Chapter 17.2:
Earth’s Early History
Age of the Earth

It is estimated that the Earth
is 4.6 billion years old
– Theories of how the Earth was “born”
vary (IE – the Big Bang Theory)
– Elements arranged themselves according
to density
Least dense make up the atmosphere, most
dense make up the core
Earth’s Early
Atmosphere

Earth’s early atmosphere probably
contained hydrogen cyanide, carbon
dioxide, carbon monoxide, nitrogen,
hydrogen sulfide, and water
– All toxic gases except the
water
The Early Earth
4 billion years ago the first solid rocks
formed
 3.8 billion years ago that the Earth was
cool enough to have liquid water

– So this is when the first oceans formed
Earth BEFORE oceans
formed

Characteristics of Earth before oceans
formed:
Comets and asteroids
Volcanic activity
Atmosphere of poisonous gas
Why do you think
oceans did not exist on
Earth 4 billion years
ago?

Water was a gas! Because Earth was
very hot!
Formation of Organic
Molecules

Early atmosphere allowed the
formation of organic molecules
– because it DID NOT have oxygen
– Would not form in today’s atmosphere
Oxygen is too reactive
 Bacteria would eat the molecules

Miller and Urey
Stanley Miller and Harold Urey
attempted to answer whether
Organic molecules could have
formed before life was present.

Filled an enclosed flask with
a mixture of gases and water to
simulate the Earth’s Atmosphere!
Used electrodes to produce a
Spark that simulates lightning!
Miller and Urey made
sure no microscopic
life existed in this
experiment. Why?

They had to make sure the original
simple substances they were testing
were the ONLY possible source for the
creation of organic molecules.
The Results from Miller
and Urey

Amino acids began to form after a
couple days.

This mixture of amino acids and other
organic compounds showed us that
molecules necessary for life could
have risen from simple compounds
present on early Earth without life.
The First Cells
Thought to have originated 200-300 million
years after oceans formed.
Why? Because the presences of liquid
water was necessary for first life.
 Earliest cells called proteinoid microspheres
were created when organic molecules
became trapped in bubbles
 Not alive but have some
characteristics of living cells
 They had SELECTIVELY
PERMEABLE MEMBRANES.
Oxygen-Free Life Forms

The first life forms are thought to have
evolved in oxygen free environments
approximately 3.5 billion years ago
First organisms on Earth were most
like today’s BACTERIA

Photosynthetic organisms evolved
in the oceans approximately 2.2 billion
years ago

Started pumping oxygen into the
atmosphere
Endosymbiotic Theory

Belief is that eukaryotes formed approx. 2
billion years ago from COMMUNITIES of
PROKARYOTES.
– Prokaryotes “moved into” prokaryotic cells

Created a symbiotic relationship
– One group was able to use oxygen to make ATP

Mitochondria
– One group was able to use light to make sugars

Chloroplasts
Evidence of the
Endosymbiotic Theory

Membranes of mitochondria and
chloroplasts are similar to that of freeliving prokaryotes

Mitochondria and chloroplasts have
– DNA similar to bacterial DNA
– Ribosomes similar to those of bacteria
– Reproduce by binary fission
Sexual Reproduction

The use of sexual reproduction by
early organisms allowed evolution to
occur at a much quicker rate
– More gene shuffling