sediment particles into the layer. Blue-green bacteria are still making stromatolitic domes in Shark Bay, Australia, 3000 million years later.
25 2500 2100 Oxygen was a waste product produced by bacteria and would have been poisonous to these early life forms. Initially the oxygen was chemically trapped in the rocks, e.g. BIFs and limestones, but eventually there was too much to store in this way and it escaped into the atmosphere. Terrestrial red beds were created 2100 Ma, by the oxidisation of iron. Eventually free oxygen began to accumulate in the atmosphere.
31 3100 1500 Small amounts of oxygen in the atmosphere. The first eukaryotes appear, the basic cell type that almost every living thing on Earth is made of — protista, fungus, plant, animal kingdoms (only bacteria, the kingdom Monera, have the simpler prokaryotic cell). Eukaryotes require oxygen for their metabolism. Sexual reproduction is said to have evolved at about this time. Rocks from 1000 Ma show an increase in diversity of these early eukaryotes, protista.
39 3900 700 Geneticists have suggested animal life began approx. 1000 Ma, but there is no evidence for this in the geological record. Choanoflagellates are protistids with genetic material also found in animals and it has been suggested that the animal kingdom evolved from something similar. The earliest trace fossils in Australia (made by the activities of presumably soft bodied animals) and Africa are from about 700 Ma.
40 4000 600 The first multicelled animal fossils including the sea-pen Charnia, worms, sea urchin-like creatures and jellyfish, are from a little over 600 Ma. Fecal pellets discovered in 600 Ma rocks in Scotland must have been left by an animal with a gut.
41 4100 500 Animals with hard parts (shells and skeletons) e.g. trilobites and molluscs evolved 545 million years ago (at the beginning of the Cambrian). Soon afterwards, all kinds of organisms with hard parts began to evolve, including corals, crinoids, brachiopods, nautiloids, graptolites and microscopic species too (e.g. foraminifera). The earliest fish evolved in the early Cambrian. The first fish with calcareous backbones (rather than cartilaginous notocords) evolved a little later. Comparison can be made to other vertebrates including ourselves.
42 4200 400 Sufficient ozone in the atmosphere allowed plants to evolve from algae and colonise the land. Invasion of land by plants began with the evolution of non-vascular bryophytes in the Mid Ordovician, about 450 Ma. Cooksonia, the first vascular plant, evolved in the late Silurian, approx. 420 Ma. Soon afterwards animals followed the plants.
43 4300 300 Animal life has been found on the marshy land associated with early plant fossils. Worms, snails and, by the late Devonian (about 350 Ma), the first amphibians (tetrapods) left the aquatic realm. Amphibians rapidly evolved into lizards. Lizards had developed a waterproof egg that did not have to be laid in water. They did not have a need to stay close to bodies of water and keep wet. The first tropical rain forests evolved (the coal forests) and began to spread about 320 Ma.
44 4400 200 Lizards evolved into dinosaurs 225 Ma. There are a number of differences, but the most obvious is in the construction of the hip so that dinosaurs were able to stand with straight legs beneath their body. Some were bipedal. Mammal-like reptiles evolved into the first mammals — shrew-like insectivores about 210 Ma.
45 4500 100 Archaeopteryx, the first bird, evolved from feathered theropod dinosaurs about 140 Ma. Soon afterwards (approx. 130 Ma) flowering plants evolved — Archaefructus was the earliest angiosperm (it had carpels but no flower), but soon afterwards species related to magnolia appeared (oldest fossil flower).
8 months ago 4535 65 Mass extinction of 65 to 70 per cent of all species, including all the ammonites, belemnites, flying reptiles and dinosaurs (although birds, the last of the evolutionary line of the dinosaurs, continued to thrive).
7 months ago 4550 50 – 60 After extinction of the dinosaurs, very rapid mammalian evolutionary radiation, especially in the Eocene (about 40 – 55 Ma), occupying land, sea and air.
4 months ago 4565 35 The first primates, related to lemurs, evolved late in the Cretaceous, just before the mass extinction and the disappearance of dinosaurs, ammonites etc. Monkeys evolved about 35 Ma and started to evolve rapidly: the dryopithecines appeared about 25 Ma and the first apes 17 Ma.
2 months ago 4585 15 Grass evolved. This is, perhaps, the most important flowering plant so far as humans are concerned as it provides us with wheat, barley, maize, rice, etc. The first grasslands evolved during a prolonged phase of climatic cooling. A number of animals took advantage of the expanding grasslands, including some of the primates that live on the ground rather than the trees.
About 3 weeks ago Almost 4600 Approx. 5 – 6 The hominid Australopithecus evolved ('hominid' and 'human' should not be confused; Australopithecus was not human). There have been several species. Recently a skull approx. 7 Ma old was discovered that has been suggested to be the earliest hominid but some believe it to be the skull of an ape. Australopithecus afarensis ('Lucy') evolved about 5 Ma and the last species of Australopithecus boisei ('Nutcracker Man'), lived from 2.3 to 1.4 Ma.
About 7 – 8 days ago 2 – 2.5 The first species of human evolve in Africa — Homo habilis. Fossils of their brain cases show that the speech centre is only just beginning to develop.
Approx. 6 – 7 days ago 2 – 1.6 Homo erectus is considered by some to be two species — H. ergaster and H. erectus. They evolved in Africa about 1.6 Ma. Homo erectus was the first human species to migrate across Europe and Asia.
Approx. 5 days ago until 'last night' 1.3 Ice ages start, but this was a period of very variable climate. Britain was sometimes buried beneath about 1 km thick ice caps, sometimes tundra developed, sometimes it was warmer than today. During warm periods, lions, hippo and rhino lived in Britain, but when the tundra developed, mammoth, wolves and giant elk lived here. The last glaciation in Britain ended about 10 000 years ago.
Approx. 2 days ago 0.5 Homo heidelbergensis evolved. The earliest fossils are approx. 500 000 years old. 'Heidelberg Man' is also known as 'Swanscombe Man' and 'Boxgrove Man' in Britain.


The timeline in 'birthdays' Approximate age of the Earth (millions of years) Approximate time before present (millions of years) (Ma) Notes on key events along the timeline
46 Approx. 13 hours ago 4600 150 000 years ago Homo neanderthalensis evolved and spread throughout Europe during the ice ages — fossils are found particularly in Mediterranean countries, but also Germany (the type area) and they also reached Britain. It possibly evolved from Homo heidelbergensis.
Approx. 12 hours ago 130 000 years ago Modern humans (Homo sapiens sapiens) evolved in Africa about 130 000 years ago, perhaps from 'Rhodesia Man', Homo rhodesiensis, but the evolution of Homo is controversial and there are a number of different evolutionary theories. The idea that we evolved from Homo neanderthalensis is flawed.
Approx. 3.5 hours ago Approx. 35 000 years ago Human populations expanded from Africa and spread to Europe and Asia approx. 35 000 years ago. Neanderthals became extinct about 30 000years ago.
Approx. 1 hour ago Approx. 11 000 years ago Man became a farmer.
Approx. 1 minute ago 250 years ago The Industrial Revolution. During the last 60 seconds we have pollution, radioactive waste, hole in ozone later, acid rain, mass extinction, etc.
The future 4600 – 9000 The next 4600 million years

What next? The world has changed so much in the last 4600 million years who knows what will happen in the next.
In the short term: more pollution? More extinctions? Changes in the atmosphere? Global warming? Rising sea levels and flooding of the continental margins?

In the medium term: the extinction of human beings. What will evolve to dominate the world then? The mammals had to wait for the extinction of the dinosaurs before their sudden evolutionary radiation. What is waiting for our extinction? Could it be the turn of the insects? Or something totally unknown?

In the longer term: reorganisation of the continents. Greenhouse and icehouse Earth? And finally the Sun becomes a red giant, destroys the planets and dies.