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3/22/2016
EK 1.D.1: There are several hypotheses about
the natural origin of life on Earth, each with
supporting scientific evidence.
EK 1.D.2: Scientific evidence from many
different disciplines supports models of the
origin of life.
Illustrative examples are titled in blue
Evidence for the
Origin of Life on Earth
• The
evolutionary
tree of life
can be
documented
with
evidence.
• The Origin of
Life on Earth
is another
story…
Hypotheses for the Origin of Life
• Special Creation
– Was life created by a supernatural
or divine force?
– not testable
• Extra-terrestrial Origin
– Was the original source of organic
(carbon) materials comets &
meteorites striking early Earth?
– testable
• Spontaneous Abiotic Origin
– Did life evolve spontaneously from
inorganic molecules?
– testable
Contributions
• Scientific evidence includes molecular building
blocks that are common to all life forms
• Scientific evidence includes a common genetic
code
Conditions on Early Earth
• Reducing atmosphere
– water vapor (H2O), CO2, N2, NOx, H2, NH3, CH4, H2S
– lots of available H & its electron
– no free oxygen
• Energy source
– lightning, UV radiation, volcanic
Provided inorganic precursors form which
organic molecules could have been
synthesized due to the presence of available
free energy and the absence of a significant
quantity of oxygen. Organic molecules did
not breakdown as quickly due to low O2
levels
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Life’s Evolutionary Timeline
• Key events in
evolutionary
history of life on
Earth
–3.5–4.0 bya: life
originated
–2.7 bya: free O2 =
photosynthetic
bacteria
–2 bya: first
eukaryotes
Miller-Urey Experiment
Produced
• amino acids
• hydrocarbons
• nitrogen bases
• other organics
Origin of Organic Materials
• Abiotic synthesis
–(1920) Oparin & Haldane
• Primordial Soup Theory (out of slime)
• states that Life began (~3.8-3.55 bya) in a warm
pond/ocean from a combination of chemicals that
forms amino acids, which then make proteins
• propose reducing atmosphere hypothesis
–(1953) Miller & Urey test hypothesis
• formed organic compounds
–amino acids
–adenine
Key Events in the
Origin of Life
• Origin of Cells (Protobionts)
• Origin of Genetics (RNA World Hypothesis)
• Origin of Eukaryotes
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Origin of Cells (Protobionts)
• lipid bubbles
– separate inside from outside
– metabolism & reproduction
Origin of Genetics
(RNA World Hypothesis)
• RNA is likely first genetic material
• multiple functions: encodes information (selfreplicating), enzyme, regulatory molecule,
transport molecule (tRNA, mRNA)
– Rapid in times of
ecological stress
– makes inheritance
possible
– makes natural
selection & evolution
possible
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Origin of
Eukaryotes
First Eukaryotes
• Development of internal membranes
– create internal micro-environments
– specialization led to increased cellular
efficiency… natural selection
• Endosymbiotic Theory
• Proposed by Lynn
Margulis
infolding of the
plasma membrane
nuclear envelope
endoplasmic
reticulum (ER)
plasma
membrane
nucleus
DNA
cell wall
Prokaryotic
cell
Prokaryotic
ancestor of
eukaryotic cells
plasma
membrane
First Endosymbiosis
Second Endosymbiosis
• Evolution of eukaryotes
• Evolution of eukaryotes
– origin of mitochondria
– engulfed aerobic bacteria, but did not
digest them
– mutually beneficial relationship led to
increased cellular efficiency…
natural selection
internal membrane
system
– origin of chloroplasts
– engulfed photosynthetic bacteria,
but did not digest them
– mutually beneficial relationship led to
increased cellular efficiency…
natural selection
aerobic bacterium
Eukaryotic
cell
Eukaryotic
cell with
mitochondrion
photosynthetic
bacterium
mitochondrion
chloroplast
Endosymbiosis
Endosymbiosis
Ancestral
eukaryotic cell
Eukaryotic cell
with mitochondrion
Theory of Endosymbiosis
• Evidence
• mitochondria & chloroplasts
resemble bacterial structure
– The environment was too hostile for life until 3.9 bya
– Earliest fossil evidence for life dates to 3.5 bya
– genetic
– functional
• mitochondria & chloroplasts
move freely within the cell
• mitochondria & chloroplasts
reproduce independently
from the cell
Geologic Time Scale
• The Earth formed approximately 4.6 billion years
ago (bya)
– structural
• mitochondria & chloroplasts
have their own circular DNA, like bacteria
mitochondrion
Eukaryotic cell with
chloroplast & mitochondrion
Lynn Margulis
– Taken together, this evidence provides a plausible
range of dates when the origin of life could have
occurred.
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Geologic Time Scale
Mass Extinction
• Mass extinctions were followed by
diversification of life-forms
– At the end of the Cretaceous period, many life-forms
disappeared, including the dinosaurs
• These mass extinctions may have been a result of an
asteroid impact or volcanic activity
• Cambrian Explosion
– Diversification of Animals
• within 10–20 million years most of the major phyla of animals appear in
fossil record
Mass Extinctions
Five Major Extinctions
• Every mass extinction reduced the diversity of life
– But each was followed by a rebound in diversity
– Mammals filled the void left by the dinosaurs
Five Major Extinctions
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