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Kathryn Fischer




MSE 2101

MSE 2101 Test #3 Study Guide
Chapter 5: The Nature of Life on Earth
The Big Picture, pg
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Thus, evolution plays a
central role in the definition of life as well as in our understanding of life on Earth
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o (2) Living cells undergo metabolism, by which we mean chemical reactions
that keep the cells alive
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Life on Earth survives under a much wider range of conditions than we would have
guessed a few decades ago, suggesting that life elsewhere might similarly be found in a
fairly broad range of environments
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Chapter 5
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173-179

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On Earth, liquid water plays 3 key roles in metabolism:
o (1) Liquid water allows organic materials to float within the cell, because the
chemicals dissolve in water
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o (2) Liquid water acts as a medium for the transport of chemicals in and out of the
cell, as well as for getting rid of waste products
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o All living cells on Earth depend on liquid water to play these three roles, and this
dependence limits the conditions under which we find life on our planet: we find
life only in places where it is neither too cold nor too hot for liquid water to exist
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o This is the third feature that seems generally needed for all life, aka a means of
storing information
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How does the structure of DNA allow for its replication?

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MSE 2101

o The molecular structure of DNA is a double helix
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§ The two strands are linked with only four DNA bases: ATGC
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Thymine attaches only to
adenine, and cytosine only attaches to guanine
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Step 1 begins with the
complete double helix, step 2 is when the strands separate (“unzip”), step 3 is when
the unzipped strands serve as a template for making a new complementary strand
(because only A attaches with T, G with C), step 4 is the end result with two
identical copies of the original DNA molecule
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Because cell division
is the key to passing down genetic material from one generation to the next,
DNA replication explains the basis of heredity
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§ Instead, most biologists believe that DNA evolved from a simpler selfreplicating molecule (RNA) that carried hereditary information in the earliest
living organisms
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How is heredity encoded in DNA?
o Besides having the ability to be replicated, DNA also determines the structure
and function of the cells within any living organism
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o The instructions representing any individual function (such as the instructions for
building a single protein) make up a gene
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A single gene
consists of a sequence of DNA bases that provides the instructions for a single
cell function
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§ Noncoding DNA makes up more than 95% of the total DNA in human beings
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o The complete sequence of DNA bases in an organism, encompassing all of the
organism’s genes as well as its noncoding DNA, is called the organism’s genome
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o Every member of a particular species has the same basic genome, but individuals
vary here or there in their precise sequence of DNA bases
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Different cell types (such as muscle cells

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MSE 2101

or brain cells) differ only because they express, or actually use, different
portions of their full set of genes
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This fact underlines the science of cloning
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The set of rules for
reading DNA is called the genetic code
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For the purpose of protein
building, each word represents either a particular amino acid or a “start reading” or
“stop reading” instruction
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o Most organisms only use 20 amino acids to make proteins (out of a possible 64
genetic words), so there is a fair amount of redundancy in the genetic code
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This suggests that the genetic code once depended
only on two-base words instead of three-base words
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o Another important feature of the genetic code is that it is the same in nearly all
living organisms on Earth
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§ This common language of the genetic code is further evidence for a common
ancestor of all life on Earth
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o A molecule of RNA is quite similar in structure to a single strand of DNA, except
that it has a slightly different backbone and one of its four bases is different from
one of the DNA bases
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o Several different types of RNA participate in carrying out genetic instructions in
the cell
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§ Ribosomal RNA (rRNA): molecules that compose the ribosome within a
cell, where amino acids are assembled into proteins
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o The process described above is known as RNA translation, because it effectively
translates the genetic instructions into an actual protein
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o The key to this knowledge lies in understanding how DNA molecules gradually
change through time
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§ Speed: copying the entire 3-billion-base sequence in human DNA takes a
human cell only a few hours
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o Errors sometimes occur however, such as the wrong base getting attached in a base
pair (like C with A instead of C with G)
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§ Absorption of ultraviolet light or nuclear radiation or the action of certain
chemicals (carcinogens) can also cause mistakes to occur
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§ Mutations that add or delete a base within a gene tend to have the most
dramatic effects on protein structure, because there is no spacing between
genetic words
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o Mutations that change proteins are often lethal, since the cell may not be able to
survive without the correctly structured protein
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§ In this case, the mutation represents a permanent change in the cell’s
hereditary information
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Mutations therefore provide the basis for evolution
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o The combination of individual variation and population pressure leads to natural
selection, in which the advantageous adaptations will be preferentially passed
down through the generations
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Evolution proceeds through the
occasional beneficial mutation
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§ This process is one of the primary ways that bacteria gain resistance to
antibiotics
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o Lateral gene transfer can change a species more rapidly than individual mutations,
but mutations are still the underlying basis, since they created the genes in the first
place
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5 – Life at the Extreme
Pgs
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What kinds of conditions can life survive?
o Black Smokers: a volcanic vent on the ocean floor that spews out extremely
hot, mineral-rich water
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§ Nearly all organisms would quickly die if placed in the extremely hot water
near a volcanic vent, yet scientists have discovered life – mostly microbes
of the domain archaea – thriving in the extremely hot water around black
smokers and other vents
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Organisms that survive in extreme environments of any kind are called extremophiles
(“lovers of the extreme”)
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o Some can live in normal and extreme conditions, while others can only survive in
the extreme conditions
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Other extreme environments include the dry valleys of Antarctica, which receive
very little rain or snowfall and have temperatures that rarely rise about freezing
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The small spaces within
the rocks occasionally contain small pockets of water from the scarce rain and
snowfall
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§ There are microbes that survive in these tiny pockets of liquid water inside
rocks in freezing cold valleys
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Microbes called endoliths (meaning “within rocks”) can live several
kilometers below the surface of Earth in water that fills the pores within rock
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Another adaptation to extreme conditions is found in endospores – special “resting”
cells produced by some bacteria
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§ Some endospores can even survive in the vacuum of space (hence why
scientists worry about spacecraft contamination)
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Earth’s atmosphere contained oxygen at a level suitable for human life for only the past
few hundred million years (10% of Earth’s history)
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o Extreme life appears to be much more the norm than is life that lives in an
environment suitable to humans
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o (2) The fact that extremophiles can survive such a broad range of conditions
suggests that life may be possible in many more places than we would have
guessed – i
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any world containing an environment in which some type of
extremophile might survive becomes a good candidate for the search for life
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6 – Evolution as Science
Pgs
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o However, we use the theory of evolution to understand how and why these
changes occur
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§ The theory of evolution clearly explains the major features of life on Earth,
but scientists still debate the details of the theory, such as the rate at which
evolution proceeds
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When considering scientific alternatives to evolution, first must examine whether the
alternatives fulfill the three hallmarks of science:
o (1) No supernatural (science seeks explanations for observed phenomena that
rely solely on natural causes)
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o (3) A scientific model must make testable predictions about natural phenomena
that would lead us to revise or abandon the model if the predictions do not agree
with observations
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§ From Judeo-Christian viewpoint, literal interpretations of the Bible
in terms of creation theories do not agree with one another (first chapter
of Genesis directly conflicts with the second chapter of Genesis)
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o Creation Science: proponents of this theory (started in 1980s) tried to find
evidence to support the idea that Earth was created a mere 6000 years ago
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• Evidence they must reject includes radiometric dating of rocks,
astronomical measurements of distances to other stars and
galaxies, and even tree ring data that go back more than 6000
years
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Thus creation
science fails hallmarks 2 and 3
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§ Fails all three hallmarks
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Second and third failures in that it
does not offer a predictive model that can be tested
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Chapter 6: The Origin and Evolution of Life on Earth
The Big Picture, pg
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However, we have found
plausible scenarios for the origin of life based on natural, chemical processes
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Earth has supported life for most or all of the past 4 billion or more years, but life
remained microbial for most of this period
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o If aliens had observed Earth during about the first 90% of its history, they would
have found a planet that was home to nothing more than microscopic species
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o Thus, the likelihood of impacts must be an important consideration in assessing
the habitability of other planets
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However, we have developed or are on the verge of
developing the capability to engineer existing species, including ourselves, and perhaps
to create entirely new species
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Chapter 6
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193-199

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When did life begin? Stromatolite and sulfur isotope evidence tells us that life existed
by about 3
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o Carbon and other isotopic evidence may push the time back to more than
3
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o Life must have existed even earlier than the oldest fossil evidence of it, though we
do not know exactly how much earlier
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o (1) Stromatolites: “rock beds”, 3
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§ Rocks that are characterized by a distinctive, layered structure
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§ Living stromatolites grow in size as sediments deposited over them,
making the microbes migrate upwards in order to remain at the depths to
which they are adapted
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• Microbes near the top generate energy through photosynthesis,
while those underneath eat the organic compounds left as waste by
the photosynthetic microbes
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The
relationship between endoliths and rock is called symbiosis à
“living rock
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5billion years
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§ Finding ancient microscopic fossils is difficult because rocks become
increasingly rare with age
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§ The oldest claimed microfossils from 3
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• The rock is not from a shallow sea as originally assumed, but
instead must have formed near a deep-sea volcanic vent similar to a
black smoker
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MSE 2101

o (3) Isotopic evidence: living organisms have a preference for Carbon 12
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§ This is because carbon-13 amounts for about 1 out of every 89 carbon
atoms… so living organisms and fossils of living organisms always show a
slightly lower fraction of carbon-13 atoms than that found in inorganic
material
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8-3
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Rocks found
here have lower carbon-13 ratio and are more than 3
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• Controversy: rocks found here are metamorphic, which means
they have been transformed substantially by high pressure or heat,
which would explain why no intact microfossils remain within
them
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o If they were volcanic (igneous) rocks, then the case for life is
more difficult to imagine
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85billion years ago
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§ This is important because we expect other worlds with similar conditions
to young Earth… so life might be quite common in the universe
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o By comparing the genomes of different organisms, we should be able to
reconstruct the evolutionary history of much of life on Earth
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§ Lateral gene transfers likely more common among early living
organisms, changing genomes even more rapidly
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§ The fact that plants and animals represent two branches that split off in
about the same place from other eukarya tells us that all animals and
plants are quite similar genetically
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Where did life begin? It seems unlikely that life could have arisen on the land
surface
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o Ozone (O3) is a form of oxygen produced in the upper atmosphere by
interactions between ordinary oxygen (O2) and ultraviolet light from the Sun
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MSE 2101

Before the ozone layer existed, any surface life would have been exposed to
high levels of this radiation
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o However it seems much more likely that life arose in the more hospitable
conditions under water (since water absorbs ultraviolet light) or in rocks beneath
the surface
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o (1) Shallow ponds: organic compounds may have formed spontaneously in
shallow ponds, and volcanic hot springs may also have offered energy to support
an origin of life
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o (2) Deep-sea vents: deep-sea or underground environments would have been
protected from high-energy radiation, making them a better candidate for the
origin of life
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o Even if life first arose in ponds at the surface, the impacts of the late heavy
bombardment probably would have allowed the survival only of life that had
migrated to deep-sea or underground environments
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§

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Chapter 6
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199-207

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Over the past few decades, laboratory experiments have given us insights into the
chemical processes that likely occurred on the early Earth
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Two important caveats with these lab experiments:
o (1) The laboratory experiments generally try to recreate the chemical
conditions that should have prevailed on the early Earth, an assumption
that makes sense if life originated here
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o (2) Science is not bothered by the possibility of life arising through divine
intervention, so these lab experiments don’t address such theories
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MSE 2101

o Such reactions do not occur naturally today, because Earth’s oxygen rich
atmosphere prevents complex organic molecules from forming readily outside
living cells
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o However, the oxygen in our atmosphere is a product of life, produced by
photosynthesis, which means it could not have been present before life arose
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The Miller-Urey experiment: in the 1950s, scientists Stanley Miller and Harold
Urey tested the hypothesis that sunlight-fueled chemical reactions could have led to the
spontaneous creation of organic molecules (because young Earth’s atmosphere was
oxygen-free)
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§ (1) One glass flask was partially filled with water to represent the
sea and heated to produce water vapor
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§ (2) These gases flowed into a second flask, where electric sparks
provided energy for chemical reactions (simulating lightning)
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§ The water soon began to turn a murky-brown, and a chemical analysis
(after letting it run for about a week) showed that it contained many amino
acids and other organic molecules
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§ This means that the experiment’s specific results don’t tell us much about
what happened on the early Earth
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o Modern tweaks to the experiment haven’t cleared up our understanding
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§ Hydrogen can play a major role in facilitating the production of organic
molecules, but the hydrogen content of the early atmosphere is a
topic of great debate
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o (1) Chemical reactions near deep-sea vents: as these undersea volcanoes
heat the surrounding water, a variety of chemical reactions can occur between the
water and the minerals
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o (2) Material from space: analysis of meteorites shows that they often contain
organic molecules, including complex molecules such as amino acids
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§ Moreover, recent research shows that ultraviolet light from the young Sun
could also have produced some of the building blocks of life
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• This dust, laden with organic molecules, could have “rained down”
on the young Earth
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• The heat and pressure generated by these impacts may have further
facilitated the production of organic molecules in Earth’s
atmosphere and oceans
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§ Given that there are 3 different ways of obtaining organic molecules, it
seems likely that at least parts of the early Earth would’ve contained
substantial amounts of the organic molecules needed for life
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o One approach is to look backward from organisms living now, starting with
heredity
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§ Therefore we are looking for a molecule that is simpler than DNA but still
capable of making fairly accurate copies of itself – most obvious candidate
is RNA
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o RNA still possesses hereditary information in the ordering of its bases and in
principle it can serve as a template for making copies of itself
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§ Solution to dilemma came from scientist Thomas Cech who found that
RNA could catalyze biochemical reactions in much the same way as
enzymes
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• Some RNA molecules can even partially catalyze their own
replication
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To create this RNA world, experiments were created to show that several types of
inorganic molecules (such as mineral clay) can facilitate the self-assembly of long,
complex organic molecules
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§ Pre-cells can be formed naturally in at least 2 different ways: (1)
by cooling a warm-water solution of amino acids so that they form bonds
among themselves to make an enclosed spherical structure or (2) by
mixing lipids with water
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Confining RNA and other organic molecules within pre-cells could have facilitated an
origin of life in two important ways:
o (1) Keeping molecules concentrated and close together should have
increased the rate of reactions among them, making it far more likely that a
self-replicating RNA would have arisen
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o (2) Once self-replicating RNA molecules came to exist, pre-cells would have
kept them isolated from the outside in a way that should have facilitated a
molecular analog to natural selection, in which RNA molecules that replicated
faster and more accurately would rapidly come to dominate the population
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The general requirements for life are much broader than the requirements for complex
beings such as humans
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Our solar system contains a vast number of worlds, but most of them are unlikely to
have life because they lack a liquid medium of any kind
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More worlds may have had liquid water or other liquids in the distant
past
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We are living during a time when many
spacecraft are simultaneously exploring different worlds in our solar system, and more
missions are being planned for the future

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