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Early Earth and the Origin of Life
Chapter 26
Evolution of Life on Earth

Life on earth originated between 3.5 and
4.0 billion years ago.
Earth formed about 4.5 billion years ago.
 The oldest fossils of prokaryotes are 3.6
billion years old.

First 3/4 of evolutionary
history- organisms were
microscopic*
*based on molecular
clocks.
Domination of Prokaryotes
Prokaryotes dominated evolutionary
history from 3.5 to 2.0 bya.
 The two domains of prokaryotes,
Bacteria and Archaea, diversified as a
variety of metabolic types living near
hydrothermal vents and in shallow water
communities that left fossils called
stomalites.

Clock Analogy for Key Events in Evolutionary History
Introduction of Oxygen

Oxygen began accumulating in the
atmosphere about 2.5 bya.


Oxygenic photosynthesis evolved in cyanobacteria.
As O2 accumulated in the atmosphere, the
reactive molecule posed an environmental
challenge for life.


Some species survived in habitats that remained
anaerobic.
Among other survivors, a diversity of adaptations to the
changing atmosphere evolved (cellular respiration).
Evolution of Eukaryotic Life

Eukaryotic life began by 2.1 bya.


The oldest fossils of eukaryotes date back
2.1 billion years.
The eukaryotic cell evolved from a
prokaryotic ancestor that hosted smaller
internal prokaryotes.

Endosymbiotic Theory
Evolution of Multicellular Life

Multicellular eukaryotes evolved by 1.2
bya.
There are fossils of multicellular algae
dating back 1.2 billion years.
 The oldest fossils of animals are about 600
million years old.

The Cambrian Explosion

Animal diversity exploded during the early
Cambrian period.
 Most phyla of animals make their first fossil
appearance during a relatively brief span from
about 540-520 million years ago.

Plants, fungi, and animals colonized land
about 500 million years ago.


A symbiotic relationship of plants with fungi
contributed to the move onto land.
Herbivorous animals and their predators followed.
Figure 26.8 The Cambrian radiation of animals
The Origin of Life
The first cells may have originated by
chemical evolution on a young Earth.
 Though life today arises by biogenesis,
the very first cells may have been
products of a prebiotic chemistry.


This idea of life emerging from inanimate
material is called spontaneous generation.
The Origin of Life

Although there is NO evidence that
spontaneous generation occurs today,
conditions on the early Earth were very
different.
Relatively little atmospheric oxygen to tear
apart complex molecules
 Energy sources such as lightening, volcanic
activity, and UV light were more intense

Four-Stage Hypothesis for the Origin of Life

According to one hypothetical scenario, the
first organisms were products of chemical
evolution in four stages:
1.
2.
3.
4.
Abiotic synthesis of small organic molecules, such as amino
acids and nucleotides.
Joining of small molecules (monomers) into polymers,
including proteins and nucleic acids.
Origin of self-replicating molecules that eventually made
inheritance possible
Packaging of these molecules into “protobionts” droplets
with membranes that maintained an internal chemistry
different from the surroundings.
BIOCHEMICAL EVOLUTION
1) The Earth and its atmosphere formed


Gasses present when the atmosphere was
first formed included CO, CO2, H2, N2, H2O,
S, HCl, HCN (hydrogen cyanide), but little or
no O2.
A.I. Oparin and J.B.S. Haldane independently
theorized that simple molecules were able to
form only because oxygen was absent. WHAM
prevalent in atmosphere… (water, hydrogen, ammonia, methane)

As a very reactive molecule, oxygen, had it been
present, would have prevented the formation of
organic molecules by supplanting most reactants
in chemical reactions.
BIOCHEMICAL EVOLUTION
2) The primordial seas formed.

As the earth cooled, gases condensed to produce
primordial seas consisting of water and minerals
(beginning of hydrologic cycle).
3) Complex molecules were synthesized.

Chemicals present in the ancient seas:

Acetic acid, formaldehyde, and amino acids. These
kinds of molecules would later serve as monomers,
or unit building blocks, for the synthesis of polymers.
How were the first organic molecules
created?
Energy catalyzed the formation of organic
molecules from inorganic molecules. An
organic “soup” formed.
1.
•
2.
NO ENZYMES WERE NEEDED.
Energy was provided mostly by ultraviolet
light (UV), but also lightening, radioactivity,
and heat- hydrothermal vents (hot volcanic
outlets in the deep-sea floor).
Figure 26.10x Lightning
Abiotic Synthesis is Testable

Laboratory experiments performed under
conditions simulating those of the
primitive Earth have produced diverse
organic molecules from inorganic
precursors.
Figure 26.9 Pasteur and biogenesis of microorganisms (Layer 3)
Stanley Miller and Harold Urey

Using an airtight apparatus, CH4 (methane), NH3
(ammonia), H2O, H2 and a high voltage discharge,
they found that after one week the water
contained various organic molecules including
amino acids.




(WHAM! Water, hydrogen, ammonia, methane)
The amino acids synthesized are the building blocks of
proteins for organisms.
Proteinoids are abiotically produced polypeptides. They
can be experimentally produced by allowing amino acids
to dehydrate on hot, dry substrates.
Adenine and other nucleotides are the building blocks
of RNA (also- Adenine for ATP).
Figure 26.10 The Miller-Urey experiment
http://bcs.whfreeman.com/thelifewire/content/chp03/0301s.swf
RNA – First Genetic Material?

The “RNA world” preceded today’s “DNA
world”.


RNA may have been the first genetic material.
The first genes may have been abiotically
produced RNA, whose base sequences
served as templates for both alignment of
amino acids in polypeptide synthesis and
alignment of complementary nucleotide bases
in a primitive form of self-replication.
Figure 26.11 Abiotic replication of RNA
The First Heterotrophs

Prokaryotic Heterotrophs feeding on
organic molecules in the seas began to
develop metabolism.

The first form of metabolism (fermentation)
using glycolysis most likely arose because
the atmosphere lacked free oxygen:
anaerobic
Autotrophic Evolution

The first autotrophs were probably
nonoxygenic photosynthesizers.


They did not split water and liberate oxygen (cyclic
only)
The first organisms to use noncyclic
photosynthesis or oxygenic photosynthesis
(water-splitting enzyme) were probably
cyanobacteria (blue-green algae)
Creating the Ozone

A byproduct of oxygenic photosynthesis was
oxygen and as it accumulated in the
atmosphere (2.7-2.2 billion years ago),
1.
2.
3.

First dissolved into the surrounding water until the seas
and lakes became saturated with oxygen.
Additional oxygen would then react with dissolved iron
and precipitate as iron oxide.
Then additional oxygen finally began to “gas out” of the
seas etc. and accumulate in the atmosphere.
The ozone layer was created.

As the ozone absorbed UV rays, the major source of
energy for abiotic synthesis of organic molecules and
primitive cells was terminated.
Effect of Oxygen on Earth

The oxygen had a tremendous impact
on Earth:
Corrosive O2 attacks chemical bonds,
doomed many prokaryotes.
 Some survived in anaerobic
environments (obligate anaerobe
survivors)
 Others adapted- cellular respiration.

The First Eukaryotes
http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter4/animation__endosymbiosis.html



Evolution of Eukaryotic organelles from
prokaryotes occurred about 2.1 billion years
ago.
Mitochondria and Chloroplasts are descendents
of “endosymbionts”- symbiotic cells living within
larger host cells.
Many eukaryotes may have evolved from
prokaryotes enjoying a mutually beneficial
relationship (symbiosis).

Endosymbiotic theory- Margulis.
Endosymbiosis Theory
(Lynn Margulis, 1970’s)
Evidence for Endosymbiosis
1.
2.
3.
4.
Mitochondria and chloroplasts resemble bacteria
and cyanobacteria with respect to their DNA,
RNA, and protein synthesis machinery.
Mitochondria and chloroplasts reproduce
independently of their eukaryotic host cell.
Ribosomes of mitochondria and chloroplasts
reproduce independently of their eukaryotic host
cell.
The thylakoid membranes of chloroplasts
resemble the photosynthetic membranes of
cyanobacteria.
Timeline of Classification
1. 384 – 322 B.C. Aristotle

2 Kingdom Broad Classification – Plants or Animals
2. 1735 - Carl Linnaeus


2 Kingdom Multi-Divisional Classification
(Kingdom, Phylum, Class, Order, Family Genus, Species)
3. Evolutionary Classification – (After Darwin)

Group By lines of Evolutionary Descent
4. Five Kingdom System – 1950s

(Whittaker) – 1950s – Plantae, Fungi, Animalia, Protista,
Monera
6. Three Domain System – late 1990s

late 1990s – Bacteria, Archaea, Eukarya
Linnaeus System Evolves from TWO Kingdoms
to FIVE

As we learned more about different kinds of life, there
needed to be more Kingdoms
 1800’s – Added Kingdom Protista
 Amoeba, Slime Molds
 1950’s – Added Fungi and Monera
 Fungi distinguished from Plants
 Prokaryotes (no nucleus) bacteria given
category
 1970’s – Split Kingdom Monera into 2 separate
Kingdoms


Eubacteria – bacteria with peptidoglycan
Archaebacteria – bacteria without peptidoglycan
The Five-Kingdom System
Reflected increased knowledge of life’s
diversity
 Kingdom is highest – most inclusive taxonomic
category
 Five Kingdoms include:







Monera
Protista
Plantae
Fungi
Animalia
Recognized 2 types of cells: prokaryotes &
eukaryotes
The Five-Kingdom System

Described classification as:






Plantae, Fungi, Animalia, Protista, Monera
recognizes only 2 types of cells: prokaryotic and
eukaryotic
sets all prokaryotes apart from eukaryotes
prokaryotes are in their own kingdom (Monera)
distinguished 3 kingdoms of eukaryotes based on
mode of nutrition
protista were all eukaryotes that did not fit the
definition of plants, fungi, or animals
Figure 26.15 Whittaker’s five-kingdom system
Figure 26.16 Our changing view of biological diversity
The Three-Domain System
http://bcs.whfreeman.com/thelifewire/content/chp27/27020.html
Molecular analyses have given rise to the
most current classification system – the
Three Domain System
 Domain is larger than kingdom
(superkingdoms)


The 3 Domain System is the most recent
classification system and includes:



Bacteria
Archaea
Eukarya
The Three Domain System

Describes classification as:






Not all prokaryotes are closely related (not
monophyletic)
Prokaryotes split early in the history of living
things (not all in one lineage)
Archaea are more closely related to Eukarya than
to Bacteria
Eukarya are not directly related to Eubacteria
There was a common ancestor for all extant
organisms (monophyletic)
Eukaryotes are more closely related to each other
(than prokaryotes are to each other)
Section 18-3
Classification of Living Things
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