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Chapter 21
The Origin and Evolution of Life
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Early Attempts at Understanding
• Ancient cultures believed life could be generated from nonliving matter (spontaneous generation)
• Later experimental evidence proved that life can only come
from other living things (biogenesis)
Francis Redi (1668)
• Two jars with decaying meat, both exposed to atmosphere, one
covered with gauze, one uncovered
– Flies settled on meat in open jar
– Maggots appeared on meat in open jar
– Did not appear in covered jar
• Conclusion: maggots came
from the eggs of the flies
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Louis Pasteur (1861)
• Placed fermentable sugar solution in a flask with a swan neck
Long swan neck open to air to allow oxygen to enter
– Mixture and flask boiled to kill organisms in solution
– Pasteur postulated:
• Spontaneous generation would not occur even in the presence of O2
• Organisms in air caused fermentation
• Organisms would settle on the bottom of the curved portion of the
neck and not reach solution
• Sugar solutions would not ferment
• Also cut off swan neck flask
– Microorganisms reached sugar solution
– Solution fermented
• Conclusion: spontaneous
generation does not occur
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Current Theories about the Origin of Life
• Extraterrestrial Origin for Life on Earth
– Life existed elsewhere in solar system (Mars)
– Possibly arrived from extraterrestrial sources via meteorites
• Earth Origin for Life on Earth
– Early earth was probably a hot planet with no oxygen in the
atmosphere
– Organic molecules could have possibly:
• Formed from inorganic ones
• Collected in oceans
• Then served as building blocks and a source of energy for simplest
organisms
– Life could have originated as:
• Heterotroph
• Autotroph
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Heterotroph Hypothesis
• Heterotrophs (animals, fungi, most protozoa, bacteria)
– Consume organic molecules from surroundings in order to:
• make new molecules
• provide an energy source
• First living things were bacteria-like anaerobic organisms that
lived off organic molecules accumulated in ocean
• As usable molecules were depleted from the environment,
these primitive cells may have mutated to be able to convert
unusable material to usable compounds
• Genetic mutations may have been common
– Amount of UV light high (no ozone to screen out UV)
• Mutated organisms survived and reproduced
• Through series of genetic mutations, complex series of
biochemical reactions originated within some cells in order for
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the cells to convert unusable to usable compounds
Autotroph Hypothesis
• Autotrophs
– Use external energy to combine inorganic molecules into
organic molecules
• Chemoautotrophs – use energy from inorganic chemical reactions
(some prokaryotic organisms)
• Photoautotrophs – use
energy
the sun
But
in from
reality,
(eukaryotic plants, algae, some prokaryotic organisms)
• First organism
was
a chemoautotroph
that used
WE
JUST
DON’T KNOW
energy released from inorganic chemical reactions to
synthesize organic molecules
• Supporting Evidence
– Most prokaryotic organisms are autotrophic and live in hot
hostile environments (hot springs, thermal vents)
• Could have originated on hotter earth
• Subsequent evolution could have led to a variety of prokaryotic cells
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The Origin of Genetic Material
How genetic information could have been stored inside early life forms
Both DNA and RNA store and transfer genetic information
– RNA is a simpler molecule
– In most cells today
• DNA stores genetic information
• RNA assists DNA in transferring genetic information
• However, in some viruses, RNA serves as the genetic material
• Evidence for RNA being the first genetic material:
– RNA can be assembled from simpler subunits that could
have been present on early earth
– RNA molecules are able to make copies of themselves
without the need of enzymes or being inside a cell
– Once a genetic material can be copied, the organism can
reproduce
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Developing of the Earth’s Atmosphere
• Initially earth was too hot to have an atmosphere
• As it cooled, an atmosphere without oxygen formed
• About 2 billion years ago, atmosphere changed to one
with oxygen
– Mainly as a result of photosynthesis
• Living things existed before oxygen was present
• First anaerobic organisms mutated to photosynthetic autotrophs
• Waste product of photosynthesis is O2
– Aerobic respiration evolved resulting in more efficient
conversion of food to energy
– Ozone also formed resulting in:
• Diminished spontaneous formation of complex organic molecules
• Reduced genetic mutations in cells
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Developing of the Earth’s Atmosphere
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Establishment of Three Major Domains of Life
1. Eubacteria
•
Many different metabolic abilities
Heterotrophs, photoautotrophs, chemoautotrophs
Prokaryotic
2. Archaea
•
•
•
•
Primarily chemoautotrophs
Usually found in harsh environments
Use inorganic chemicals to generate energy
Reactions often produce methane (CH4) or hydrogen
sulfide (H2S)
3. Eucarya
•
•
•
Plants, animals, fungi, protozoa, algae
Cells are larger than that of the prokaryotics
Cells appear to have incorporated cells of other
organisms
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From Prokaryotes to Eukaryotes
The Endosymbiotic Theory
• Eukaryotic cells came from combining several
different types of prokaryotic cells
– Some eukaryotic organelles may have originated as
free-living prokaryotes
– Cells formed a symbiotic relationship,
• Adapted, reproduced as a team and evolved into present
eukaryotic cells
– Examples: mitochondria, chloroplasts
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The
Endosymbiotic
Theory
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Role of Natural Selection
• Evolution – continuous genetic adaptation of a population of
organisms to its environment over time
– Involves changes in genes present in a population (not individual organisms)
• Natural Selection - process that encourages passage of
beneficial genes and discourages passage of harmful (or less
valuable) genes to future generations
• Theory of Natural Selection – (proposed by Darwin and Wallace)
Individuals whose gene combinations favor life in their surroundings will
be likely to survive and pass their genes on to the next generation
• Theory of Evolution – populations of organisms become
genetically adapted to there surroundings over time
– Natural selection brings about evolution by selecting which genes are
passed on
Natural
Selection
Genetic
selection
Evolution
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Misinterpretations Associated with
Natural Selection
• “Survival of the Fittest”
– Survival alone is not good enough
– Survival and reproduction is the key
• Struggle for life
– Not necessarily open conflict
– But finding the necessary resources for survival and
reproduction
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Genetic Diversity
influences the process of natural selection
Resulting from mutation
• Spontaneous mutation – changes in DNA that cannot
be tied to any particular causative agent
– Cosmic radiation, mutagenic chemicals
– Naturally occurring genetic mutation rates are low (~1 in 100,000)
• Possible outcomes when a gene is altered
– Mutation so minor that it has no effect
– Mutation harmful
– Mutation is beneficial
• In order for mutations to be passed to the next generation, they
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must happen in cells that will become sex cells
Genetic Diversity
Resulting from sexual reproduction
• Does not generate new genetic information
• Genes recombine into new mixtures
Genetic recombination: each individual has unique
set of genes, half donated by mother, half by father
Acquired characteristics do not
influence natural selection
not genetically determined
gained during life of organism
cannot be passed on to other generations
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Processes that drive natural selection
Selecting agents – specific environmental factors that
favor and influence the likelihood that certain
characteristics will be passed on
• Differential Survival
– Some individuals possess characteristics enabling them to
preferentially survive and pass on their genes
• Differential Reproductive Rates
– Some organism may be able to better utilize resources to
produce more offspring
• Differential Mate Selection
– Some individuals possess characteristics enabling them to
be more frequently chosen as mates
• Size, aggressiveness, attractiveness to opposite sex
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Differential Reproductive Rates
Cows act as the
selecting agent by
eating the taller
plants first
Two identical fields of clover
Tall plants rarely
reproduced
Seeds from both
fields were
collected and
grown under
identical conditions
Only short plants
produced seeds
Cows selectively ate
plants with tall gene
Seeds from grazed
field produced
mostly short plants
Seeds from
ungrazed field
produced tall, short
but mostly medium
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sized plants
Hardy-Weinberg Concept
Conditions necessary for genetic makeup to remain constant
1. Mating must be completely random
2. Mutations must not occur
3. Migration of individual organisms into and out of
the population must not occur
4. The population must be very large
5. All genes must have an equal chance of being
passed onto the next generation (no natural selection)
Concept allows for comparisons
of genes within a population to determine if genetic changes are occurring
of genes of two different populations to determine similarities in genetic makeup
Concept is an argument for evolution because most of these conditions cannot be met
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Processes That Drive Natural Selection
All the genes of all
individuals in a
population
Constant genetic makeup over several
generations (unchanging gene pool)
indicates that evolution is not taking place
A changing gene pool indicates evolution
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is taking place
Evidence for the Theory of Evolution
1. Species and populations are changing genetically
through mutations, adaptations, environmental
changes, selective breeding, extinctions
2. Evolution occurs by small steps
–
–
–
All species use same DNA code and amino acid building blocks
Difficult to eliminate a structure that becomes part of a process
controlled by genes (example: appendix)
Closely related species have similar DNA and embryological
development
3. Fossil records show changes in kinds of organisms.
–
Species appear and subsequently become extinct
4. New techniques and discoveries support evolution
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Misconceptions about the Theory of Evolution
1. Evolution happened only in the past
–
Lots of evidence exists for current genetic changes
2. Evolution has a specific goal
–
–
Natural selection favors organisms that best fit the current environment
Random events can have major influence on natural selection
3. Changes in environment cause mutations that are
needed for an organism to survive
–
Mutation are random events (not necessarily adaptive)
4. Individual organisms evolve
–
Individuals may adapt by changing behavior or physiology but they
cannot change their genes
5. An existing species may have arisen from another
present-day species
–
The species in question may have had a common ancestor
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Species
• Population of organisms whose members have the
potential to interbreed naturally to produce fertile
offspring but do not interbreed with other groups
– Populations that demonstrate gene flow between them
• Gene flow - movement of genes from one generation to the next or
from one region to another
– Smallest irreversible step in the evolutionary process
• Two key ideas in these definitions
– Species is a population of organisms (not individuals)
– Offspring must be fertile
• Identifying species
– Biological species concept
• Species can be identified by their inability to interbreed
• Not a practical way to distinguish species
– Morphological species concept
• Method of using physical characteristics to identify species
• Useful but not foolproof
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Speciation
• Range – geographical area over which a species is
found
• Geographic isolation – portion of a species becomes
reproductively isolated from the rest of the gene pool
by geographic change
– No gene flow occurring
• Geographical barriers - geological features that keep
portions of the species from exchanging genes
• Speciation - process of generating new species
– Over a long period of time, accumulated genetic
differences may result in subspecies
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How New Species Originate
Three step process
1.
2.
3.
•
•
Geographic isolation occurs
Selective agents favor specific valuable genetic combinations
Genetic differences become so great that reproduction
between two groups is impossible
– Populations become separate genetically
Process has occurred only if gene flow between
isolated populations does not occur even after
barriers are removed
Separation of species is not enough to generate new
species
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Theory of Natural Selection as a
Mechanism for Evolution
•
•
1.
2.
3.
4.
Proposed by Darwin and Wallace (1858)
Based on the following assumptions
All organism produce more offspring than can survive
No two organisms are exactly alike
Organisms are in a constant struggle for survival
Individuals that possess favorable characteristics for
their environment have higher rate of survival and
produce more offspring
5. Favorable characteristics become more common in
the species and unfavorable characteristics are lost
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End of Chapter 21
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