The document outlines the formation and evolution of Earth, starting from the Big Bang approximately 14.4 billion years ago, through the nebular hypothesis explaining the solar system's development. Key points include the formation of the moon through a giant impact, the development of Earth's internal structure due to heating and differentiation, and the evolution of the atmosphere and early life, particularly through the emergence of cyanobacteria and stromatolites. It emphasizes the transition from a CO2-rich atmosphere to an oxygen-rich one, driven by photosynthesis, and provides evidence of life dating back 3.5 billion years.

1. P RO F. D R . S . S H A F I Q - U R - R E H M A N
D E PA RT M E N T O F E N V I RO N M E N TA L S C I E N C ES
U N I V E RS I T Y O F P ES H AWA R
Planet Earth:
from origin to present

2. TheGeologicalTimeScale

3. 1. Planets revolve around the Sun in same direction
2. The orbits are all nearly circular
3. The orbits are in the same flat plane
4. The Sun rotates in almost the same plane as the
planets and in the same direction as the they
revolve
5. Most of the planets rotate in the same direction as
the sun
6. Seven of the nine planets have moons
Six facts worth considering about
the origin of Earth

4. Origin of the Universe
The universe began about
14.4 billion years ago
The Big Bang Theory states
that, in the beginning, the
universe was all in one
place
All of its matter and energy
were squished into an
infinitely small point, a
singularity
Then it exploded and
material blown out by the
explosion eventually
formed stars and galaxies

5. Nebular Hypothesis:
The currently accepted
argument regarding how
Solar System may have
formed
5 billion years ago a cloud
of gas and dust (nebula),
at least 10 billion km in
diameter, rotated slowly
in space
As time passed the cloud
condensed under its own
gravity and shrank in size
The Nebular Hypothesis

6. Bulk of the mass concentrated
in the center of nebula while its
outer parts were turbulent
The turbulent eddies collected
matter and small chunks grew
in size and collided, eventually
forming large aggregates of gas
and solid chunks comprising
elementary particles
The compression made the
interior so hot that fusion
began and the core of the cloud
formed the sun
The Nebular Hypothesis

7. After sufficient mass and density
was achieved in the Sun, the
temperature rose to one million
°C, resulting in thermonuclear
fusion:
H + H = He + energy
The Sun

8.  Gravitational forces allowed the inner planets to
accrue and compact solid matter (including light and
heavy atoms)
 Solar radiation blew gases (primarily H & He) away
from the inner planets
 These gases were collected and condensed into the gas
giants (Jupiter, Saturn, Uranus, Neptune)
 Beyond Neptune, ice and frozen gases formed Pluto,
Sedna and the Kuiper Belt Objects
 The left-over debris formed comets and asteroids
Proto-planets

9. Size of the Planets

10. Bombardment From Space
For the first half billion
years of its existence, the
surface of the Earth was
repeatedly pulverized by
asteroids and comets of all
sizes
One of these collisions
formed the Moon

11. Formation of the Moon
The Giant Impact Hypothesis
predicts that around 50 million
years after the initial creation of
Earth, a planet about the size of
Mars collided with Earth
This idea was first proposed about
30 years ago, but it took
calculations by modern high-speed
computers to prove the feasibility

12. Formation of the Moon
Part of the material from the collision remained in orbit around the
Earth
By the process collision and accretion, this orbiting material
coalesced into the Moon
The early Moon orbited very close to the Earth

13. Internal Heating of Early Earth
1. Collisions (Transfer of kinetic
energy into heat)
2. Compression
3. Radioactivity of elements (e.g.
uranium, potassium, or
thorium)
Three major factors that caused heating and melting in the early Earth’s
interior:

14. Internal Heating
About 100 million years after
initial accretion, temperatures at
depths of 400 to 800 km below
the Earth’s surface reach the
melting point of iron
In a process called global
chemical differential, the
heavier elements, including the
melted iron, began to sink down
into the core of the Earth, while
the lighter elements such as
oxygen and silica floated up
towards the surface

15. Global Chemical Differentiation
This global chemical differential was completed by about 4.3 billion
years ago, and the Earth had developed a inner and outer core, a
mantle and crust

16. Chemical Composition of Earth
Whole Earth:
Fe+O+Si+Mg = 93%
Crust:
Si+O+Al = 82%
Each of the major layers has a distinctive chemical
composition, with the crust being quite different from
the Earth as a whole

17. Continents: Formed from
solidified magma that floated up
from the Mantle
Continent, Oceans & Atmosphere
Oceans and Atmosphere: Fluid
and gaseous outer layers believed
to have been created by out-
gassing of gases and fluids from
volcanic eruptions (in a process
called volatile transfer)

18. The Continents
By 2.5 billion years ago, the continents
had been formed
The density of the continental crust
(2.8 g/cm3) is lighter that the crust
found on ocean bottoms (3.2 g/cm3),
so the continents rise above the ocean
floor
A question that remains unanswered
is, when did plate tectonics start?

20. Evolution of Atmosphere
Initially the Earth is thought to have had a thin atmosphere made
up primarily of helium and hydrogen gases, but they escaped into
outer space as the Earth’s gravity could not hold them together
For the next several hundred million years, volcanic out-gassing
began to create a thicker atmosphere composed of a wide variety
of gasses, similar to modern volcanic eruptions
By 3.5 billion years ago, when the
Earth was a billion years old, it had
a thick atmosphere composed of
CO2, methane and water vapors etc.
By human standards this early
atmosphere was very poisonous as
it contained almost no oxygen

21. These volcanic eruptions include:
Water vapor (H2O)
Sulfur dioxide (SO2)
Hydrogen sulfide (H2S)
Carbon dioxide (CO2)
Carbon Monoxide (CO)
Ammonia (NH3)
Methane (CH4)
Atmosphere
Note that oxygen (O2) gas is not created by volcanic eruptions

22. It is hypothesized that water vapor escaping from the interior of
Earth via countless volcanic eruptions created the oceans (this
took hundreds of millions of years). The earliest evidence of
surface water on Earth dates back about 3.8 billion years
Oceans
Astronomers also
hypothesize that comets
impacting the Earth were a
major source of water that
contributed to creation of
the oceans
Remember, that comets are
best described as “dirty ice
balls”

23. Origin Of Life on Earth
These 3.5 billion year old fossilized algae mats, found in
Western Australia, are called stromatolites, and considered to
be the earliest known life on earth

25. Stromatolites
 Stromatolites are fossil remnants of early life
 They are mounds, columns and sheet-like
sedimentary rocks originally formed by the growth of
layer upon layer of cyanobacteria
 These are uni cellular photosynthesizing microbes
found in aquatic environment
 They produce oxygen as a by product. As such
photosynthesis is the only major source of free
oxygen in atmosphere

26. Stromatolites
 As stromatolites became more common 2.5 b.y.a.
they gradually changed the Earth’s atmosphere from
CO2 rich mixture to the present day O2 rich
composition
 Microfossils in rocks dating between 2.7 and 3.0 Ga
show more conclusive evidence for life
 It seems unlikely that life started on land because
early atmosphere was highly deficient in O2

27. Evidence of Life
 Stromatolites offer evidence for the existence of
microbial colonies as far back as 3.5 Ga
 Individual fossilized cells were found in a rock 3.5 Ga
formation in northwestern Australia
 Microscopic photos of sections of this rock show
structures that resemble individual cells
 The structure and chemical analyses show the presence
of organic carbon, possibly from sedimentary rocks
formed in shallow seas
 As such the protective layer of ozone in the upper
atmosphere was lacking and early forms of life were
exposed to high level of UV radiation

28. Life on Earth
 Thus the microfossil evidence clearly points to
existence of life before 3.0 Ga
 Isotopic carbon (C12) is far more common than C13
(typically 1 out of 89 atoms). As such low C13 ratio
indicate organic percentage
 Rocks recovered from sea near Greenland indicate a
lower ratio of C13 isotope from a rock that was dated
more than 3.85 Ga
 Recent evidence suggests that life already existed by
3.85 b.y.a.

29. Prokaryota
Bacteria are cells without nuclei and are called Prokaryota
While prokaryotes are nearly always unicellular, some are capable of
forming groups of cells called colonies

30. Cyanobacteria
Cyanobacteria, commonly called blue-green algae, is a phylum of bacteria
that obtain their energy through photosynthesis
This was the first life on Earth

32. Banded Iron Formations
It is hypothesized that the banded iron layers were formed in sea
water as the result of free oxygen released by photosynthetic
cyanobacteria combining with dissolved iron in the oceans to form
insoluble iron oxides, which precipitated out, forming a thin layer
on the seafloor

33. Nucleus Bearing Cells
Then, at the start of the Cambrian, 570 million years ago, there was an
explosion in the diversity of life on Earth by Nucleus-bearing cells