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The BGS Geological Timeline
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
www.bgs.ac.uk/discoveringGeology/time/timeline/home.html
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.
www.bgs.ac.uk/discoveringGeology/time/timeline/home.html
Approximate Approximate
The Timeline age of the time before
in Earth (millions present Notes on key events along the Timeline
‘birthdays’ of years) (millions of
years)
Earth formed from a dust cloud with the Sun in centre. When Earth
0 0 4600 was about 80 per cent of its present size, it crashed with another
planetoid. Debris around the Earth fused together to form the Moon.
Earth's core formed when dense metals sank to the centre.
1 100 4500 Eventually the stony crust cooled and solidified. Little is known
about Earth (no crustal rocks survive).
Oldest known minerals to form on Earth are zircon crystals found in
2 200 4400 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.
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
9 900 3700 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.
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
11 1100 3500 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.
Stromatolites formed by blue-green cyanobacteria: microscopic,
single celled, photosynthesising organisms. The organic mats
16 1600 3000 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.
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
25 2500 2100 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
www.bgs.ac.uk/discoveringGeology/time/timeline/home.html
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.
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
39 3900 700 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.
The first multicelled animal fossils including the sea-pen Charnia,
40 4000 600 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.
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,
41 4100 500 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.
Sufficient ozone in the atmosphere allowed plants to evolve from
algae and colonise the land. Invasion of land by plants began with
42 4200 400 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.
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.
43 4300 300 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.
Lizards evolved into dinosaurs 225 Ma. There are a number of
differences, but the most obvious is in the construction of the hip so
44 4400 200 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.
Archaeopteryx, the first bird, evolved from feathered theropod
dinosaurs about 140 Ma. Soon afterwards (approx. 130 Ma)
45 4500 100 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 Mass extinction of 65 to 70 per cent of all species, including all the
ago 4535 65 ammonites, belemnites, flying reptiles and dinosaurs (although
birds, the last of the evolutionary line of the dinosaurs, continued to
www.bgs.ac.uk/discoveringGeology/time/timeline/home.html
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