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Chapter 19/20
Section 19-3: Earth’s Early History
The Mysteries of Life’s Origins
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Earth formed as pieces of cosmic debris collided
Young planet struck by one or more huge objects and
melted
Elements redistributed by density
Millions of years of violent volcanic activity,
comets/asteroids hitting surface
About 4.2 bya surface cooled enough for solid rocks to
form, water to condense, permanent oceans form
The Mysteries of Life’s Origins
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Early atmosphere had little to no oxygen
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Mostly carbon dioxide, water vapor, nitrogen
Smaller amounts of carbon monoxide, hydrogen sulfide,
hydrogen cyanide
Sky pinkish-orange
Oceans brown with dissolved iron
The First Organic Molecules
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In 1953, chemists Stanley
Miller and Harold Urey
tried recreating conditions
on early Earth to see if
organic molecules could
be assembled under these
conditions
The First Organic Molecules
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After a week they had produced 21 amino acids
Showed how mixtures of organic compounds necessary
for life could have arisen
Idea of atmospheric composition incorrect
Formation of Microspheres
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Geological evidence shows that about 200-300 mya after
Earth cooled enough to carry liquid water cells similar to
bacteria were common
Large organic molecules form bubbles called proteinoid
microspheres under certain condition
They are not cells, but have some living characteristics –
selectively permeable membranes, means of
storing/releasing energy
Thought to have acquired characteristics of living cells
about 3.8 bya
Evolution of RNA and DNA
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Central dogma
The “RNA World” hypothesis about the origin of life
suggests RNA evolved before DNA
Simple RNA-based system underwent several changes to
DNA-directed protein synthesis
Experiments show how small RNA sequences could have
formed from simpler molecules
Under certain conditions, RNA sequences help DNA
replicate, process mRNA after transcription, catalyze
chemical reactions
Some can even grow/replicate on their own
Production of Free Oxygen
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Microfossils,of prokaryotes that resemble bacteria have
been found in rocks more than 3.5 billion years old
Evolved in the absence of oxygen
Photosynthetic bacteria became common and producing
oxygen by 2.2 bya
Oxygen combined with iron in the oceans, producing iron
oxide which sank to ocean floor and formed great bands
of iron that are the source of most iron ore mined today
Oceans changed blue-green
Production of Free Oxygen
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Next oxygen started accumulating in the atmosphere
Ozone layer formed, skies turned blue
Early atmosphere thought to be similar to volcanic gases
Production of Free Oxygen
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First cells evolved in absence of oxygen
Deadly poison , many cells went extinct
Some evolved metabolic pathways to use the oxygen
(cellular respiration) or ways to protect themselves from
it
Evolution of Eukaryotic Cells
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It is believed that about 2 bya some ancient prokaryotes
began evolving internal membranes – ancestors of
eukaryotes
According to endosymbiotic theory, prokaryotic cells
entered and began living inside those ancestral
eukaryotes
Over time, they developed symbiotic relationships
Evolution of Eukaryotic Cells
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Microscopists saw that the membranes of mitochondria
and chloroplasts resembled the cell membranes of freeliving prokaryotes
Two related hypotheses:
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Mitochondria evolved from endosymbiotic prokaryotes that
were able to use oxygen to generate energy-rich ATP
molecules (now could use oxygen)
Chloroplasts evolved from endosymbiotic prokaryotes that
had the ability to photosynthesize
Modern Evidence
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During the 1960s, Lynn Margulis of Boston University
noted that mitochondria and chloroplasts contain DNA
similar to bacterial DNA
Also have ribosomes that resemble those of bacteria
Mitochondria and chloroplasts, like bacteria, reproduce by
binary fission
Significance of Sexual Reproduction
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During asexual reproduction (prokaryotes) , genetic
variation is restricted to mutations in DNA
When eukaryotes reproduce sexually, offspring receive
genetic material from two parents
Meiosis and fertilization shuffle genes, generating genetic
diversity.
Offspring of sexually reproducing organisms are never
identical to parents or siblings
Increases the likelihood of a population’s adapting to new
or changing environmental conditions
Multicellularity
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Multicellular organisms evolved a few hundred million
years after the evolution of sexual reproduction
Even greater diversity