this presentation provides an
introduction to Earth's history we'll
zoom through 4.6 billion years of
geologic time in the next six or seven
minutes we have one major learning
objective for this lesson we want you to
be able to explain how the primitive
earth formed described how it developed
its oceans and atmosphere and discuss
what
early life looked like on the planet
Earth's history can be divided into
three big chunks of time we call eons
during the first two yawns the archean
and proterozoic the planet's barren
surface would have looked much different
from today with no plants or animals on
land and no fish in the sea it was only
during the last twelve percent of
geologic time during the phanerozoic eon
when earth began to resemble the planet
we see around us today as we move
through this presentation keep an eye on
the time line along the left margin of
the screen as an indicator of where we
are in Earth's history earth formed by
4.6 billion years ago as our solar
system was forming from a collapsing
cloud of dust and gas the remaining
materials organized into a flattened
disk of rotating debris gravity from
larger clumps pulled in adjacent dust
and rocks to form planetesimals which
eventually collided together to form the
early Earth Hut yawn earth was
characterized by widespread melting and
rapid separation of the internal
compositional layers of mantle and core
within the first 30 million years of a
planet's life volcanic gases including
water vapour from Earth's interior
formed a new atmosphere by outgassing
processes the planet's surface was too
hot for liquid water resulting in a
constant cycle of precipitation and
evaporation relatively early on in
Earth's history there was a collision
with a smaller mars-sized planet that
blasted out the material that came to
form the moon this resulted in planet
scale melting and differentiation of the
crust the very first atmosphere was
helium and hydrogen leftover from the
formation of the planet these light
gases were quickly lost to space then
outgassing from volcanic eruptions
contributed to a secondary atmosphere
which would have differed substantially
from the air we breathe today there was
no oxygen at high concentrations of
carbon dioxide ammonia and methane
differentiation of Earth's core resulted
in the development of a magnetic field
to protect this new atmosphere from
solar
about 4 billion years ago the inner
planets including earth were subjected
to a bombardment of comets and asteroids
these impacts transferred water to earth
that contributed to the early oceans the
rest of the water came from outgassing
volcanic eruptions as the hot earth
cooled all life on Earth is cellular and
the most primitive life is represented
by prokaryotic cells such as bacteria
there are less than one micron in size
the early atmosphere was modified by two
processes first bacteria evolved to use
photosynthesis to consume carbon dioxide
and produce oxygen second sunlight broke
down ammonia to form nitrogen and
hydrogen the hydrogen was lost to space
while the nitrogen stayed around to form
the major components of today's
atmosphere communities of cyanobacteria
precipitated limestone between cells to
form algal mats that created mind like
structures similar to the modern
stromatolite sin this image so early
bacterial life in the ocean starts to
consume carbon dioxide and produce
oxygen by photosynthesis but as we will
see next
none of this oxygen makes it to the
early atmosphere which was dominated by
nitrogen and carbon dioxide banded iron
formations or Biff's are the most common
sedimentary rocks formed during the
Archaean these are chemical sedimentary
rocks formed when oxygen reacted with
dissolved iron in the early oceans this
process continued until all with salt
iron was oxidized this took more than a
billion years it was only then when the
dissolved iron was used up did oxygen
become available to be added to the
atmosphere gradually over time the
carbon dioxide concentration in the
atmosphere was reduced and the oxygen
and nitrogen levels increase
multicellular organisms first appeared
around this time in the early
Proterozoic and represented a
fundamental biological change these were
larger than prokaryotic organisms with
more complex internal structures
eukaryotes consumed oxygen and produce
carbon dioxide so we can infer that
oxygen had begun to accumulate in the
atmosphere albeit at low concentrations
little happened for the next billion
years as Earth's early continents
amalgamated into a relatively stable
supercontinent this has been dubbed the
boring billion because these modest
changes in the plate tectonic cycle
resulted in future
in Earth's atmosphere hydrosphere and
biosphere life was a mix of single and
multi celled organisms living in an
oxygen deficient ocean however the
breakup of the supercontinent Rodinia
kicked off a series of dramatic changes
beginning around 750 million years ago
there is extensive evidence of
glaciation in the rock record from
around 700 million years ago during
snowball earth it is thought that only
the lowest latitudes around the tropics
didn't freeze over geological evidence
has been interpreted to indicate that
the planet seesaw back and forth between
the extreme cold of snowball earth and
the warmth of hot ice earth conditions
when no ice would be present it's
thought that these conditions alternated
twice during about 100 million years
some potential reasons for such
significant climatic contrasts include
changes to the output of the Sun
fluctuations and the carbon dioxide
concentrations of the atmosphere or
changes in ocean circulation around this
time oxygen was more abundant in the
atmosphere at about 50% of its current
concentration changing global conditions
were a bite to spark a great surge in
evolution and the development of
organisms with hard skeletons that would
be readily preserved as fossils fossils
became much more abundant in the rocks
of the phanerozoic eon representing the
last 542 million years of geologic time
the presence of fossils allows
geologists to further subdivide that
yawn into three shorter eras the
paleozoic mesozoic and cenozoic the
breaks between the paleozoic and
mesozoic and the Mesozoic and Cenozoic
eras represent two major extinction
events that significantly reduce the
number of organisms present on the
planet generally you can see that this
graph shows an increase in the number of
general with time except for five major
extinction events the most significant
of which occurred between the eras
extinction can be due to various causes
all of which result in a dramatic change
in some part of the Earth's system that
affects key populations of organisms so
summarizing a few points from Earth's
early history the oceans came from
volcanic eruptions comets and asteroids
our atmosphere was very different than
the Archaean Earth's climate has
fluctuated dramatically and we have
recent record of evolutionary change is
separated by dramatic extinctions we had
one major learning objective for this
lesson how confident are you that you
could complete this task
you