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Learning Objectives
The Origin and Evolutionary
History of Life
1. What conditions do geologists think existed on
early Earth?
2. The prebiotic soup hypothesis and iron–sulfur
world hypothesis
3. Hypotheses for the origin of cells?
4. Evolution of photosynthetic autotrophs and
their affect?
Chapter 21
Early Earth
The Origin of Life
• Biologists generally agree:
1. life originated from nonliving matter
2. by chemical evolution
•How did life begin?
•Earth is ~ 4.6 billion years old!!
•CO2, H2O, CO, H2, N2, (& ammonia, hydrogen
sulfide, methane)
•No O2
Things to consider
• There is no direct fossil evidence of the
origin of life
• But - • Experiments have demonstrated
molecules found in all living
organisms, may have formed
abiotically.
• Origin of life can not be tested experimentally
• But - • hypotheses about chemical evolution can be
tested
4 Requirements needed for
Chemical Evolution of life
1. Absence of oxygen!!!
• oxygen would oxidize organic molecules
- Atoms make molecules
- Organic molecules have Carbon atoms
- So! O2 would have broken down organic
molecules
- O2 would not have been available
(bonded to other molecules)
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4 Requirements for needed for
Chemical Evolution of life
2. Energy - to form organic molecules
- Earth was violent!
- Sun
- Lightning
- Volcanos
4 Requirements for
Chemical Evolution
3. Chemical building blocks to build organic
molecules
• water, minerals, gases in atmosphere
• to form organic molecules
4. Lots of time - for molecules to accumulate
and react
- Earth ~4.6 bya; Life ~ 3.5 bya
Electrodes
Miller and Urey’s
How did Organic Molecules occur?
Experiment
NH3
CH4
2 Hypothesis:
1. Prebiotic soup hypothesis
• organic molecules formed near Earth’s
surface “sea of organic soup” or on rock
or clay surfaces
• - smaller organic molecules bonded to
make bigger molecules
Spark chamber
Condenser
Boiling
chamber
Heat source
How did Organic Molecules occur?
2. Iron–sulfur world hypothesis
• organic molecules formed at hydrothermal vents in
deep ocean floor
• Diverse communities exist here
• Produce precursers of biological molecules
• Ammonia! Precurser of Proteins & Nucleic Acids
• Also: in Lab Iron & Nickel cause reax with CO and
hydrogen sulphide to make acetic acid/other simple
organic molecules
H2
H2O
To
vacuum
Organic molecules
collect in the trap
The Origin of Cells
• Scientists have made Protobionts:
• macromolecule assemblages made abiotically (not
from living things)
• macromolecules formed spontaneously!
• We have found Protobionts:
• Can divide
• Organized
• Internal environment differs from external
environment
• Metabolism (chemical/energy changes)
Did Cells arise from protobionts??????
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Protobionts: e.g.,Microspheres
•Made from Water + abioticically produced
protein!
• Can divide!!!!
•Excitable – electrical charge across their surface
•Can absorb materials from environment – acting
like they have a membrane
•“Precells??”
Molecular reproduction was important step in
origin of cells
DNA → RNA → protein
• DNA and RNA can replicate if right molecules are
present
• Important with respect to cell division
• Was RNA here first. Many think so.
Scientists (1980) working with single-celled organism
observed RNA (not enzymes) – catalyzing chemical
reactions!!!!!!!!
- They called these Rybozymes
- Many other found
- Some catalyze replication of RNA!!!!!!!!!!!!!!!!!!!!!!!!
Molecular reproduction was important step in
origin of cells
DNA → RNA → protein
• DNA and RNA can replicate if right molecules are
present
• Important with respect to cell division
MANY hypothesize RNA occurred 1st.
- what if by chance an RNA strand was/became a
Rybosyme?
- what if RNA strand replicated - - DNA?
- what if membranes formed spontaneously?
- exps. H2O+lipids+proteins shaken = vescicles
The First Cells
• 2 types of cells
Prokaryotes (no nucleus, no organelles)
Eukaryotes (nucleus & organelles)
• When did first cells appear on Earth?
• Who was first Prokaryotes or Eukaryotes?
Microfossils ~3.5 MYA or rock squiggles?
Evidence suggests 1st cells were Prokaryotes
Did this happen? Time & molecules were there?????
Stromatolites
• Prokaryotic heterotrophs
• “consumed” organic molecules from environment
• probably anaerobic bacteria (No O2 then!)
• Autotrophs - evolved later
• produced organic molecules by photosynthesis
- Put O2 in atmosphere!!!!
- Created Ozone layer
- changed early life
- permitted evolution of efficient aerobes
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Ozone
ORIGINAL
PROKARYOTIC
HOST CELL
DNA
Endosymbiont
Hypothesis
Aerobic bacteria
• Provided
UV protection
• UV from sun breaks
double bonds of Oxygen
Molecules and forms O3
•i.e., O2 from
photosynthesis resulted
in Ozone – protection for
life
Multiple invaginations of the
plasma membrane
Aerobic bacteria
become mitochondria
Photosynthetic
bacteria...
... become
chloroplasts
EUKARYOTIC CELLS:
PLANTS, SOME
PROTISTS
EUKARYOTIC CELLS:
ANIMALS, FUNGI,
SOME PROTISTS
Fig. 21-7, p. 454
Evidence for endosymbiotic hypothesis
• Biochemistry of eurkaryotic cells and bacteria
• Centrioles, mitochondria, and chlorplasts have DNA
• Remnants of the DNA that occurred in engulfed bacteria???
• Intermediates?? e.g., ameoba has no mitochondria but
does have bacteria that performs same role
• Animals that photosynthesize: coral, clam, snail,
paramecium (photosynthetic algae in their cells_
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