Page 72 - The World About Us

Dinosaur fossils
1.1.1
1.1.1
2.2.32.3.1

What is geological time and the history of life on Earth?

Our planet is approximately Earth forms
4,600,000,000 years old. For humans, with fig.199 Life and geological me. Cambrian Moon forms
a lifespan of less than 100 years, these Explosion
numbers are almost impossible to Oceans form
comprehend. Yet to fully understand why
our planet is like it is today we do need to Evolu on of grass 541 million years PHANEROZOIC HADEAN
understand its full history. What were the Ex nc on of dinosuars PROTEROZOIC 600 million years First
key me periods and events through which 1,958 million years life
our world evolved? Breakup of Pangea Today Green
algae
This history is recorded in the Earth's Life moves onto evolves
rocks, which act as the planet's history the land 1,500 million years
book. Geological me is the way that
ARCHEAN
scien sts have divided Earth’s history into Red-brown
541 million b.p. algae
chapters, separated by significant events. appears Beginning of
These events are preserved as changes in photosynthesis
rocks. However, instead of using years, Mass ex nc on Evolu on Increasing ‘Snowball by bacteria
days, hours and minutes, which are too event of the atmospheric Earth’
brief to encompass the mescale of stone, dinosuars oxygen Evidence of
geological me uses eons, eras, periods atmospheric oxygen
and epochs. Stromatolites, ‘living rocks’, formed flowering plants. This, in turn, saw the
Figure 199 shows the four eons of from a build-up of microbial mats, appear diversifica on of pollina ng insects into
geological me. They are unimaginably in the fossil record 3.7 billion years ago, bu erflies, moths, midges, bees.
long periods of me. One way to give a just before 05:00 hours. They became the Dinosaurs were wiped out in another
sense of the enormity of this geological dominant life of the shallow seas for over mass ex nc on just before 23:40, crea ng
me is to imagine the history of the Earth twelve hours in our imaginary day. an evolu onary niche for early mammals
compressed into a single 24 hour day. It is not un l 13:00 hours that more to exploit. In the post-dinosaur Cenozoic
The Hadean Eon complex, single-celled organisms, with a era it took another 30 million years for
cell nucleus emerge. Mul -cellular grass to evolve at 23:49. Grasses then
In this model the Earth was created organisms appear at around 16:10 and spurred the evolu on of many species of
from space debris orbi ng the Sun at 00:00
heralded the beginning of the decline of grazing animals.
hours. The first big event happened at the stromatolites.
about 00:30 hours when the Earth was Our earliest hominid ancestors may
involved in a cosmic collision with another The Phanerozic Eon have walked the Earth by 23:58 but our
new planet. This collision knocked the Just a er 21:10 there was a sudden own species, Homo sapiens, did not
Earth off its ver cal axis. The debris from diversifica on of life in the seas. Known as emerge un l 200,000 years ago, at just
that collision was thrown back into space the ‘Cambrian explosion', this period of four seconds to midnight. Our human
to form the Moon. It took 600 million years me saw a vast expansion of aqua c life ancestors invented agriculture and began
for the surface of the Earth to cool down forms. By 21:41 sharks had evolved, to live in ci es just one fi h of a second
sufficiently for the rock to become solid. hun ng the seas for other early fish. before midnight. Neil Armstrong set foot
on the Moon (4.5 billion years a er its own
The Archean Eon At 21:14 plants began to colonise the forma on) at just 0.000925 seconds before
land, followed soon by the first land
Life may have evolved in its most animals. However, they would not become the end of our imaginary day.
primi ve form between 02:30 to 04:30 fully-established un l 21:44. Land plants at
hours. Photosynthesising bacteria evolved this me resembled simple mosses which KEY TERMS
around 06:15. The first evidence of
evolved into large tree ferns and primi ve
atmospheric oxygen, a waste product of trees by 22:00. These trees covered the Eon: the longest division of geological
photosynthesis, is found the end of the Earth in dense forests by 22:10 hours. time dividing the history of Earth into four
Archean at 11:30 hours. sections: Hadean, Archean, Proterozoic
A mass ex nc on, probably caused by and Phanerozoic.
The Proterozoic Eon a meteorite strike, wiped out 90% of life Era: subdivision of an eon, usually marked
The Proterozoic witnessed the first forms at the end of the Permian period at by a significant change in the
mass ex nc on on Earth as the planet’s 22:26. This event marked the start of the development of life on the planet.
first atmospheric pollutant, oxygen gas, Triassic period and saw the evolu on of Period: subdivision of era, often marked
wiped out most early life forms. The the dinosaurs. Dinosaurs were highly by a change in the fossil record, eg.
oxygen reduced methane levels in the successful, domina ng the world for 183 Carboniferous period.
atmosphere, plunging the world into a 300 million years throughout the Triassic, Epoch: subdivision of a period, marked by
million year-long glacia on, known as Jurassic and Cretaceous periods. change in the conditions for, and of, life
‘Snowball Earth’. At 23:15 the Earth saw the evolu on of on the planet, eg. Holocene epoch.
Age: subdivision of an epoch.
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The climate has changed from the start of the Quaternary period.