Origin of Cells (Protobionts)
 Non-living molecules surrounded by a
membrane had some properties of life: simple
reproduction & metabolism, separation of
internal environment from surroundings

Could have formed spontaneously from organic
compounds
 Phospholipid (contain carbon 
Bubbles…
Tiny bubbles…
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organic!) bilayers can
form when lipids are placed
in water
 May have taken up additional organic
molecules from environment
RNA World Hypothesis: Origin of Genetics
 RNA is likely first genetic material


Able to self-replicate and store protobionts’
genetic information
multi-functional
 codes information
 makes inheritance possible
 Dawn of natural selection &
evolution
 enzyme functions (catalyst)
 Ribozymes

RNA enzymes that can make
short pieces of RNA
 Involved in cell replication
 Involved in protein synthesis

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Likely lead to a “DNA World”
Geological Evidence Supports the Models
for the Origin of Life
 Fossils found in sedimentary rocks tell
us which organisms lived first

Rocks occur in “strata”, or layers
 Younger sediments are closer to surface
than older ones
 Method for determining age of fossils:

Radiometric dating
 Involves analyzing amount of certain
radioactive isotopes remaining
 Each isotope has a unique half-life: # of years it
takes for 50% of the original sample to decay
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Key Events in Origin of Life
 Key events in
evolutionary history
of life on Earth



Earth formed
approximately 4.6 bya
Environment became
suitable for life 3.9
bya
Earliest fossils are
from 3.5 bya
 … provides evidence
for when life could
have originated
(probably 3.5- 4.0
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bya)
Prokaryotes
 Prokaryotes dominated life
on Earth from 3.5–2.0 bya
3.5 billion year old
fossil of bacteria
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modern bacteria
chains of one-celled
cyanobacteria
First prokarotes: Stromatolites
 Rocklike structures composed of
layers of prokaryotes & sediment
 Oldest known fossils
 Existed 3.5 bya & formed complex
communities

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So life on earth must have originated
earlier than that
Lynn Margulis
 Prokaryotes were the first life forms on
earth.

Protobionts were replaced by
autotrophs – organisms that can
produce all their needed compounds
from molecules in the environment
 Often use light as an energy source

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Autotrophs likely led to heterotrophs–
organisms which live on products
excreted by autotrophs, or on
autotrophs themselves
Oxygen atmosphere
 Oxygen began to accumulate 2.7 bya

reducing  oxidizing atmosphere
 Produced via photosynthesis
 Photosynthetic prokaryotes called cyanobacteria
 makes aerobic respiration possible
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~2 bya
First Eukaryotes
 Development of internal membranes


create internal micro-environments
advantage: specialization = increase efficiency
 natural selection!
infolding of the
plasma membrane
plasma
membrane
endoplasmic
reticulum (ER)
nuclear envelope
nucleus
DNA
cell wall
Prokaryotic
cell
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Prokaryotic
ancestor of
eukaryotic
cells
plasma
membrane
Eukaryotic
cell
Endosymbiosis
 Process explaining the origin of
mitochondria and chloroplasts

Mitochondria & chloroplasts were formerly small prokaryotes
living within larger cells
 Evolution of eukaryotes

Mitochondria & chloroplasts became a single, interdependent
organism w/ their host
internal membrane
system
aerobic bacterium
mitochondrion
Endosymbiosis
Ancestral
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eukaryotic
cell
Eukaryotic cell
with mitochondrion
Endosymbiosis: Origin of Mitochondria
 Proposed ancestors of mitochondria:
aerobic (oxygen-using) heterotrophic
prokaryotes
 Cells engulfed aerobic bacteria, but
did not digest them
 mutually beneficial relationship: aerobic
cells could benefit from having a
structure that itself utilized oxygen
 natural selection
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Endosymbiosis: Origin of Chloroplasts
 Proposed ancestor: photosynthetic
Eukaryotic
cell with
mitochondrion
prokaryotes
 Cells engulfed photosynthetic
bacteria, but did not digest them

mutually beneficial relationship: hetertrophic
“host” could use nutrients released from
photosynthesis
 natural selection!
photosynthetic
bacterium
chloroplast
Endosymbiosis
Eukaryotic cell with
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chloroplast & mitochondrion
mitochondrion
Evidence of Endosymbiosis

structural
 mitochondria & chloroplasts
resemble bacterial structure

genetic
 mitochondria & chloroplasts
have their own circular DNA, like bacteria

functional
 mitochondria & chloroplasts
move freely within the cell
 mitochondria & chloroplasts
reproduce independently
from the cell via binary fission (the same
process that some prokaryotes use)
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The Origin of Multicellularity
 The evolution of eukaryotic cells allowed


for a greater range of unicellular forms
(what we call protists today)
A second wave of diversification occurred
when multicellularity evolved and gave rise
to algae, plants, fungi, and animals
The oldest known fossils of multicellular
eukaryotes are of small algae that lived
about 1.2 billion years ago
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