Handout: Geological timescale
Strand 4
INTRODUCTION:
The History of the Earth and all life that it encompasses makes a wonderful, interesting
story. We can learn about this story if we read what the rocks that make up our Earth have
to tell us. Important aspects to understanding the history of the Earth are the fossil records
that are discovered and then dating them. With relative dating we’re able to simply use the
principle that the deeper the fossil, the more layers that are above a fossil, the older it is. A
more specific type of dating, radiometric dating, is able to give us more precise dates.
Simply put certain elements ‘degrade’ at known rates, some take billions of years to
degrade. Scientists therefore find out how much of the element found in a fossil or rock has
‘degraded’ from its original state and then date it.
Geological timescale is a chronological measurement that relates geology to time and is used
by geologists, palaeontologists, biologists and other earth scientists to investigate the timing
of events that occurred in the History of Earth.
Bullet Key:
= Mass extinction event
= South African fossil example
= Continental drift
Eon
Era
Period
Hadean Eon
Millions
of years
ago
4,570 –
3,800
Major biological and geographical events
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Evidence of Earth’s formation from meteorite and lunar rock
found on Earth which is 4.5 billion years old.
No known life.
Earth’s crust cools and solidifies.
The moon forms.
Archean Eon
3,800 –
2,500
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Earliest known life on Earth, bacteria forms 3.5 billion years ago.
Earth is very different from today.
Atmosphere is mostly methane and ammonia.
Continents only just have begun to form.
Bacterial micro-fossil, Archaeospheroides barbertonis, that is 3.2
billion years old has been found in Barberton, Mpumalanga.
Proterozoic Eon
2,500 - 543
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Precambrian Era
First multi-celled organisms like sponges appear.
Earliest complex life forms are algae from 1.4 billion years ago.
Oxygen begins to accumulate in the atmosphere due to
photosynthesising algae and bacteria.
Soft-bodied fossils have been found in Northern Cape and
Namibia dating to the Ediacaran Period (late Precambrian).
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Cambrian
543 – 488
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Ordovician
488 – 443
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Paleozoic Era
Phanerozoic Eon
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Cambrian Explosion: a large number of new animal species
appear in a relatively short time.
First fish appear.
No known life on land yet.
By the end of period, all phyla of animal kingdom existed besides
invertebrates (backboned animals).
First land plants appear.
Primitive fungi and sea weed appears.
The oceans are full of corals, molluscs, worms, primitive fish, and
echinoderms like starfish.
End of this period saw emergence of first vertebrates in primitive
fish.
Ordovician – Silurian mass extinction. 27% of all families and 57%
of all genera disappear.
Silurian
443 – 417
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First air breathing animal appears - scorpions.
Spiders, insects and fish with bony jaws appear.
Fish adapt to living in rivers and fresh water for the first time.
Simple land plants, lacking separate stems and leaves begin to
diversify.
Devonian
417 – 359
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Fish dominated the oceans including sharks and lung fish.
Amphibians appear and develop from lung fish.
The first trees and other plants spread across the land creating
the first forests.
Fossils of club mosses and simple vascular plants found in
Devonian Shale in Grahamstown, Eastern Cape.
Devonian – Carboniferous mass extinction. 19% of families and
50% of genera disappear.
Carboniferous
359 – 299
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Coal beds containing coal balls were laid down during this period.
Many swamps on land and sponge reefs in the oceans.
Reptiles appear.
Early winged insects and cockroaches appear.
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Permian
299 – 251
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Triassic
251 – 200
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Mesozoic Era
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Jurassic
200 – 145
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Amphibians dominate the land.
Early cone-bearing plants like pine trees appear.
Fossils of glossopteris that existed in primitive forests found near
Escourt and Mooi River in Kwa-Zulu Natal.
The ‘living fossil’ Coelacanth is believed to have come from this
period and was first discovered close to East London.
Therapsid (mammal-like reptile) fossils from the Permian Period
have been discovered in the Karoo. Important branch of reptiles
as they later evolved into the first mammals.
Midway through Permian Period all continents were joined
together forming the supercontinent, Pangaea.
Permian – Triassic mass extinction (250mya): Period ends with
the largest mass extinction known with 96% of all marine species
and 70% of all animals becoming extinct. In total 57% of all
families and 83% of genera disappear.
Causes for this mass extinction are believed to be from massive
volcanic activity, increased CO2 and methane levels causing global
warming and a decrease in global oxygen levels.
The few survivors of the Permian extinction go on to populate the
land and oceans.
New species like mammals, dinosaurs and crocodiles appear.
First bony fish found in the oceans.
Supercontinent Pangaea begins to split into northern Laurasia
and southern Gondwanaland which have completely formed by
200mya.
Triassic – Jurassic mass extinction. 23% of all families and 48% of
genera become extinct.
Dinosaurs dominate the land.
Mammals are common but small.
Feathered dinosaurs and birds appear.
The most common land plants are ferns, palm-like trees called
cycads, and grasses.
Fossils of many dinosaurs from this era are found in the
Drakensberg and Maluti Mountains as well as the Euskylsaurus
fossils from Lady Grey.
Fossils of the first mammals such as the Megazostrodon found in
the Eastern Cape and Lesotho.
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Cretaceous
145 – 65
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Paleogene
65 – 23
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Neogene
23 – 2.6
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Cenozoic Era
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Quaternary
2.6 - Today
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Dinosaurs continue to dominate the land.
Marsupials, snakes, modern sharks, bees and butterflies appear.
Flowering plants (angiosperms) appear.
90% of all modern trees and shrubs were present by this period.
Earliest primate fossils discovered from 70 mya that were shrewlike.
By the end of the Cretaceous Period the 7 continents were
almost completely separated.
Period ends with Cretaceous – Paleogene mass extinction which
ended 17% of all families and 50% of all genera including all the
dinosaurs. Regarded as the second largest extinction event.
Extinction thought to be caused by asteroids or comet impact
which created devastating climatic effects.
Rise of the mammals and birds.
45 mya advanced primates, similar to lemurs, evolved and
protomonkeys and apes became dominant.
Rodents, pigs, cats, dogs, bears and whales appear. Flowering
plants spread across the globe.
Mammals and birds continue to dominate Earth.
6 – 8 mya early hominids evolved and branched off from the
great apes.
The first hominids, australopithecines, appeared 5 – 6 mya.
Australopithecus africanus fossils have been found in Taung and
the Cradle of Humankind. Australopithecus sediba fossils found
close by at Malapa.
North and South America join together at Panama while India
collides further into Asia forming the Himalaya’s.
Period begins with the Northern Hemisphere glaciation 2.588
mya which coincides approximately with the dawn of the Homo
hominid genus.
Officially recognized as the third period of the Cenozoic era only
in May 2009.
Australopithecines became extinct 1.5 mya and the emergence of
the Homo genus began 2.5 mya.
2 mya 25% of the Earth was covered with ice.
Mammoths, sabre-toothed cats, and giant camels dominated
until 10,000 years ago when many large mammals went extinct
(possibly due to human hunting).
Anatomically Modern humans appeared and developed
civilization by the end of this period.
Early modern human fossils found in various parts of South
Africa.
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OTHER GRAPHICAL REPRESENTATIONS OF THE HISTORY OF LIFE ON EARTH:
1st Jan: 3.8 billion years ago, the first forms of life, bacteria form.
1st Aug: The appearance of
Eukaryotes.
1st Nov: Complex Invertebrates evolve.
8th Dec:
Dinosaurs
dominate
the Earth.
15th Nov: Jawless fish
form.
1st Dec: Bony fish
and sharks evolve.
12th Dec:
Mammals
arise.
30th Dec:
Mammals
radiate.
31th Dec: (7pm)
Australopithecus
arises. (11:56pm)
Modern Humans
have evolved.
Fig 1. A calendar of a typical year with 365 days, marking specific events as they would occur if the History of the
Earth was condensed into one year.
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GEOLOGICAL CLOCK
Fig 2. A geological clock representing the History of Earth and showing certain important geological and
biological events(Anon n.d.)1.
2. Anon, Geologic Clock with events and periods [image online]. Available at: http://en.wikipedia.org/wiki/File:Geologic_Clock_with_events_and_periods.svg
[Accessed November 26, 2011b].
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TREE OF LIFE DIAGRAM
Fig 3. This diagram is not a geological timescale; however it illustrates Darwin’s Tree of Life idea where all life is
connected and shows how species arise from different species through descent with modification. This particular
image puts a time frame on these life forms and shows how long each species has been living on the Earth (Anon
n.d.)2.
3. Anon, BBC Nature - Tree of Life. Available at: http://www.bbc.co.uk/nature/life [Accessed November 26, 2011a].
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