geological time scale

The first geologic time scale was proposed in 1913 by
the British geologist Arthur Holmes (1890 - 1965). This
was soon after the discovery of radioactivity, and using
it, Holmes estimated that the Earth was about 4 billion
years old - this was much greater than previously
believed.
Dividing Earth History into Time Intervals
Geologists have divided Earth's history into a series of
time intervals. These time intervals are not equal in
length like the hours in a day. Instead the time intervals
are variable in length. This is because geologic time is
divided using significant events in the history of the
Earth.

Examples of Boundary "Events"
For example, the boundary between the Permian
and Triassic is marked by a global extinction in
which a large percentage of Earth's plant and
animal species were eliminated. Another example
is the boundary between the Precambrian and
the Paleozoic which is marked by the first
appearance of animals with hard parts.

Eons
• Eons are the largest intervals of geologic
time and are hundreds of millions of
years in duration. In the time scale above
you can see the Phanerozoic Eon is the
most recent eon and began more than
500 million years ago

Eras
• Eons are divided into smaller time intervals
known as eras. In the time scale above you
can see that the Phanerozoic is divided into
three eras: Cenozoic, Mesozoic and Paleozoic.
Very significant events in Earth's history are
used to determine the boundaries of the eras.

Periods
• Eras are subdivided into periods. The events
that bound the periods are wide-spread in
their extent but are not as significant as those
which bound the eras. In the time scale above
you can see that the Paleozoic is subdivided
into the Permian, Pennsylvanian,
Mississippian, Devonian, Silurian, and
Ordovician and Cambrian periods.

Epochs
• Finer subdivisions of time are possible and the
periods of the Cenozoic are frequently
subdivided into epochs. Subdivision of periods
into epochs can be done only for the most
recent portion of the geologic time scale. This
is because older rocks have been buried
deeply, intensely deformed and severely
modified by long-term earth processes. As a
result, the history contained within these
rocks cannot be as clearly interpreted.

Geological timeline
• During its dramatic 4.5 billion year history, Earth
has gone through a series of major geological and
biological changes. The timescale below
highlights a number of notable prehistoric events
and the geological periods in which they
occurred. As things didn't get interesting from a
biological perspective until around 570 million
years ago, we've included a couple of zoomed in
timelines to show the detail of more recent
evolutionary history.

4.6 billion years ago The origin of the Earth
3.8 billion years ago First life arises
2.1 billion years ago Eukaryotes evolved
1.1 billion years ago
First sexually reproducing
organisms
570 million years ago First arthropods evolve
530 million years ago The first fish
475 million years ago First land plants
385 million years ago First forests
370 million years ago The first amphibians
320 million years ago The earliest reptiles

225 million years ago The dinosaurs evolve
200 million years ago The mammals evolve
150 million years ago First birds
130 million years ago Flowering plants evolve
100 million years ago The first bees evolve
Dinosaurs and ammonites become
extinct
14 million years ago The first great apes appear
2.5 million years ago Genus Homo evolves
200 thousand years ago Our species, Homo sapiens evolves
10 thousand years ago End of the last Ice Age

Archean era
• It was during the Archean era that life first arose on
Earth. At this time there were no continents, just small
islands in a shallow ocean. There was a vast amount of
carbon dioxide in the atmosphere, but since the sun
was much fainter back then, the combined effect did
not raise Earth's temperature to an extreme. Such
levels of carbon dioxide would be toxic to the majority
of animals alive today - as would the low oxygen levels.
• Began:3.8 billion years ago
• Ended:2.5 billion years ago

Cryogenian period
• A succession of incredibly harsh ice ages waxed
and waned during the Cryogenian. It is nicknamed
Snowball Earth as it's been suggested that the
glaciation was so severe it may even have reached
the equator. Life during the Cryogenian consisted
of tiny organisms - the microscopic ancestors of
fungi, plants, animals and kelps all evolved during
this time.
• Began:850 million years ago
• Ended:635 million years ago

Ediacaran period
• Known also as the Vendian, the Ediacaran was the final
stage of Pre-Cambrian time. All life in the Ediacaran was
soft-bodied - there were no bones, shells, teeth or other
hard parts. As soft bodies don't fossilise very well,
remains from this period are rare. The world's first ever
burrowing animals evolved in the Ediacaran, though we
don't know what they looked like. The only fossils that
have been found are of the burrows themselves, not the
creatures that made them. This period gets its name
from the Ediacara Hills in Australia, where famous fossils
of this age were found.

Cambrian period
• The Cambrian is famed for its explosion of abundant and
diverse life forms. Life had diversified into many forms and
many ways of living: animals now swam, crawled,
burrowed, hunted, defended themselves and hid away.
Some creatures had evolved hard parts such as shells,
which readily fossilised and left a clear record behind.
However, sometimes geologists get lucky and find beautiful
fossils of soft and squishy creatures - as at the Burgess
Shale site. In Cambrian times there was no life on land and
little or none in freshwater - the sea was still very much the
centre of living activity.

Ordovician period
• During the Ordovician, a few animals and plants began
to explore the margins of the land, but nothing
colonised beyond these beachheads, so the majority of
life was still confined to the seas. The Ordovician began
with shallow, warm seas but the end of the period
experienced a 500,000 year long ice age, triggered by
the drift of the supercontinent, Gondwana, to the
south polar regions. The Ordovician ended with a mass
extinction.
• Ended: Ordovician-Silurian mass extinction

Silurian period
• The Silurian period was the time when reefs got their
act together, grew really big and created a completely
new type of ecosystem for marine life. Silurian reefs
weren't built by the same types of coral around now,
but by a host of tabulate and rugose corals, crinoids
and sponges. As the Ordovician ice ages ended, sea
levels rose, making the Silurian a period of extensive
seas. Bony fish made their first appearance.
Meanwhile, on land, plants became more established,
and grew in a zone along the edges of rivers and lakes
to give Earth its first riverine and wetland habitats.
• Began: Ordovician-Silurian mass extinction
• Ended: 417 million years ago

Devonian period
• The Devonian is also known as the Age of Fishes, since
several major fish lineages evolved at this time. Sea
levels were high and the global climate was warm. Sea
surface temperatures in the tropics averaged 30
Celsius, much like the warmer parts of the Pacific
today. Growth rings from corals living during the
Devonian period have provided evidence that there
were more than 365 days in the year back then -
about 404 at the start of the period, falling to 396 by
the end.
• Began: 417 million years ago
• Ended: Late Devonian mass extinction

Carboniferous period
• The Carboniferous is famed for having the highest
atmospheric oxygen levels the Earth has ever
experienced and for the evolution of the first reptiles.
Plants grew and died at such a great rate that they
eventually became coal. The period was originally
called the Coal Measures after its proliferation of coal-
bearing rocks. Though the Carboniferous started off
warm - hence its lush coal forests - the temperature
began to drop and the polar regions were plunged into
an ice age that lasted millions of years. In North
America, the Carboniferous is divided into two epochs,
the Mississippian and the Pennsylvanian.
• Began: Late Devonian mass extinction

Permian period
• The Permian started with an ice age and ended with
the most devastating mass extinction the Earth has
ever experienced. In fact, at least two mass extinctions
occurred during this time. It's also when all the
continents of the world finally coalesced into one
supercontinent, named Pangaea (meaning 'the entire
Earth'). As the globe warmed up and the ice retreated,
many areas of Pangaea became very arid. The oxygen
level plummeted too, from a high of 35% of the total
atmosphere to around 15%. For comparison, today's
oxygen content is 21%.
• Began: 290 million years ago
• Ended: Permian mass extinction

Triassic period
• The Triassic began after the worst mass extinction ever, at
the end of the Permian. Life on Earth took a while to recover
and diversify. The Triassic was characterised by heat, vast
deserts and warm seas. Even the polar regions were warm,
so lush forests grew there. However, the lack of other life,
coupled with the period's particular environmental
conditions, opened up some evolutionary opportunities. As a
result, the very first mammals and dinosaurs evolved. During
this time, the giant supercontinent of Pangaea began to
break apart. The period ended as it had begun, with an
extinction event that wiped out many species.
• Began:Permian mass extinction
• Ended:Triassic-Jurassic mass extinction

Jurassic period
• The Jurassic began after the mass extinction event that
ended the Triassic. Life, however, was quick to recover
from this blow and the Jurassic eventually became host
to the most diverse range of organisms that Earth had
yet seen. Amongst them were the first birds and some
of the dinosaurs. Continental break-up during this time
gave rise to the sea that would eventually widen to
become the Atlantic Ocean. The ocean floor that
formed at this time is the oldest surviving on the planet
- all older ones having now been 'recycled' through
plate tectonics.
• Began:Triassic-Jurassic mass extinction

Cretaceous period
• The Cretaceous ended with the most famous mass
extinction in history - the one that killed the dinosaurs.
Prior to that, it was a warm period with no ice caps at
the poles. Much of what we now know as dry land -
such as southern England and the midwest of the USA -
was underwater, since sea levels reached their highest
ever during this time. The Atlantic Ocean grew much
wider as North and South America drew apart from
Europe and Africa. The Indian Ocean was formed at this
time, and the island that was India began its journey
north towards Asia.
• Ended:Cretaceous-Tertiary mass extinction

Palaeocene epoch
• The Paleocene epoch was a time of dense forests and
evolutionary experiments. The extinction of the
dinosaurs and other giant reptiles at the end of the
Cretaceous paved the way for mammals and birds to
evolve to fill those empty niches, so many new
creatures appeared. During the Paleocene the island
continent of India moved north and collided with Asia.
At the end of the epoch, an abrupt rise in temperature
across the planet made the climate much wetter and
caused a sea level rise.
• Began: Cretaceous-Tertiary mass extinction
• Ended:54.8 million years ago

Eocene epoch
• The Eocene began as a time of global warming, with
temperatures across the planet soaring. Forests thrived
and trees grew even in polar regions. Eventually, the
Eocene became cooler and drier. As India continued to
drift northwards, pushing against the Eurasian
continent, the mass of rocks thrust up between them
formed the Himalayas. At this time Africa was an island,
not yet joined to the Middle East and Europe, but its
own journey north was to trigger the formation of the
Alps. Many species of grass evolved in the Eocene
epoch, but were very limited in extent - today's grassy
plains were still far in the future.
• Began:54.8 million years ago
• Ended:33.7 million years ago

Oligocene epoch
• Over 30 million years ago, the Oligocene epoch saw
the start of the global cooling that would eventually
shift the Earth's climate to one where glaciers were
present and ice ages were possible. Worldwide, this
was the time when grasslands began to expand and
forests - especially tropical ones - shrank
correspondingly. Animals evolved to fit the new,
open landscape and many fast-running prey and
predator species arose as a result.
• Began: 33.7 million years ago
• Ended: 23.8 million years ago

Miocene epoch
• The apes arose and diversified during the Miocene
epoch, becoming widespread in the Old World. In
fact, by the end of this epoch, the ancestors of
humans had split away from the ancestors of the
chimpanzees to follow their own evolutionary path.
As in the Oligocene before it, grasslands continued
to expand and forests to dwindle in extent. In the
Miocene seas, kelp forests made their first
appearance and soon became one of Earth's most
productive ecosystems.

Pliocene epoch
• The Pliocene world looked very similar to Earth today
as North and South America had been drifting ever
closer and the gap between them was sealed in this
epoch. At the start of the Pliocene, over 5 million years
ago, the north polar ice cap came and went with the
seasons and with fluctuations in climate. However, as
the world cooled in the late Pliocene, ice at the North
Pole became permanent and grassland and tundra
thrived. The human lineage split away from the
chimpanzees' early on in the epoch.

Pleistocene epoch
• During the Pleistocene, glaciers came and went,
resulting in a series of ice ages punctuated by warmer
periods. There were at least 20 cycles of this advance
and retreat. During the ice ages, global temperatures
were 5 degrees centigrade cooler than today and it
was much drier, since much of the world's water was
locked up in massive ice sheets. The expansion of the
deserts and the action of glaciers grinding up rocks
meant that dust storms would have been a lot more
common in the Pleistocene than they are now. Our
species evolved during this epoch.
• Ended: 11.7 thousand years ago

Holocene epoch
• The Holocene (or Recent) is the current geological
epoch which started some 11,500 years ago when the
glaciers began to retreat. This retreat marked the end
of the glacial phase of the most recent ice age. Its
character was set by the spread of forests as the ice
retreated and then by their shrinkage as mankind's
demand for timber and agricultural land grew.
Although we think of the Holocene as a warm time for
the planet, we are still in an ice age. This is indicated by
the presence of ice caps at the poles - the planet as a
whole is just in an interglacial phase.
• Began: 11.7 thousand years ago
• Ended: Present day

geological time scale

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